Very Large Array Radio Study of a Sample of Nearby X-Ray and Optically Bright Early-type Galaxies

We reduce and analyze VLA data obtained both before (historical VLA data) and after the major upgrade in 2011 (Karl G. Jansky VLA/EVLA data; Perley et al. (2009, 2011)).

The major data size difference is in the increased bandwidth of the observations due to the upgrade to the Wideband Interferometer Digital ARchitecture (WIDAR) correlator system. The older historical data consisted of only one or two spectral windows with tens of channels with a channel width of 100–1000 kHz, whereas the Karl G. Jansky VLA data have tens of spectral windows with up to hundreds of channels with a channel width of 3000 kHz, which makes the size of the data set significantly larger. This affects our approach to the calibration methods we used for the two different data sets. For the Karl G. Jansky VLA data (∼20–100 GB), we used the more effective pipeline calibration method, and in the case of historical data a manual calibration approach was chosen, following the "Jupiter continuum calibration tutorial" available on the VLA NRAO website. ¹⁶

4.1.1. Historical VLA Data

4.1.2. Karl G. Jansky VLA Data

The new Karl G. Jansky VLA observations at 1–2 GHz presented here include 20 sources in A configuration from our project (ID: 15A-305, P.I.: Werner) and 10 archival observations in various VLA configurations (Table C1). The details of the data reduction and imaging were described in our recent paper on the giant elliptical galaxy IC 4296 (Grossová et al. 2019).

A similar approach is used to calibrate ¹⁸ and image all sources from the 2015 project. The corresponding flux-density calibrators for each observation are listed in Table C1.

The imaging was performed with the MultiTerm MultiFrequency synthesis (MTMFS) clean algorithm (Rau & Cornwell 2011) with the briggs (robust=0) weighting scheme (Briggs 1995). The second-order Taylor polynomial (nterms=2; McMullin et al. 2007) was used to account for the spectral behavior of the sources. The various spatial scales of radio emission require an individual approach to each source with different combinations of weightings, gridders, convolvers, and UV-tapers. When the dynamic range of the total-intensity images reaches high enough values, ¹⁹ a few cycles of phase and possibly one cycle of amplitude and phase self-calibration are performed.

An example of the final total-intensity map is presented for the giant elliptical galaxy NGC 4472 in Figure 2.

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4.1.3. Total Flux Densities and Uncertainties

The final total flux densities centered at 1.5 GHz (S_1.5GHz) for each source are derived using the imstat casa task. It is worth noting that, for sources in the sample observed only in A or B configuration, the final flux density might be lower than the "true" total flux density, because of the missing short baselines. On the other hand, we are mainly interested in central cores, where the flux density values are not affected by the missing short baselines.

The corresponding measurement uncertainties for the flux densities (ΔS_{1.5 GHz}) are determined as follows (e.g., Klein et al. 2003; Rajpurohit et al. 2018):

$$\begin{eqnarray}\begin{array}{rcl}{\rm{\Delta }}{S}_{1.5\mathrm{GHz}} & = & \sqrt{{\left({S}_{1.5\,\mathrm{GHz}}\cdot {S}_{\mathrm{cal}}\right)}^{2}+{S}_{{\rm{n}}}^{2}+{S}_{{\rm{z}}}^{2}}\\ & = & \sqrt{{\left({S}_{1.5\mathrm{GHz}}\cdot {S}_{\mathrm{cal}}\right)}^{2}+{\left({\sigma }_{\mathrm{rms}}\cdot \sqrt{{npts}/{A}_{\mathrm{beam}}}\right)}^{2}},\end{array}\end{eqnarray} \tag{ 1 }$$

where S_cal is the specific calibration uncertainty, which is about 4% for VLA observations (Perley & Butler 2013). S_n, the noise uncertainty, is defined as the off-source rms noise (σ_rms) of the image multiplied by the square-root of the number of points (npts) in pixels per area, which covers the entire radio emission of the source divided by the beam area (A_beam) in pixels. The term S_z accounts for a possibly wrong zero level, but may be neglected for interferometric observations (Klein et al. 2003).

The rms noise values were determined from four circular off-source regions in the image using casaviewer, and the median was taken as the final σ_rms. The casa tool for image analysis, ia.beamarea, gives an area that is covered by the beam, or A_beam, in pixels. Finally, we create a polygonal region enclosing the source's entire radio emission and save it into a casa region file with the extension ".crtf". The casa task imstat with a parameter region='*.crtf' is used to determine the integrated flux density at 1.5 GHz (S_1.5GHz) and the number of points (npts) or pixels within that specific region of the source emission.

The value of the peak intensity at 1.5 GHz (S_peak) and the corresponding uncertainties are determined accordingly.

4.1.4. Spectral Index Maps

To investigate the most-recent activity of the AGN in our sample, we produce in-band spectral index maps from our 15A-305 VLA A configuration project for 18 giant elliptical galaxies. Each observation contains 16 spectral windows spanning the frequency range between 1–2 GHz.

We adopted two different approaches to estimate the spectral index. First, we used the MTMFS deconvolution algorithm in the tclean casa task. It produces two Taylor coefficient images: tt0 as an equivalent of a Stokes I image, thus containing the information on the emission at a reference frequency, and tt1 as a Taylor expansion term. The spectral index, α, is then defined as α = tt1/tt0. The final output of the spectral index map is stored in the casa product file, ".alpha.image.tt0" with the corresponding measurement uncertainty values saved in the casa product, ".alpha.image.alpha.error.tt0". To focus only on the emission above 5 × σ_rms, we applied this threshold value through the task widebandpbcor to the final "alpha image" and with a parameter calcalpha we recalculated the spectral index map. In the casaviewer, a circular region for each source with the diameter of 0.7 kpc is defined to investigate the spectral index at the same physical scale in all 18 galaxies. The size of the central region (0.7 kpc) was chosen to be applicable to all of our sources taking into account the different distances. This region was then used to extract the mean spectral index values from the threshold-adjusted casa "alpha images".

For our second approach, we estimated the spectral index fitting flux densities measured in the central regions as described in the following. We split the 12 spectral windows ²⁰ into four chunks and "cleaned" them separately with specific reference frequencies (1.1, 1.3, 1.7 and 1.9 GHz). Then, we smoothed the image chunks with the casa task imsmooth to gain the same restoring beam size (resolution). Lastly, in casaviewer, we extracted the flux densities from a circle of 0.7 kpc diameter centered at the radio core (consistent with the peak of radio intensity or the point from which the symmetrical jets/lobes are streaming out and are confirmed by the position of the center listed in the literature or NASA/IPAC Extragalactic Database (NED) ²¹ ) and calculated the spectral indices as a slope of the log-log fit with the extracted flux densities on the y-axis and the corresponding frequencies on the x-axis.

A comparison between the two methods adopted to estimate the spectral index is shown in Figure 3 for the case of NGC 4472; the results appear in good agreement.

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However, the latter method is simpler, because the spectral index maps ("alpha images") are automatically created during the tclean task with the parameter nterms=2. For further analysis of spectral indices, we concentrate only on this approach.

We use archival Chandra Advanced CCD Imaging Spectrometer (ACIS) observations first reported and described in Lakhchaura et al. (2018).

The archival Chandra observations were processed using the standard CIAO (v4.13; Fruscione et al. 2006) procedures and most-recent calibration files (CALDB 4.9.4). For galaxies observed multiple times, the individual observations were reprojected and merged. Most galaxies were observed in the VFAINT mode using the ACIS-S chip; however, for a couple of galaxies we combined the ACIS-S and ACIS-I observations, and for some of the galaxies there were only ACIS-I observations available.

The observations were deflared using the lc_clean algorithm within the deflare routine. The images were generated from the merged observations using the fluximage procedure with a binsize of 1 pixel (0.492 arcseconds). The images were background subtracted using blanksky background files, which were scaled to match the observations in the 9–12 keV band, and exposure corrected using the corresponding exposure maps. The point sources were found using the wavdetect tool, inspected visually and filled with a mean-surrounding surface brightness using the dmfilth procedure (CIAO 4.13).

The two-dimensional surface brightness distribution of each image obtained by this procedure was modeled using a projected version (Ettori & Fabian 2000) of a classical beta model (Cavaliere & Fusco-Femiano 1976). The fitting was performed in the SHERPA (v4.13; Freeman et al. 2001) package using the Cash statistics (Cash 1979) and Monte Carlo optimization method. The fitted models were subtracted from the original images and the resulting residual images were used for further analysis. Individual X-ray cavities were searched by eye using residual images, and their reliability was checked using the Poisson statistics to be at least 4σ under the surrounding background. The detection of these cavities is further supported by a novel machine-learning method (a.k.a CADET; T. Plšek et al. 2021, in preparation).

5.1.1. Central Radio Emission

5.1.2. Extended Radio Emission

The morphology of the radio emission at 1–2 GHz varies widely within our sample of early-type galaxies.

On the one hand, the total-intensity radio maps reveal well-collimated, large-scale radio jets and lobes extending up to hundreds of kiloparsecs (e.g., NGC 315, Figure B3(c)) or small-scale jets residing within a few kiloparsecs from the radio nucleus (e.g., NGC 5129, Figure B5(h)). On the other hand, we also observe many galaxies where the jets and lobes are disturbed by the influence of the surrounding hot gas or interaction with other galaxies: narrow- or wide-angle tails (e.g., NGC 507, Figure B3(d); and IC 4296, Figure B3(b)), tails tracing the path of the interaction with another galaxy (e.g., NGC 741, Figure B3(g)), S-shaped radio emission (e.g., NGC 1316, Figure B4(a)), diffuse morphology with no clear jets or lobes (e.g., NGC 1407, Figure B4(c)), and relic-like, large-scale diffuse emission (e.g., NGC 5419, Figure B6(a)). A more detailed definition of our categories is given in Section 3.

For IC 4296 (Figure B3(b)), NGC 57 (Figure B1(b)), NGC 533 (Figure B3(e)), NGC 1550 (Figure B4(d)), NGC 4261 (Figure B4(f)), NGC 4374 (Figure B4(g)), NGC 4472 (Figure B4(h)), NGC 4552 (Figure B5(b)), NGC 5129 (Figure B5(h)), and NGC 5419 (Figure B6(a)), we found new, previously unobserved and undescribed radio morphology at the high resolution of about 1''–2'' in the frequency range of 1–2 GHz (red contours in Figures B3–B6).

Details of radio morphologies together with relevant information on the multifrequency data for each source individually are given in Appendix A.

5.3.1. X-ray Cavity Rate

5.3.2. Central X-ray Point-source Rate

We investigate the correlations and trends between the radio power at ∼1.5 GHz (P_1.5GHz) and the X-ray luminosity within 10 kpc from the center of the galaxy (Figure 4; top), the luminosity of the central X-ray point source or AGN (Figure 4; bottom), entropy and cooling time of the hot X-ray-emitting gas (Figure 5; top and bottom, respectively), the power of the jet estimated from X-ray cavities (Figure 6; top), the luminosity of Hα+[N ii] nebulae (Figure 6; middle), the mass of the central supermassive black hole (Figure 6; bottom), and the largest linear size of the radio emission (Figure 7). Moreover, the flux-density distribution for pointlike and extended radio sources is presented in Figure 8.

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The details of the Hα+[N ii] nebulae emission extent and other information about multiphase gas is given in Table C2.

The Spearman and Pearson correlation coefficients are used to derive the trends between two investigated quantities without accounting for the measurement uncertainties. Moreover, a Bayesian approach to linear regression, taking into account the corresponding measurement uncertainties for both the x and y variables, as well as the upper limits, was used within the linmix package ²³ (Kelly 2007).

The statistical significance of the trends is represented by the probability derived from the null hypothesis test (i.e., the p-value).

Significant trends, with a Spearman and Pearson correlation factor of 0.71 and corresponding p-value of 0.004, are obtained when comparing the radio power with the luminosity of the central X-ray point source (Figure 4; bottom) and the radio flux density with the largest linear size of the detected radio emission (Figure 7).

Negligible correlations are found when comparing the radio power with the jet power required to inflate the cavities; the luminosity of the X-ray atmosphere (Figure 4; top), and the luminosity of the warm ionized nebulae, traced by Hα+[N ii] line emission (Figure 6; middle), the mass of the supermassive black hole (Figure 6; bottom) as well as various thermodynamical properties of the X-ray-emitting hot gas such as entropy (Figure 5; top) and cooling time (Figure 5; bottom) (Lakhchaura et al. 2018).

To estimate the age of the radio-emitting plasma in the innermost central regions (within 0.7 kpc radius of the radio core) of the early-type galaxies, we determined in-band spectral indices for 18 out of 42 galaxies observed with the upgraded VLA A configuration. The description of the approach we used to determine the spectral indices for the subsample of our galaxies can be found in Section 4.1.4.

The final central spectral indices are presented in Table 3 and Figure 9. Interestingly, about one-third of our sources have inverted or flat radio spectra (α ≥ −0.5), while the majority showed steep spectral indices (α < −0.5). This result is consistent with a scenario where some of the sources were recently active, while, for some, the spectral shape is in agreement with synchrotron cooling.

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The early-type galaxies in our sample show a very high radio-detection rate at frequencies between 1–2 GHz. For 41 out of the sample of 42 galaxies, we detected at least pointlike emission in the central region. A significant fraction, 27/42 galaxies, shows extended radio emission from jets and lobes. For 26 out of these 42 galaxies, the presence of X-ray cavities was detected. Moreover, X-ray cavities were also detected for eight sources without jets/lobes. Altogether, 34 sources show signatures of interaction with the X-ray-emitting medium. This suggests that most of these systems are operating in radio-mechanical or maintenance-mode AGN feedback.

Our high detection rate of radio sources appears to be consistent with previous findings (e.g., Dunn et al. 2010; Brown et al. 2011; Sabater et al. 2019). At first glance, these results indicate a very high AGN duty cycle. It is, however, worth noting that the detected extended radio emission most likely does not always represent the current state of AGN activity and is often a remnant from a previous cycle.

The 14 sources with pointlike radio emission are potential FR 0 sources (Baldi et al. 2015), however, some of them could also belong among young or aged radio sources, or alternatively, they can be linked to star formation (Condon 1992; Lacki & Thompson 2013). To investigate whether the origin of the observed radio emission is solely related to AGN activity or could also be linked to star formation, we follow the analysis of Kolokythas et al. (2018). We determine the far-ultraviolet (FUV) fluxes of the galaxies with point-source-like radio emission using the Galaxy Evolution Explorer (GALEX) Survey GR6 catalog ²⁴ to estimate the star-formation rate (SFR) (Bell 2003), from which we determine the expected radio power (Salim et al. 2007). If the expected radio power from the FUV-estimated SFR is at least half of the observed radio power, star formation could potentially be the dominant radio-emission mechanism.

Our analysis shows that for 5 out of the 14 galaxies with pointlike radio morphology, namely NGC 1404, NGC 3091, NGC 3923, NGC 4073 and NGC 4406, star formation could dominate the observed radio emission (see Table 4). However, three out of five sources where the radio emission could potentially be dominated by star-formation activity also host ghost cavities, indicating that radio-mode AGN activity is also present in these galaxies (see Section 6.5.2). Importantly, 7/14 pointlike galaxies display cavities, indicating that despite the lack of extended radio structures at 1–2 GHz, these galaxies host a radio AGN capable of inflating lobes and cavities. The two galaxies that lack observable signatures of radio-mode AGN feedback are NGC 1404 and NGC 4406, which are falling through and being stripped by the intracluster medium (ICM) of the Fornax and Virgo clusters, respectively. X-ray cavities were also previously detected for FR 0 in the brightest cluster galaxy of the galaxy cluster Abell 795 by Ubertosi et al. (2021).

Table 4. The Star-formation Contribution to the Radio Emission in the Case of Early-type Galaxies with Point-source Radio Morphologies

Source	FUVflux a	SFRFUV	${P}_{1.5\mathrm{GHz}\_\exp }$	50% of P1.5GHz_obs
Name	(μJy)	(10−2 M⊙ yr−1)	(W Hz−1)	(W Hz−1)
(1)	(2)	(3)	(4)	(5)
IC1860	$\left(60.3\pm 6.3\right)$	$\left(7.10\pm 0.70\right)$	$\left(1.29\pm 0.13\right)\times {10}^{20}$	$\left(7.13\pm 0.33\right)\times {10}^{21}$
NGC 57	$\left(94.5\pm 7.2\right)$	$\left(7.30\pm 0.60\right)$	$\left(1.32\pm 0.10\right)\times {10}^{20}$	$\left(1.93\pm 0.10\right)\times {10}^{20}$
NGC 410	$\left(77.8\pm 9.3\right)$	$\left(4.40\pm 0.50\right)$	$\left(7.92\pm 0.90\right)\times {10}^{19}$	$\left(1.29\pm 0.05\right)\times {10}^{21}$
NGC 410	$\left(97.6\pm 2.7\right)$	$\left(5.49\pm 0.15\right)$	$\left(9.95\pm 0.28\right)\times {10}^{19}$	$\left(1.29\pm 0.05\right)\times {10}^{21}$
NGC 1132	$\left(59.7\pm 9.4\right)$	$\left(6.00\pm 0.90\right)$	$\left(1.08\pm 0.17\right)\times {10}^{20}$	$\left(1.98\pm 0.13\right)\times {10}^{21}$
NGC 1404	$\left(550.1\pm 27.2\right)$	$\left(2.85\pm 0.14\right)$	$\left(5.16\pm 0.26\right)\times {10}^{19}$	$\left(5.20\pm 0.80\right)\times {10}^{18}$
NGC 1404	$\left(702.9\pm 1.5\right)$	$\left(3.64\pm 0.01\right)$	$\left(6.60\pm 0.01\right)\times {10}^{19}$	$\left(5.20\pm 0.80\right)\times {10}^{18}$
NGC 1404	$\left(778.2\pm 3.9\right)$	$\left(4.03\pm 0.02\right)$	$\left(7.30\pm 0.04\right)\times {10}^{19}$	$\left(5.20\pm 0.80\right)\times {10}^{18}$
NGC 1404	$\left(732.3\pm 5.7\right)$	$\left(3.79\pm 0.03\right)$	$\left(6.90\pm 0.06\right)\times {10}^{19}$	$\left(5.20\pm 0.80\right)\times {10}^{18}$
NGC 2300	$\left(150.8\pm 6.7\right)$	$\left(3.35\pm 0.2\right)$	$\left(6.07\pm 0.27\right)\times {10}^{19}$	$\left(2.05\pm 0.20\right)\times {10}^{20}$
NGC 2300	$\left(163.0\pm 4.2\right)$	$\left(3.62\pm 0.01\right)$	$\left(6.56\pm 0.17\right)\times {10}^{19}$	$\left(2.05\pm 0.20\right)\times {10}^{20}$
NGC 3091	$\left(186.4\pm 3.4\right)$	$\left(5.62\pm 0.1\right)$	$\left(1.02\pm 0.02\right)\times {10}^{20}$	$\left(7.60\pm 0.40\right)\times {10}^{19}$
NGC 3091	$\left(163.5\pm 4.2\right)$	$\left(4.93\pm 0.34\right)$	$\left(8.90\pm 0.60\right)\times {10}^{19}$	$\left(7.60\pm 0.40\right)\times {10}^{19}$
NGC 3923	$\left(438.4\pm 6.1\right)$	$\left(2.49\pm 0.03\right)$	$\left(4.51\pm 0.06\right)\times {10}^{19}$	$\left(1.29\pm 0.11\right)\times {10}^{19}$
NGC 3923	$\left(357.5\pm 21.5\right)$	$\left(2.03\pm 0.12\right)$	$\left(3.68\pm 0.22\right)\times {10}^{19}$	$\left(1.29\pm 0.11\right)\times {10}^{19}$
NGC 4073	$\left(191.0\pm 3.9\right)$	$\left(8.91\pm 0.18\right)$	$\left(1.61\pm 0.03\right)\times {10}^{20}$	$\left(1.54\pm 0.09\right)\times {10}^{20}$
NGC 4073	$\left(130.0\pm 10.2\right)$	$\left(6.1\pm 0.05\right)$	$\left(1.10\pm \right)0.09\,\times \,{10}^{20}$	$\left(1.54\pm \right)0.09\,\times \,{10}^{20}$
NGC 4125	$\left(206.80\pm 16.6\right)$	$\left(1.22\pm 0.10\right)$	$\left(2.22\pm 0.18\right)\times {10}^{19}$	$\left(2.36\pm 0.11\right)\times {10}^{19}$
NGC 4406	$\left(1004.0\pm 9.0\right)$	$\left(4.16\pm 0.04\right)$	$\left(7.53\pm 0.07\right)\times {10}^{19}$	$\left(5.30\pm 1.00\right)\times {10}^{18}$
NGC 4406	$\left(1029.6\pm 12.6\right)$	$\left(4.26\pm 0.05\right)$	$\left(7.72\pm 0.09\right)\times {10}^{19}$	$\left(5.30\pm 1.00\right)\times {10}^{18}$
NGC 4406	$\left(800.7\pm 17.2\right)$	$\left(3.32\pm 0.07\right)$	$\left(6.01\pm 0.13\right)\times {10}^{19}$	$\left(5.30\pm 1.00\right)\times {10}^{18}$
NGC 4936	$\left(70.0\pm 9.2\right)$	$\left(0.89\pm 0.12\right)$	$\left(1.61\pm 0.21\right)\times {10}^{19}$	$\left(1.96\pm 0.08\right)\times {10}^{20}$
NGC 7619	$\left(81.2\pm 6.6\right)$	$\left(2.31\pm 0.22\right)$	$\left(4.20\pm 0.40\right)\times {10}^{19}$	$\left(3.82\pm 0.15\right)\times {10}^{21}$
NGC 7619	$\left(95.4\pm 3.3\right)$	$\left(2.31\pm 0.11\right)$	$\left(4.18\pm 0.20\right)\times {10}^{19}$	$\left(3.82\pm 0.15\right)\times {10}^{21}$
NGC 7619	$\left(81.6\pm 7.0\right)$	$\left(2.31\pm 0.23\right)$	$\left(4.20\pm 0.40\right)\times {10}^{19}$	$\left(3.82\pm 0.15\right)\times {10}^{21}$

Notes. The columns are as follows: (1) the source name; (2) the far-ultraviolet flux from the GALEX-DR5 (GR5) catalog (Bianchi et al. 2011); (3) the estimated star-formation rate (SFR) according to the relation of Bell (2003); (4) the expected radio power from the SFR (Salim et al. 2007); (5) the half value of the radio power; if the expected power is greater than 1/2 of the observed power, star formation dominates.

^a Multiple measurements of the FUV flux were found in the catalog for some sources.

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Using FUV fluxes as an indicator of star formation, Kolokythas et al. (2018) found that for 5 of the 26 galaxies in their high-richness subsample of the CLoGS, star formation could have a significant contribution to the radio emission.

To study the duty cycle of radio-mode AGN activity in more detail, we estimate the spectral indices in the nuclear regions for a subsample of our early-type galaxies (Table 3). The distribution of the central spectral indices could help us place better constraints on the duty cycle of the AGN.

From in-band VLA analysis of the mean spectral indices of the radio emission within 0.7 kpc from the central region, we determined that approximately one-third of the sources have a relatively flat (α ≥ −0.5) spectral index. The rest of the galaxies have steep spectra. This result indicates that the age of the radio-emitting plasma in the centers of these galaxies spans a range of values, and while the radio-mode AGN activity is variable, it has a relatively high duty cycle.

This analysis could also motivate a more in-depth investigation of the nuclear activity of these AGN. For example, higher-resolution very-long-baseline interferometry (VLBI) observations would offer a closer look at the central regions and their most recent states of activity. From our sample, an example is provided by the recently active source NGC 5044, where Schellenberger et al. (2020) confirm ongoing jet activity using the VLBA and summarize the presence of multiple generations of X-ray cavities.

Another example in the sample is NGC 1407, where we see a small-scale young jet in the core (Giacintucci et al. 2012). A recent study of NGC 1316 (Maccagni et al. 2021) using the upgraded Karoo Array Telescope (MeerKAT), as well as VLA and Atacama Large Millimeter/submillimeter Array (ALMA) data, found a highly variable central radio source in the last three cycles of its activity. One cycle, forming large diffuse radio lobes, could have started around 20 Myr ago. The second cycle started possibly around 3 Myr ago, forming a flattened S-shaped structure. The current activity appears to be 1 Myr old.

We investigated the possible correlations between the radio power and the X-ray luminosity of the hot halo as well as the central point source, the thermodynamical properties of the hot gas, the Hα+[N ii] luminosity, and the jet power calculated from the X-ray cavities and between the radio flux density and the largest linear size of the radio emission (Table 2 and Figures 4–7).

We see a significant correlation with a Spearman and Pearson coefficient of 0.64 with p-value of 0.004 between the total radio flux density and the largest linear size of the radio emission. The larger the total extent of the radio emission, the more powerful the source is (Singal 1993; Lara et al. 2001; Shabala & Godfrey 2013; Tang et al. 2020). While the distances of our sources span a factor of 6, the radio fluxes span four orders of magnitude. The range of radio power is thus too large to be accounted for by the more powerful sources being more distant.

Similarly, a significant correlation with a Spearman and Pearson coefficient of 0.71 with p-value of 0.004 is found between the radio power and the X-ray luminosity of the central X-ray point source. ²⁵ The correlation is consistent with a scenario where the X-ray emission comes from an unresolved X-ray jet or the base of the jet.

The relation between the luminosity of the hot X-ray atmospheres within 10 kpc radius from the central region and the radio power at 1.5 GHz shows no correlation. The lack of trends could, at first glance, appear surprising. It is worth noting that the lack of any trend detected here could be affected by the limited range of X-ray luminosities in the innermost regions up to a radius of 10 kpc spanning over approximately one order of magnitude in comparison with a large span of radio power values (∼six orders of magnitude; Figure 4; left). Moreover, for a stable continuous accretion of the hot atmospheric gas, at a given black hole mass, one would expect higher accretion rates and thus more powerful jets for lower entropies and shorter cooling times. However, the gas with typical cooling times seen in early-type galaxies is expected to be thermally unstable (e.g., Fabian & Nulsen 1977; Nulsen 1986). Assuming that the ambient medium cools and forms dense clouds with very small volume-filling fractions that fall toward the central black hole, the accretion rate can for short moments rise by orders of magnitude, triggering a feedback response (Gaspari et al. 2013; Voit et al. 2015; McNamara et al. 2016). Since the infall of dense clouds with a range of masses is essentially chaotic, the power of a given triggered outburst can also have a range of values. Thus, we are not expecting any trend between the thermodynamic properties of the atmospheric gas and the observed radio emission.

The radio power showed no correlation with the radio power and the jet power computed using X-ray cavities (Lakhchaura et al. 2018, and references therein), and Hα+[N ii] luminosity (Lakhchaura et al. 2018), and the mass of the central supermassive black hole. ²⁶

The Hα+[N ii]-emitting gas represents only a fraction of the cold/warm gas mass in the centers of these galaxies, and the observed nebulae are not necessarily located in the vicinity of the supermassive black hole. Therefore, the lack of correlation is not entirely surprising. Results of the statistical analysis are consistent with previous ones found in the literature (Franceschini et al. 1998; Liu et al. 2006). Weak correlations with CO-emitting cold molecular gas have previously been observed by Babyk et al. (2019).

6.5.1. Widening Radio Jets

One of the signatures of the interaction between the radio plasma and the hot X-ray gas could be seen in the widening of the jets after they pierce through the relatively dense galactic atmosphere.

The radio contours in 3C 449, NGC 315, NGC 4261 and IC 4296 ²⁷ by, e.g., Killeen & Bicknell (1988) and Grossová et al. (2019) (Figure 10) widen significantly at about 10–20 kpc as the relativistic plasma in jets is released from the higher-pressure environment of the hot X-ray gas. In other words, if the ambient gas pressure drops below a certain value, then the jets can widen significantly.

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The widening and corresponding brightening of the jets could be connected to the lower ambient ICM pressure and possibly to additional thermal energy from the interaction of the radio and X-ray plasma, which will increase the pressure of the radio jet, causing it to expand (e.g., Gizani & Leahy 1999).

6.5.2. Ghost X-Ray Cavities

6.5.3. Possible X-Ray Tail

We observe an X-ray tail in the so-called cross-cone region between the jets (yellow circles in the images of IC 4296 and 3C 449; Figure 11) due to the relative motion of the intracluster/group medium and the centrally located jetted galaxy in a system that experiences ICM sloshing. With a large opening angle of the jets and the presence of potential "warm" spots, the galaxies can be categorised as Wide-angled Tail (WAT) galaxies (e.g., Owen & Rudnick 1976; O'Donoghue et al. 1990; Leahy 1993; Missaglia et al. 2019).

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In some cases, the radio emission is observed to be offset from the optical/X-ray emission of the host galaxy. Several mechanisms can be responsible for such misalignment. One possibility is that the offset radio emission might belong to a merging galaxy where only one nucleus is active (a so-called "offset AGN"; Steinborn et al. 2016) or to a recoiled black hole kicked out of the system at large velocities (ranging from hundreds to thousands of kilometers per second) when the central black holes of merging galaxies coalesce.

To date, there have been tens of observed candidates for such black holes ejected by gravitational wave recoil (also supported by simulations, e.g., Campanelli et al. 2007; Blecha et al. 2016, 2013; Chiaberge et al. 2017; Condon et al. 2017; Komossa 2012). The broad-line region can be carried away by the recoiling black hole and the AGN can continue to be active (e.g., Komossa 2012). Moreover, due to previously published candidates for an offset black hole within our sample, namely NGC 4486 and NGC 5813, we investigate this mechanism for three more potential candidates: NGC 3923, NGC 5129 and NGC 4125.

6.6.1. Small Offsets: NGC 3923, NGC 4486, NGC 5846, NGC 5129

Multifrequency studies were carried out to search for such recoiling black holes (Lena et al. 2014; Barrows et al. 2016; Skipper & Browne 2018; Ward et al. 2021). Even in the cases of the giant elliptical galaxies M 87 (NGC 4486) and NGC 5846, a parsec-scale displacement of the supermassive black hole from the center of the host galaxy was observed and ascribed to the recoil mechanism (Batcheldor et al. 2010; Lena et al. 2014). Although, the apparent supermassive black hole offset in M 87 could be just due to the variation of the flux, as suggested by López-Navas & Prieto (2018).

In our analysis, we present a previously unpublished candidate for the radio/X-ray offset from the optical core of 12 (0.5 kpc), which may be the result of a displaced supermassive black hole from the central potential of the host giant elliptical galaxy NGC 5129. In addition, we confirm the previous findings for NGC 4486 and NGC 5846, where we also find small offsets of the central radio emission relative to the optical/X-ray emission. Moreover, we found a previously unpublished small offset of 0.21 kpc for the radio emission in the central regions of NGC 3923.

All small-scale offsets are shown in Figure 12.

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6.6.2. Intriguing Radio Lobe Emission in NGC 4125

The most intriguing is the offset radio emission located 46'' (4 kpc) from the radio core of NGC 4125 (Figure 13; top). The unusual position of the radio emission has been previously reported by Krajnović & Jaffe (2002), who claimed that due to the missing optical counterpart, it is probably a background source. However, our analysis, where we estimated how distant a potential background source would need to be if its optical counterpart is not seen by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), ²⁹ showed that it is quite unlikely that the observed radio emission is connected to a background source.

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Our analysis takes into account the limiting apparent magnitude of the Pan-STARRS survey of 23 mag. We derived that a galaxy similar to M 87, with an absolute magnitude of −21.5, would need to be at least at a luminosity distance of ∼6300 Mpc (corresponding to a redshift of 0.95) to be missed by the optical survey. At this luminosity distance, the potential background radio source would have a size of ∼1 Mpc (assuming a linear-scale 8 kpc arcsec^–1) and large radio power of 10 × 10²⁵ W Hz^–1, corresponding to twice the power of the giant radio halo in the cluster merger "El Gordo" (Lindner et al. 2014) at the redshift of z = 0.87.

Moreover, we examined the archival data from the TIFR GMRT Sky Survey (TGSS) at 148 MHz and Westerbork Synthesis Radio Telescope (WSRT) data at 1.4 GHz and created a spectral index map between these two frequencies (Figure 13; bottom). We found a flat spectral index at the position of the radio/X-ray/optical core of NGC 4125 and steeper spectral indices in a clear double morphology of the radio emission resembling a FR II radio galaxy, which would potentially disfavor the recoiled black hole scenario. A "kicked-out" black hole, which recoils and moves with a high velocity relative to the galaxy, would be expected to form narrow tail structures along its path (Blecha et al. 2011), whereas we observe an apparently untouched radio emission from potential hotspots (especially in the northwestern side).

The analysis of the X-ray Chandra data reveals possible X-ray cavities (T. Plšek et al. 2021, in preparation) perpendicular to the core of NGC 4125, thus they do not correspond to the offset radio emission in the form of radio lobes. Burke-Spolaor et al. (2017) found a similar offset radio source in the central dominant galaxy (CDG) of the galaxy cluster Abell 2261 with a missing optical and X-ray counterpart (Gültekin et al. 2021). Interestingly, the radio morphology of the emission resembles the one in NGC 4125 with its pear-like shape.

Further analysis needs to be carried out to investigate the origin and source of this unusual double-lobe radio emission.

Most of the early-type galaxies in our sample are low-power Fanaroff–Riley Class I (FR I; Capetti et al. 2017a) radio sources. Although two giant ellipticals, NGC 533 and NGC 1600, show morphological similarities to Fanaroff–Riley Class II (FR II; Capetti et al. 2017b) radio sources, they still have low power, which is more typical for FR Is (see Section 6.7).

6.7.1. NGC 533

The radio morphology of NGC 533 (Figure 14; top) was observed within our 15A-305 project in the VLA A configuration and has a resolved central region and two inflated, almost symmetric small-scale spherical radio jets/lobes with a possible hot spot in the western lobe. We suggest that this source resembles the radio morphology of FR II radio galaxies based on visual comparison with sources in the FR II FIRST Catalog (FRIICAT; Capetti et al. 2017b). Further analysis of the in-band spectral index map revealed a flattening of the spectral index at the position of the radio lobes, especially in the region west of the radio core visible in Figure 14 (bottom left). Potentially, due to its small physical size (less than 30 kpc), it could fall into the category of Compact Double Radio galaxies (COMP2CAT; Jimenez-Gallardo et al. 2019). NGC 533 could resemble the compact radio source J132031.47-012718.5 (Figure 14; bottom right).

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6.7.2. NGC 1600

The radio emission of the giant elliptical galaxy NGC 1600 was previously published by Birkinshaw & Davies (1985), where they used VLA C configuration data at 4.885 GHz. They defined the source as a double radio source (in other words, FR II) and stated that the galaxy's core is located close to the northern radio lobe-like feature. The higher-resolution VLA A configuration data at 1.4 GHz shows more clearly the jet/lobe morphology and potential hotspots.

Additionally, we suggest that the radio morphology in NGC 1600 at both frequencies resembles the radio morphology of the source J1423+2501, identified as an FR II radio source in the FRIICAT catalog (Capetti et al. 2017b) with an unidentified radio core (Figure 15).

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The low-power FR I radio source 3C 449 is the CDG of the 1.5 keV group/cluster Zwicky 2231.2 + 3732 at a luminosity distance of ∼76 Mpc. The radio emission extends out to almost 500 kpc (Perley et al. 1979; Andernach et al. 1992; Feretti et al. 1999) in the form of relativistic (up to ∼3.3 kpc) and well-collimated (up to ∼16.7 kpc) radio jets terminating in large diffuse radio lobes with a slight bend to the west (Figure B3(a)).

Associated X-ray emission from the hot gas and its interaction with the radio source has been studied by Hardcastle et al. (1998), Croston et al. (2003), and Lal et al. (2013). The X-ray cavities were observed coincident with the inner southern radio jet, and the Chandra X-ray Observatory detected cold fronts that likely originate from sloshing of the hot gas, which could also contribute to the bending of the radio jets.

Hubble Space Telescope (HST) observations revealed a 600 pc dusty disk, the orientation of which is not perpendicular to the orientation of the radio jet axis (Feretti et al. 1999; Tremblay et al. 2006). The CO(1–0) transition line, as a tracer of molecular gas, was detected by Ocaña Flaquer et al. (2010). The study of SOAR data revealed Hα+[N ii] line emission within the innermost 2 kpc from the core (Lakhchaura et al. 2018).

IC 1860 is the CDG of the 1.3 keV group IC 1860 (Abell S301; Abell et al. 1989).

The observed radio emission in the VLA A configuration has the form of a pointlike ³⁰ radio source slightly elongated in the northeastern and southwestern directions(Figure B1(a)).

The XMM-Newton and Chandra data were analyzed by Gastaldello et al. (2013), who found sharp surface brightness discontinuities (cold fronts) that they interpreted as evidence for sloshing, which could be a result of a minor merger with the spiral galaxy IC 1859 (this is also consistent with numerical simulations for relaxed clusters; Ascasibar & Markevitch 2006). Moreover, their radio observations at lower frequencies with the GMRT revealed a steep-spectrum diffuse radio emission enclosed by these cold fronts.

IC 4296 is a giant elliptical galaxy and the CDG of Abell 3565.

The radio emission identified with the radio source PKS 1333-33 extends well beyond the host galaxy IC 4296. Our VLA A and D configuration data (Figure B3(b)) show an unresolved central radio source and extended radio emission in the form of two well-collimated (within the first ∼10 kpc from the core) radio jets expanding up to ∼150 kpc on both sides with many knots and wiggles and terminating in large radio lobes with a radius of ∼60–80 kpc. IC 4296 and its radio source (in VLA C configuration) was extensively studied by Kileen et. al (1986a, 1986) and Killeen & Bicknell (1988).

The X-ray emission from this giant elliptical galaxy was described by Pellegrini et al. (2003). Moreover, our analysis of the archival XMM-Newton data revealed an X-ray cavity associated with the northwestern radio lobe (Grossová et al. 2019). More in-depth X-ray spectral analyses of the thermodynamical profiles show an unusually low central entropy and cooling time.

The presence of cooling gas in the form of warm Hα+[N ii] (at kiloparsec scale) (Lakhchaura et al. 2018), molecular CO(2–1) emission (at 100 pc scale) (Boizelle et al. 2017; Ruffa et al. 2019) and a warped dust disk (at <1 pc scale) (Schmitt et al. 2002) has been revealed in the central region.

More details of the potential unbalanced cooling in the nucleus and the relationship between the radio, X-ray, and optical emission for IC 4296 is summarized in our paper (Grossová et al. 2019).

NGC 57 is another giant elliptical galaxy from our proposal VLA A configuration data and identified as an isolated elliptical galaxy (Smith et al. 2004; O'Sullivan et al. 2007).

Our high-resolution VLA A configuration total-intensity image (Figure B1(b)) reveals, for the first time, a pointlike radio morphology previously unseen in the TIFR GMRT Sky Survey (TGSS), the NRAO VLA Sky Survey (NVSS) or in the study by O'Sullivan et al. (2007). No dusty features (Goullaud et al. 2018) or warm gas (Lakhchaura et al. 2018) were found in this galaxy.

NGC 315 is well-known giant elliptical galaxy classified as a FR I radio source (Fanaroff & Riley 1974), which resides in the central region of the NGC 315 (WBL 22) group.

The radio emission has been thoroughly studied at various radio frequencies by many research groups (Bridle et al. 1979; Willis et al. 1981; Klein et al. 1994; Mack et al. 1997; Laing et al. 2006; Worrall et al. 2007; Giacintucci et al. 2011, and references therein) and has the form of relatively symmetric radio jets with several wiggles and knots, which are inflating wiggles and knots, and which extend almost 200 kpc. The southeast jet is inflating a radio lobe, while the northwest jet undergoes an apparent 180 degree bend and forms an extended plume (Figure B3(c)). Chen et al. (2012) investigated the kinematic and spatial properties of the close galaxy members of NGC 315 and found a low-density ambient X-ray gas environment, which could be the reason why the radio jet can expand to such a large distance from the core.

The X-ray analysis revealed a strong X-ray jet, which traces the radio synchrotron emission and extends up to about ∼10 kpc (Worrall et al. 2003, 2007; Donato et al. 2004; Figure B3(c)).

The optical emission is represented by a dusty disk perpendicular to the direction of the jets and a few dust patches (Capetti et al. 2000). Moreover, at the position of the HST dust patches, Morganti et al. (2009) found two H i absorption components. The cold molecular gas, traced by CO(1–0) line emission, was found with the IRAM telescope (Ocaña Flaquer et al. 2010). Moreover, Boizelle et al. (2021) detected a sub-kpc CO disk. The warmer nebulae, traced by a Hα+[N ii] line, was observed by Ho et al. (1997), but not confirmed in the more recent study as presented in Lakhchaura et al. (2018), combing narrowband imaging with long-slit spectroscopic observations with SOAR.

Another galaxy from our VLA A configuration project is a giant elliptical, NGC 410 (Figure B1(c)), which is the CDG of the LGG 18 group (Kolokythas et al. 2018).

The radio emission with a point-source-like morphology was previously observed with the VLA at various frequencies (Condon et al. 1998; Filho et al. 2002). A more recent study by Kolokythas et al. (2018) at lower frequencies (610 MHz) with the GMRT found a small extension of the radio emission to the northeast with a total extent ∼10 kpc.

González-Martín et al. (2009) claimed no point-source detection in the X-ray hard band (4.5–8.0 keV) with XMM-Newton (together with other evidence from the optical, UV and radio bands) and therefore classified NGC 410 as a galaxy without an AGN. The X-ray halo emission was detected in the 0.5–7 keV band with Chandra (Lakhchaura et al. 2018).

A recent study with SOAR data, as summarized by Lakhchaura et al. (2018), revealed Hα+[N ii] line emission within the innermost 2 kpc from the core.

NGC 499 is the only one galaxy in our sample of 42 early-type sources without radio emission (Figure B1(d)) observed in the nucleus or in the extended diffuse region.

A faint radio source was observed within the 1.4 GHz NVSS survey (Condon et al. 1998) with a flux density of 0.7 mJy at 2 σ_rms (Brown et al. 2011), which is well above the sensitivity limit of our VLA observations (for more details, see Section 5.1.1).

NGC 499 is a CDG of the NGC 499 group, which is a part of the Pisces cluster (Kim & Fabbiano 1995). Paolillo et al. (2003) have shown that the NGC 499 group is in the process of merging with the NGC 507 group (Appendix A.8).

The luminosity of the X-ray halo is relatively high (∼ 2.3 · 10⁴² erg s⁻¹; Dunn et al. 2010; Lakhchaura et al. 2018). Moreover, the Chandra image shows possible ghost cavities at about ∼10–15 kpc to the north and south of the nucleus, a sign of a previous AGN activity (Panagoulia et al. 2014; Kim et al. 2019).

Lakhchaura et al. (2018) present a detection of a centrally located warm nebular emission.

NGC 507 is the CDG of the NGC 507 group, which belongs to the Pisces cluster. As mentioned in the previous section, the NGC 507 group is likely to be merging with the NGC 499 group (Appendix A.7).

The large-scale diffuse radio emission from a low-power FR I radio source B2 0120 + 33 (Parma et al. 1986) has a shape resembling a letter "Γ" formed by two radio lobes with diameters of ∼40 and ∼20 kpc (Dunn et al. 2010; Murgia et al. 2011). Interestingly, this emission has a very steep radio spectrum, possibly due to "fossil" radio material from previous AGN activity (Murgia et al. 2011).

The central part of the brighter western lobe was resolved also by our high-resolution A configuration data (Figure B3(d)) and coincides with a Chandra X-ray cavity at ∼7 kpc to the southwest of the galaxy core (Dong et al. 2010).

Further, the eastern radio lobe is bending to the south with its edge tracing a Chandra X-ray cold front (Fabbiano et al. 2002; Kraft et al. 2004), a result of sloshing. The disturbed morphology could be due to the ongoing merger with NGC 499 as studied and simulated by Ascasibar & Markevitch (2006). A different scenario is proposed by Kraft et al. (2004): the sharp-edged X-ray surface brightness profile could be entrained ICM material from the expanding eastern radio lobe.

XMM-Newton data also confirms the observed discontinuity (Fabbiano et al. 2002).

No warm (Lakhchaura et al. 2018) and cold gas was found inside of this giant elliptical galaxy.

NGC 533 is the CDG of the NGC 533 group of galaxies.

The new high-resolution A configuration data observed within our project 15A-305 revealed a central bow-tie-shaped radio emission with a total extent of ∼5 kpc (Figure B3(e)), which resembles the radio morphology of FR II radio sources (see Discussion 6.7).

NGC 533 is one of the two galaxies in our sample (besides NGC 708; Appendix A.10) which shows signatures of being a young radio galaxy (O'Dea 1998): a compact morphology of the radio emission within the host galaxy without extended features, cavities in X-rays as a sign of small-scale interaction of the radio and X-ray plasma, a double morphology of the radio emission.

In comparison with the central X-ray emission, previously studied using Chandra X-ray images by Dunn et al. (2010) and Shin et al. (2016), we suggest that the radio lobes visible in the red contours in Figure B3(e) could correspond to the presence of the northeastern and especially southwestern X-ray cavity.

The optical analysis presented in Ferrari et al. (1999), Temi et al. (2007a), and Lakhchaura et al. (2018) showed co-spatial Hα+[N ii] emission coincident with a filamentary dust distribution.

The giant elliptical galaxy NGC 708 is a CDG of the cooling flow cluster Abell 262. Parma et al. (1986) and Blanton et al. (2004) defined this galaxy as a low-power FR I radio source.

VLA archival data in the A and C configurations (Figure B3(f)), also studied by Parma et al. (1986), Blanton et al. (2004), and Clarke et al. (2009), reveal a diffuse two-sided radio lobe-like emission with a total extent of ∼40 kpc from the central radio source B2 0149 + 35.

The radio emission from NGC 708, like NGC 533 (see Appendix A.9), resembles the emission of a young radio galaxy (O'Dea 1998) due to its confinement within 10 kpc of the host galaxy without extended emission, interaction with the inner X-ray gas in the form of cavities (Blanton et al. 2004; Clarke et al. 2009; Panagoulia et al. 2014), double radio morphology, and presence of the optical emission line [O iii] (Liuzzo et al. 2013).

The X-ray emission shows surface-brightness dips, which could be signatures of cavities on both sides (especially on the eastern side of the core) consistent with the expanding diffuse radio emission (Blanton et al. 2004; Clarke et al. 2009). The X-ray analysis shows that the enthalpy of the radio lobes is not sufficient to balance radiative losses from the ICM, suggesting that NGC 708 may have shut down or be in a low-power phase of activity. This is also supported by the nondetection of radio emission on smaller scales with VLBI (Liuzzo et al. 2010).

A search for multiphase gas in NGC 708 was performed by Sahu et al. (2016). A molecular CO gas disk, with a total extent of ∼3 kpc, was discovered with ALMA by Braine & Dupraz (1994) and in more detail studied also by Russell et al. (2019), Olivares et al. (2019), and North et al. (2021). The peak of the molecular gas emission coincides with the AGN core and is perpendicular to the projected orientation of the radio jets. The cool gas is also surrounded by dust lanes in the nuclear region (Sahu et al. 2016; Kulkarni et al. 2014).

The extended (∼13 kpc) warm nebula traced by Hα+[N ii] emission (Lakhchaura et al. 2018) is also consistent with the Chandra X-ray surface brightness peaks, which suggests that at least part of the hot X-ray gas is cooling at even larger distances from the core (Blanton et al. 2004).

NGC 741 is the CDG in the group holding the same name.

The radio structure, first studied by Venkatesan et al. (1994) and Birkinshaw & Davies (1985), was probably formed by the close passage of the second-brightest galaxy in the group, the head–tail radio galaxy NGC 742 with the same redshift as NGC 741 (Schellenberger et al. 2017).

Our analysis, using a new high-resolution A configuration total-intensity image, confirms the radio emission in the form of a bridge connecting NGC 471 and NGC 742 and a prominent bent radio tail (to the southwest) in the C configuration total-intensity image (Figure B3(g)).

X-ray analysis by Jetha et al. (2008) and Schellenberger et al. (2017) has shown several generations of X-ray cavities.

The warm gas was found in the nuclear regions of NGC 741 by Macchetto et al. (1996), but more recent narrowband and long-slit spectroscopic SOAR observations are disfavoring their findings (Lakhchaura et al. 2018). Dust is missing in this giant elliptical galaxy (Verdoes Kleijn & de Zeeuw 2005).

Compact radio emission is detected in our new VLA A configuration total-intensity image with a resolution of ∼1.5 arcsec for NGC 777 (Figure B3(h)), the giant elliptical galaxy and the CDG of LGG 42 group. This compact morphology was also detected by Kolokythas et al. (2018) with the GMRT.

Additionally, we also imaged the archival VLA C configuration data, where no radio emission was detected at the angular resolution of ∼3.2'', possibly due to the flux density being very close to the sensitivity limit of the VLA observations (see Section 5.1.1).

The X-ray-emitting halo has an almost undisturbed morphology (∼40 kpc across) with a quite high luminosity of ∼ 4 · 10⁴² erg s⁻¹ (when compared to the rest of the galaxies in the sample Lakhchaura et al. 2018). The X-ray surface brightness depressions were detected in the hot atmosphere of NGC 777 by Panagoulia et al. (2014).

Lakhchaura et al. (2018) reports a nondetection of any trace of warm ionized nebulae.

NGC 1132 is a giant elliptical, which resides inside the well-known fossil group NGC 1132.

Radio emission from the archival VLA C configuration data (Figure B1(e)) shows a point-source morphology from the unresolved central source.

The outer contour level (5 × σ_rms; see Table 1) coincides with the possible shock front in the X-rays noticed by Kim et al. (2018) and could be connected to the merging history of NGC 1132 as suggested by Mulchaey & Zabludoff (1999) (and predicted by the simulations of Barnes 1989 and Governato et al. 1991). Moreover, Dong et al. (2010) found an X-ray cavity at a distance of 4 kpc south of the host galaxy's center, which is consistent with the potential set of cavities detected in the southern regions by T. Plšek et al. (2021, in preparation).

No emission from warm gas was detected for this source (Lakhchaura et al. 2018).

The giant elliptical galaxy NGC 1316 hosts a well-known strong radio source, Fornax A, located at the outskirts of the Fornax cluster.

Radio emission in the higher-resolution BA configuration image extends from the northwest to southeast in the form of an oblong letter "S" and represents the most recent of its AGN outbursts with the southeastern extension also visible in the C configuration image (Figure B4(a)); both previously published (Fomalont et al. 1989; Maccagni et al. 2020, 2021).

Several generations of X-ray cavities are visible in the XMM-Newton and Chandra images (Lanz et al. 2010).

The S-shaped jet morphology together with the disturbed morphology of the multiphase gas (Morokuma-Matsui et al. 2019; Lakhchaura et al. 2018) and dust (Duah Asabere et al. 2014) support the fact that NGC 1316 is in the process of merging into the Fornax cluster (Appendix A.15).

The radio emission of NGC 1399, the CDG of the Fornax cluster, has a two-sided extended radio morphology in the form of two well-collimated radio jets terminating in two mildly diffuse radio lobes (Figure B4(b)). Similar morphological features were previously discussed in Shurkin et al. (2008) and Dunn et al. (2010).

Moreover, Paolillo et al. (2002), Shurkin et al. (2008), and Panagoulia et al. (2014) found signatures of the interaction between the AGN and the hot gas, in the form of X-ray cavities, with bright rims coincident with the outer edges of both radio lobes.

Even though the presence of CO(2–1) line emission was confirmed by Prandoni et al. (2010), no dust features were found in the core of NGC 1399. Moreover, Werner et al. (2014) showed that the central regions of this galaxy lack [C ii] line emission.

The giant elliptical NGC 1404, also part of our new A configuration VLA observation, is another member of the Fornax cluster. Machacek et al. (2005) suggested that NGC 1404 is falling toward the center of the Fornax cluster, where NGC 1399 (Appendix A.15) resides.

Our new VLA A configuration data (Figure B1(f)) at high sensitivity with the rms noise of the total-intensity image reaching 25 μJy per beam revealed a faint (flux density of 210 μJy per beam) central radio source, thus supporting the previous findings of Dunn et al. (2010), where the detection was very close to the sensitivity limit. Moreover, the central radio source was also detected in VLBI observations, which can be found in the archive of Leonid Petrov ³¹ . On the other hand, a more compact archival VLA CD configuration observation did not detect radio emission, maybe due to a lower flux density than the sensitivity limit (see Section 5.1.1).

There are no signatures of ongoing AGN–gas interactions in this system, as supported by a quite smooth circumnuclear morphology of the hot X-ray halo (Machacek et al. 2005; Dunn et al. 2010). Cold (Werner et al. 2014) and warm (Lakhchaura et al. 2018) gas is missing in this galaxy.

Another object within our sample with a newly observed VLA A configuration data is NGC 1407. This giant elliptical galaxy is located inside in the dynamically relaxed NGC 1407 (Eridanus A) group, part of the Eridanus supergroup.

The radio emission from the new A configuration data does not show a very distinctive radio point source in the nucleus (Figure B4(c)). More prominent is the diffuse morphology with asymmetric radio jet-like structures, extending especially to the east, as seen also in the archival VLA B and C configuration data (Giacintucci et al. 2012).

From multiband and multifrequency study (Giacintucci et al. 2012), reoccurring activity of the AGN has been suggested, similar to that found for another fossil giant elliptical galaxy in our sample, NGC 5044 (Appendix A.37).

Only an upper limit on the flux density of CO(1–0) emission is found for NGC 1407 (Babyk et al. 2019).

NGC 1550 is the CDG of the group of galaxies having the same name.

The radio emission observed within our new high-resolution A configuration observation revealed asymmetric small-scale jets with a physical extent of ∼4 kpc. At the lowest contour level created at 5 × σ_rms, we find a small hotspot-like feature at the distance of 6 kpc. This feature seems not to be the extension of the jet and is located within the diffuse lobe-like emission visible in the compact archival C configuration total-intensity image (Figure B4(d)); previously published by Dunn et al. (2010).

The lower-frequency GMRT study supports the presence of the diffuse lobe-like morphology (Kolokythas et al. 2018). They also found that the inward-bended eastern jet seems to be aligned with the potential hotspot feature seen in the VLA A configuration contours (Figure B4(d)). Kolokythas et al. (2020), in their recent radio and X-ray study, found a sign of sloshing of the ambient IGM around NGC 1550, previously known as relaxed. They present arc-shaped cold fronts together with a potential X-ray cavity (supported by the findings of Panagoulia et al. 2014) coincident with the position of the radio lobe.

Neither molecular gas nor cold dusty features have not yet been observed, and Lakhchaura et al. (2018) presents nondetection of warm ionized nebulae for this giant elliptical galaxy.

NGC 1600 is a relatively isolated CDG of the NGC 1600 group of galaxies (Smith et al. 2008).

The radio emission was previously published by Birkinshaw & Davies (1985) at 4.85 GHz with VLA in C configuration and shows a potential double radio source without a clearly defined radio core. The higher-resolution A configuration data at 1.4 GHz (Figure B4(e)) shows in more detail the radio jets/lobes features and potential hotspots, which could resemble the morphology of a FR II radio source (see Discussion 6.7.2).

The X-ray emission from the Chandra X-ray image shows a diffuse structure, which seems to show two surface brightness depressions at the positions of the two radio lobe structures, previously studied by Sivakoff et al. (2004).

Moreover, they found excess emission in the X-rays, which more or less coincides with the Hα+[N ii] emission (Singh et al. 1995; Lakhchaura et al. 2018) of the ionized gas (extending up to ∼30 arcsec from the core) and dust (extending up to ∼10 arcsec from the core; Ferrari et al. 1999).

The signatures of properties similar to the "fossil group" were for NGC 1600 identified by Santos et al. (2007) and Smith et al. (2008).

The radio emission from our new VLA A configuration data for NGC 2300, the central dominant galaxy of the NGC 2300 group, shows a pointlike morphology (Figure B1(g)). The archival VLA D configuration data reveals radio point-source emission, as well (Figure B1(g)).

NGC2300 is known to undergo tidal interactions with a late-type active star-forming galaxy NGC 2276 (Wolter et al. 2015). T. Plšek et al. (2021, in preparation) detected potential "ghost" X-ray cavities in NGC 2300.

The optical study of the HST and SOAR data, done by Xilouris et al. (2004), showed that there is no detection of dust or warm ionized nebulae (Lakhchaura et al. 2018) inside NGC 2300.

NGC 3091 is a part of our VLA high-resolution project from 2015 and is the CDG of a poor Hickson Compact Group (HCG 42; Hickson 1982; Colbert et al. 2001).

The radio morphology, seen in the highest-resolution ∼2'' VLA A configuration data (Figure B1(h)), is represented by a faint point-source-like radio emission in the center of the galaxy. The AB configuration data with ∼14'' resolution at the same frequency (1.4 GHz) reveals no detection, perhaps because of a lower flux-density limit of our VLA observation (see Section 5.1.1).

Even though radio emission has a pointlike morphology, T. Plšek et al. (2021, in preparation) detected potential "ghost" cavities in the X-ray hot atmosphere.

In the analysis of the optical and NIR morphology, no H i, dust features (Colbert et al. 2001) or warm gas (Lakhchaura et al. 2018) have been detected in the central regions of NGC 3091.

The biggest early-type galaxy, NGC 3923, located in the optical group (LGG 255) with only a galaxy-scale X-ray halo has an oval-shaped radio morphology in our new VLA A configuration data (Figure B2(a)), while the archival C and CD configuration data show no radio emission. This source was observed in the 5 GHz survey done by Disney & Wall (1977).

Chandra data have shown potential X-ray surface brightness decrements in the form of "ghost" cavities without corresponding radio emission from radio jets and lobes in this giant elliptical galaxy (T. Plšek et al. 2021, in preparation).

NGC 3923 is a famous shelled ³² giant elliptical with dusty filamentary features and Hα emission in the central region (Bílek et al. 2016; Miller et al. 2017). Further, Lakhchaura et al. (2018) reported nondetection of warm gas in the core.

The giant elliptical NGC 4073 is a dominant member of the poor cluster MKW 4 and shows a point-source-like radio morphology in our new high-resolution VLA A configuration observation (Figure B2(b)). A weak detection of the central radio source has been reported by Hogan (2014) with VLA in the C band (4–8 GHz).

The X-ray emission was studied by O'Sullivan et al. (2003), who found a 1.7 keV halo within the relaxed system. Moreover, T. Plšek et al. (2021, in preparation) found potential "ghost" cavities in the X-ray atmosphere. Warm emission-line nebula was not detected in this source (Lakhchaura et al. 2018).

NGC 4125 is a luminous giant elliptical galaxy residing in the NGC 4125 group (alternatively called LGG 266).

The radio emission (Figure B2(c)), detected in the archival D configuration data has a diffuse morphology with an extension to the northeast, corresponding to a pointlike radio source visible in the archival 1.4 GHz Westerbork Synthesis Radio Telescope (WSRT) observations. Its more prominent structure has a form of inflated radio lobes, which are offset by 4 kpc (46'') in projection with respect to the radio, optical, and X-ray core of NGC 4125. This offset was also noticed in the NVSS data at 1.4 GHz by Rupen et al. in their online notes ³³ about VLA H i data of early-type galaxies and by Krajnović & Jaffe (2002) in the VLA C configuration observations at 8.3 GHz. In both studies, only the central regions of the offset radio lobes were detected. The radio core corresponding to the X-ray and optical center of NGC 4125 was missing and is only clearly visible in the WSRT images. Moreover, due to the missing optical counterpart of the extended radio lobe-like emission, Krajnović & Jaffe (2002) defined NGC 4125 as a background source (see Discussion 6.6.2 for a possible explanation of this unusual emission).

González-Martín et al. (2006) studied NGC 4125 as a part of the X-ray study of LINER galaxies and identified a centrally located hard X-ray point-source-like emission, which is a sign of ongoing AGN activity. The extended cold emission from the large amount of dust (Kulkarni et al. 2014), [C ii] and [N ii] has been observed, but no CO, ³⁴ H i (Welch et al. 2010; Wilson et al. 2013) or Hα+[N ii] emission (Lakhchaura et al. 2018). This could be explained by a merger-triggered star-formation outburst providing heated gas and dust to the surrounding medium of the galaxy.

NGC 4261 is the optical counterpart to the radio source 3C 270 (Birkinshaw & Davies 1985), which is a very-well-known FR I radio source inside the Virgo cluster (W cloud; Garcia 1993) with well-defined radio jets and large radio lobes piercing through the atmosphere of the host galaxy and extending beyond from the core (Figure B4(f)).

A detailed study, combining data from the GMRT and VLA, as well as comparing the radio emission with the X-ray emission of the hot gas, was published by Kolokythas et al. (2015) and O'Sullivan et al. (2011).

From the X-ray point of view, NGC 4261 has an extended X-ray halo (Davis et al. 1995) with features showing interaction with the radio source in the form of X-ray cavities (Croston et al. 2008; O'Sullivan et al. 2011). On small scales, X-ray jets are present, corresponding to the innermost collimated radio jets (Gliozzi et al. 2003; Zezas et al. 2005; Worrall et al. 2010).

On the parsec scale, Jaffe & McNamara (1994) and Ferrarese et al. (1996) found a warped dusty disk with the rotational axis perpendicular to the direction of the radio jets streaming out of the nuclear region and extending up to ∼100 pc from the core. The presence of atomic and molecular gas from H i and CO (2–1) was detected too. Moreover, a sub-kpc CO disk was recently detected by Boizelle et al. (2021).

Very similar morphology and multiband features are observed for the giant elliptical galaxy IC 4296 (Appendix A.3), which has, however, ∼6 × larger radio lobes that leads to ∼30 × larger total energy output supplies for the inflation of those radio lobes (Grossová et al. 2019; Frisbie et al. 2020).

Messier 84 or NGC 4374 is a giant elliptical galaxy in the Virgo A group, which is a part of the Virgo cluster together with NGC 4406 (Appendix A.27), NGC 4486 (Appendix A.29), and NGC 4552 (Appendix A.30).

The radio emission in the archival B configuration data (Figure B4(g)) shows well-defined and, in the central regions, well-collimated jets terminating in diffuse plum-like lobes (first published by Laing & Bridle 1987) with the northern lobe brighter than the southern and bent to the east. VLA D configuration data reveal pear-like diffuse emission extending beyond the B configuration data. Moreover, the VLBA data analyzed by Ly et al. (2004) revealed a jet-like extension in the direction consistent with the northern radio jet from the VLA.

The extended radio lobe emission is filling the region of the drops in the surface X-ray brightness, the X-ray cavities (Finoguenov et al. 2001), and extending far beyond the host galaxy (Devereux et al. 2010).

The nuclear region in NGC 4374 revealed a small ionized gas disk (Bower et al. 1997), faint circumnuclear CO(2–1) absorption and emission together with an asymmetric dust lane (Verdoes Kleijn et al. 1999; Boizelle et al. 2017). All features are perpendicular to the innermost radio jets.

NGC 4406, known as M 86, is located in the Virgo cluster and is flying in the direction toward us. Our new VLA A configuration data at 1.5 GHz (Figure B2(d)) reveal a faint, pointlike radio emission in the center of this giant elliptical. Similarly, Dunn et al. (2010) found pointlike radio emission in the nuclear region at higher frequencies at around 4.9 GHz in the C configuration data. On the other hand, complementary archival data in VLA D configuration at 1.4 GHz have not detected any radio emission from the core, possibly due to the high noise level of the observation (for more details see Table 1).

NGC 4406 is the second X-ray brightest giant elliptical in the Virgo cluster (after M 87) and its negative radial velocity suggests a supersonic movement through the ICM. This could create the observed X-ray features (plume and tail) by ram pressure stripping (Forman et al. 1979; Randall et al. 2008; Ehlert et al. 2013; Kim et al. 2019).

Signs of possible galaxy–galaxy interactions are observed in the distribution of the atomic gas and dust (Smith et al. 2012) as well as the Hα features connected to the neighboring spiral galaxy NGC 4438 (Kenney et al. 2008; Lakhchaura et al. 2018).

NGC 4472, or M 49, is an early-type elliptical and CDG of the M 49 group falling from the south into the Virgo cluster and it is possibly in the second turn around the cluster center (Su et al. 2019).

NGC 4472 was observed in our 2015 project in the VLA A configuration (Figure B4(h)). Additionally, we also reduced archival VLA C configuration data (Condon & Broderick 1988). The high angular resolution of the A configuration total-intensity image (Figure B4(h) contours) showed the radio emission from the innermost central region in the form of an elongated amorphous feature with the extended tail to the west and a small sign of radio emission to the northeast.

On the other hand, using the more compact C configuration array, an extended radio emission with a total extent of ∼10 kpc is seen (Ekers & Kotanyi 1978). The central regions in the C configuration image are consistent with the radio features in the A configuration. Additional extended lobe-like structures are observed to the east and west of the nucleus.

The eastern lobe is consistent with a decrement in the X-ray surface brightness, one of the X-ray cavities observed in this source (for more details see, e.g., Biller et al. 2004; Kraft et al. 2011; Panagoulia et al. 2014; Su et al. 2019). Warm gas is missing in the central or extended regions of this giant elliptical galaxy (Lakhchaura et al. 2018).

NGC 4486, or Messier 87, is the central dominant galaxy of the Virgo cluster. Here, we summarize the results obtained from the archival A, B, and C configuration data (Figure B5(a)).

The bright radio core, 3C 274, was detected along with its innermost radio jet, which is coincident with the X-ray emission of the jet (Marshall et al. 2002). On the opposite side of the core, an elongated lobe-like emission is visible and coincides with an X-ray cavity (Young et al. 2002; Forman et al. 2005, 2007). The archival C configuration VLA map, which is more sensitive to extended structures, presents a western tail extending up to ∼10 kpc from the nucleus. However, we note that at 300 MHz the entire structure of NGC 4486 is far more extended (up to 80 kpc, e.g., Owen et al. 2000).

The presence of multiphase gas in the form of cold molecular CO clouds (e.g., Simionescu et al. 2018), cool [C ii] (Werner et al. 2014), nuclear ionized gas (Arp 1967; Macchetto et al. 1996), as well as extended warm ionized Hα+[N ii] nebulae (Lakhchaura et al. 2018) has been reported.

The radio emission from the giant elliptical NGC 4552, or M89, which resides inside of the subgroup A of the Virgo cluster, is presented in Figure B5(b).

The red contours show the innermost radio pointlike emission from the archival high-resolution VLA A configuration. More recent (Project ID: 16A-275) previously unpublished C configuration data revealed a more diffuse butterfly-like emission extending on both sides to about 0.8 kpc.

The relativistic plasma from the jet-like emission is clearly interacting with the hot X-ray-emitting gas and forming on both sides X-ray cavities. Machacek et al. (2006) and Allen et al. (2006) analyzed the Chandra X-ray data and found two bright rings surrounding the two cavities to the north and south, which are perpendicular to the radio structures seen in the VLA images. Moreover, Machacek et al. (2006) also found signatures of ram pressure stripping of hot gas as the galaxy moves through the ICM of the Virgo cluster (see also Kraft et al. 2017).

Ferrari et al. (1999) has shown that the emission from the ionized gas extends up to 10'', whereas the dust absorption is very weak. The warm ionized gas, traced by Hα+[N ii], has not been confirmed in a more recent study described in Lakhchaura et al. (2018).

NGC 4636 is a central early-type galaxy within a poor group in the far outskirts of the Virgo cluster. VLA A configuration images, as presented in Figure B5(c), have been previously published and discussed by Dunn et al. (2010). The additional archival C configuration VLA data from 2017 revealed a more diffuse, but still quite compact and weak radio jet-like emission with a total extent of ∼3.5 kpc. Although the majority of detected radio emission in VLA is consistent with the radio emission at lower frequencies (235 and 610 MHz) with GMRT observed by Giacintucci et al. (2011), they also found a more extended northeastern radio lobe, which is coincident with the observed X-ray (northeastern) cavity (Stanger & Warwick 1986; Jones et al. 2002; Allen et al. 2006; Baldi et al. 2009; Panagoulia et al. 2014). The southwest radio lobes are filled only partially with relativistic plasma.

There have been reports of very weak CO(2–1) emission (indicating a cloud of cold molecular gas; Temi et al. 2018), [CII] emission and warm ionized Hα+[NII] nebulae (Werner et al. 2014), as well as dusty features (Temi et al. 2003).

The giant elliptical galaxy NGC 4649, or M60, is the third most luminous galaxy in the Virgo Cluster. Archival VLA data obtained in the A configuration reveal pointlike emission from a weak central radio source in NGC 4649.

Using D configuration data, Shurkin et al. (2008) and Dunn et al. (2010) found radio lobes filling the innermost X-ray cavities.

Sharp edges seen in the X-ray images are spatially coincident with the radio emission detected in the D configuration (Figure B5(d)). The X-ray properties of the hot gas are discussed in more depth by Shurkin et al. (2008), Dunn et al. (2010), Paggi (2014), Wood et al. (2017) and Kim et al. (2020).

Molecular gas, traced by CO(1–0) emission, was detected by Sage & Wrobel (1989). However, Young et al. (2002) failed to confirm their findings with the observations from IRAM and provide only upper limits on CO flux. The galaxy does not show the presence of warmer gas traced by Hα+[NII] emission.

Figure B5(e) reveals a radio emission in NGC 4696, the CDG of Abell 3526 (Centaurus cluster) from archival VLA observations obtained in the A and BC configurations.

A thorough study of the radio properties was performed by Taylor et al. (2002) and our total-intensity image of VLA A configuration data is consistent with their results, where the radio emission shows a bright nuclear source, radio jets and lobes with the western lobe bending toward east.

The interaction with the hot X-ray atmosphere is clearly present, as the radio plasma pushed out the hot gas and created cavities (Taylor et al. 2006; Panagoulia et al. 2014; Sanders et al. 2016). Interestingly, González-Martín et al. (2006) found only a diffuse morphology without a clear pointlike emission in the hard X-ray bands as a sign of a central star-bursting activity. They also noted that the X-rays could be just obscured by the dust and gas. The ongoing AGN activity is supported by our VLA A configuration data together with the VLBA observation of a small-scale one-sided jet detected by Taylor et al. (2006).

The cold extended molecular gas and warm ionized gas and dust were discussed in the recent publication by Olivares et al. (2019) and references therein. Moreover, Mittal et al. (2011) observed a cooling [C ii] line in the central region.

NGC 4778 or NGC 4761 is the only one lenticular (S0) galaxy in our sample and the CDG of a bright compact group, HCG 62 (Hickson 1982). It is in the process of merging with its companion NGC 4776 (or NGC 4759; Spavone et al. 2006). A small point-source-like central radio emission is observed in both archival VLA A- (Vrtilek et al. 2002) and D configuration total-intensity images (Figure B2(e)).

Moreover, looking at the lower radio frequencies with GMRT, Gitti et al. (2010) found the expected radio lobes, coincident with the observed X-ray cavities (Morita et al. 2006; Panagoulia et al. 2014) and Giacintucci et al. (2011) detected a possible second outer set of radio lobes.

The Hα emission in NGC 4778 was detected and studied by Valluri & Anupama (1996).

The radio source known as 3C 278 is hosted by the giant elliptical galaxy NGC 4782 in the center of the group LGG 316.

Radio emission, shown in the archival high-resolution A configuration image (Figure B5(f)), is rather peculiar as already noted by Borne & Colina (1996) and Machacek et al. (2006). The radio jets emanating from the core are at first invisible due to the high relativistic velocities. After ∼2 kpc the eastern side of the jet is tilted to the north, whereas the western side continues in a straight line up to 15 kpc, where it bents to the north as well. When using a more compact B configuration, a more diffuse radio emission is observed, which not only traces the peculiar bend of the jets but also fills the space between and around the jet structure.

The X-ray images reveal an interaction with the neighbor NGC 4783, which could be responsible for the observed bend of the jet. Moreover, X-ray cavities have been observed at the position of both radio lobes (Borne & Colina 1996; Machacek et al. 2006).

A nuclear (within 2 kpc from the core) warm ionized nebula was detected with SOAR (Lakhchaura et al. 2018), otherwise no cool gas or dust was found in this giant elliptical galaxy.

The radio emission from the giant elliptical galaxy NGC 4936 in our new VLA A configuration total-intensity image (Figure B2(f)), shows a pointlike radio emission from the nucleus.

Macchetto et al. (1996) studied the warm ionized gas and found Hα+[N ii] emission concentrated in a small disk similar to NGC 4872 (Appendix A.35). The neutral gas, in the form of a double-peaked H i line profile, was observed for this giant elliptical by Reid et al. (1994, and references therein), together with strong emission from N ii, S II and O i in the core of NGC 4936.

NGC 5044 is the central dominant galaxy of the X-ray bright group NGC 5044, which is well known for its large cold gas reservoir.

Archival radio observations in the A, BnA and D configurations of the VLA presented in Figure B5(g) reveal emission from the central radio source with small-scale jets pointing to northeast and southwest (partly visible in the images analyzed by Dunn et al. 2010). The previously unobserved indications for diffuse jet-like emission extending to about 6 kpc from the core in the east–west direction are seen in VLA B configuration contours. More diffuse oval-shaped radio emission extending up to ∼20 kpc is seen in the D configuration.

NGC 5044 has undergone (at least) three AGN outbursts. Gastaldello et al. (2009), David et al. (2011), Giacintucci et al. (2011), David et al. (2017), and Schellenberger et al. (2020) presented GMRT and Chandra data, which revealed that the radio emission is consistent with the presence of the innermost southern X-ray cavity. They also found a second pair of more extended (up to ∼10 kpc) ghost X-ray cavities, which suggest another episode of an AGN outburst in NGC 5044. The last, the third, and most-recent activity is traced in the most-recent study (Schellenberger et al. 2021), where they also show the presence of small-scale radio jets seen in VLBI data.

Sloshing features are also observed at further distances from the center caused by the accretion of a less-massive group (Gastaldello et al. 2009).

The filamentary morphology of the Hα+[N ii] ionized nebulae extends up to 6 kpc (Ferrari et al. 1999), whereas the corresponding dust emission extends to only 1.3 kpc (Temi et al. 2007a, 2007b), similar to NGC 4472 (Appendix A.28).

Studies done by David et al. (2014), Temi et al. (2018), and Schellenberger et al. (2020) showed that CO-emitting molecular gas clouds within ∼10 kpc from the core coincide well with the presence of strong emission from the warm ionized gas (Hα filaments), the hot X-ray-emitting gas, the innermost X-ray cavities (Schellenberger et al. 2021) and the cold dust features in the central regions of NGC 5044 as well as [C ii] line emission (Werner et al. 2014). Moreover, two CO absorption features were also detected within 5 pc from the core (Schellenberger et al. 2021).

NGC 5129 is a radio-quiet giant elliptical and the dominant galaxy of a small group.

Our new A configuration data (Figure B5(h)) revealed a bright radio core with small-scale radio jets extending to about 2 kpc to the northwest and southeast from the core. We categorized this source as a compact radio source with radio jets. We find that the radio emission is offset by ∼1.2 arcsec from the X-ray and optical core of the galaxy. The pointlike radio emission in NGC 5129 was previously observed by the NVSS survey (Condon et al. 1998).

The hot X-ray gas in the Chandra images has a disturbed morphology, which is not aligned with the direction of the radio jets. The X-ray gas properties were previously studied by Bharadwaj et al. (2014).

Warm ionized gas extending up to 2 kpc from the core was detected (Lakhchaura et al. 2018).

NGC 5419 is the CDG of the poor cluster A753, which hosts the radio source PKS B1400-33.

Our new A configuration VLA observations revealed extended radio emission in the core of the galaxy. Archival VLA CD configuration data (Figure B6(a); Goss et al. 1987) show a more extended emission, which forms an L-shaped radio tail to the south extending to about 35 kpc and a second prominent diffuse 120 kpc in diameter steep-spectrum radio relic. This is a unusual feature for a poor cluster (previously observed by Subrahmanyan et al. 2003).

From the multifrequency data analysis, Subrahmanyan et al. (2003) suggests that this diffuse extended emission could be the remnant plasma of a radio lobe (although no parent optical source has been identified) injected into the poor cluster.

X-ray images showed that the hot gas coincides with the central radio emission of NGC 5419 (Balmaverde & Capetti 2006).

NGC 5813 resides in the NGC 5846 group, where NGC 5846 (Appendix A.41) is the brightest central galaxy.

Archival data in the VLA A, B, and C configurations at ∼1.4 GHz (Randall et al. 2011) revealed signatures of AGN activity in radio images, when compared with the Chandra X-ray images (Randall et al. 2015; Figure B6(b)). The position of the central radio core is consistent with the X-ray brightness peak and the radio lobe-like emission corresponds well with the innermost X-ray cavities, especially the northern part of the inner region (Randall et al. 2011; Panagoulia et al. 2014). Additionally, the compact C configuration data reveal more diffuse emission with small ripples to the northeast and southwest, but without a clear interaction with a second pair of more extended X-ray cavities (Randall et al. 2015). The radio emission filling the second generation of cavities is found at lower frequencies with LOFAR (Bîrzan et al. 2020) and GMRT (Giacintucci et al. 2011).

The ionized gas shows a filamentary morphology and is coincident with dust emission (Ferrari et al. 1999). The ionised gas appears to be located in the wake of the rising AGN-inflated bubbles (Randall et al. 2011). Cooling lines of [C ii], [N ii], and O i and warm nebulae were observed, too (Werner et al. 2014; Lakhchaura et al. 2018).

NGC 5846 is a giant elliptical galaxy and a CDG of the massive group holding the same name.

The innermost central region hosts a radio core surrounded by a more diffuse radio emission (Figure B6(c)), which also appears to fill the innermost X-ray cavities (Trinchieri & Goudfrooij 2002; Dunn et al. 2010; Machacek et al. 2011; Panagoulia et al. 2014). The extended and diffuse emission, with a total extent of ∼12 kpc, is traced by the contours of the archival CnD configuration image. The results of the presented VLA data were previously published in Machacek et al. (2011).

The X-ray image analysis revealed multiple signatures of sloshing in NGC 5846 (Machacek et al. 2011), although without a distinctive X-ray core in the central regions (Trinchieri & Goudfrooij 2002; Satyapal et al. 2005; González-Martín et al. 2006).

The ionized and molecular plasma coincides with the disturbed nuclear X-ray emission, [C ii] line emission and cold, dusty features in the central regions of NGC 5846 (Macchetto et al. 1996; Caon et al. 2000; Trinchieri & Goudfrooij 2002; Temi et al. 2007b, 2009; Mathews et al. 2013; Werner et al. 2014; Lakhchaura et al. 2018).

The giant elliptical galaxy NGC 7619 is the CDG of the Pegasus I group.

Our new VLA data in the A configuration have revealed a central radio source with a small extension to the east of the core (Figure B2(g)), consistent with previously observed pointlike morphology in the archival VLA C configuration data. This findings are consistent with the emission in NVSS (Condon et al. 1998) and with GMRT observations at lower frequencies (at 610 MHz) by Giacintucci et al. (2011).

The X-ray image analysis revealed a ram-pressure-stripped tail (to the southwest of the nuclear region) as a result of NGC 7619 falling toward the center of the Pegasus cluster and interacting with the companion NGC 7626 with radio jets/lobes morphology (Randall et al. 2009) located 415'' (102 kpc) from NGC 7619.

The study of ionized gas performed by Macchetto et al. (1996) showed a circumnuclear emission with an additional extended tail from the northeast to southwest, although the subsequent long-slit spectroscopy and narrowband imaging observations with SOAR did not confirm the previous findings of Hα+[N ii] emission (Lakhchaura et al. 2018). CO line emission was detected by Temi et al. (2007a) for this giant elliptical galaxy.