Environmental Effects in Herschel Observations of the Ionized Carbon Content of Star-forming Dwarf Galaxies in the Virgo Cluster^∗

Data were downloaded from the Herschel Science Archive in the form of fully calibrated and flat-fielded Level 2 products output by the PACS pipeline reduction. ⁵

The Herschel [C II] data cubes were analyzed using SOSPEX (Fadda & Chambers 2018). Spatially integrated spectra were created in SOSPEX using an aperture that took in the whole map and line profiles were fitted to these spectra in SOSPEX using Gaussian profiles; baseline regions were defined visually to be clear of the emission lines and of the noisy end regions of the spectra. These fits were used to obtain the integrated [C II] flux and its associated error. With the exceptions of VCC 334, VCC 737, and VCC 1725, baselines were assumed to be flat and were set to the median of the data (excluding the line region and the noisy ends of the spectra). For the three galaxies mentioned, visual inspection showed that the flat baseline initially fitted was not a good match for the actual spectral baseline. These three galaxies were fitted with a sloped baseline: for VCC 737 this reduced the line flux by 4% while, for VCC 334 and VCC 1725, the reduction was less than 1% and was within the measurement errors. The fitted line profile is shown together with the spectrum in Figure 1. The values of the [C II] intensity derived from these fitted line profiles are used for the rest of the analysis presented here (see Table 2). Line maps were created using line fitting to each pixel in SOSPEX. The line maps are used to illustrate and examine the extent of the [C II] but measurements taken from these maps were not used in the analysis presented here.

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Table 2. Measured Values for The Galaxies from The [C II] Observations and The SED Fitting

Galaxy ID	Herschel (C II)	(C II) flux	TIR Luminosity	TIR flux	(C II)/TIR
	OBSID	(10−18 W m−2)	log(LTIR/${{\boldsymbol{ \mathcal L }}}_{\odot }^{{\bf{N}}}$)	(10−15 W m−2)	(%)
VCC 144	1342224404	232.8 ± 1.6	${9.28}_{-0.01}^{+0.05}$	${60.2}_{-0.7}^{+8.1}$	${0.387}_{-0.052}^{+0.005}$
VCC 213	1342224405	338.4 ± 1.4	${8.68}_{-0.02}^{+0.04}$	${53.0}_{-2.4}^{+5.8}$	${0.638}_{-0.070}^{+0.029}$
VCC 324	1342225152	217.9 ± 1.8	${8.90}_{-0.01}^{+0.00}$	${88.0}_{-1.0}^{+0.0}$	${0.248}_{-0.002}^{+0.003}$
VCC 334	1342225154	48.6 ± 0.3	${7.94}_{-0.00}^{+0.00}$	${9.6}_{-0.0}^{+0.1}$	${0.504}_{-0.007}^{+0.003}$
VCC 340	1342225156	147.4 ± 1.1	${8.89}_{-0.00}^{+0.02}$	${24.2}_{-0.3}^{+0.9}$	${0.608}_{-0.022}^{+0.008}$
VCC 562	1342225158	44.5 ± 0.5	${8.01}_{-0.06}^{+0.04}$	${11.2}_{-1.4}^{+1.2}$	${0.397}_{-0.044}^{+0.051}$
VCC 693	1342225160	50.9 ± 0.5	${8.33}_{-0.00}^{+0.01}$	${23.7}_{-0.0}^{+0.6}$	${0.215}_{-0.005}^{+0.002}$
VCC 699	1342225162	363.6 ± 1.9	${9.18}_{-0.00}^{+0.00}$	${91.1}_{-0.0}^{+0.0}$	${0.399}_{-0.002}^{+0.002}$
VCC 737	1342225165	70.6 ± 0.7	${7.79}_{-0.00}^{+0.00}$	${6.8}_{-0.1}^{+0.0}$	${1.046}_{-0.011}^{+0.016}$
VCC 841	1342225166	67.6 ± 0.7	${7.82}_{-0.01}^{+0.04}$	${7.2}_{-0.2}^{+0.7}$	${0.934}_{-0.091}^{+0.023}$
VCC 1437	1342225221	103.7 ± 1.5	${8.35}_{-0.08}^{+0.01}$	${24.8}_{-4.4}^{+0.6}$	${0.418}_{-0.011}^{+0.075}$
VCC 1575	1342225223	486.5 ± 1.5	${9.11}_{-0.00}^{+0.00}$	${141.1}_{-0.0}^{+0.0}$	${0.345}_{-0.001}^{+0.001}$
VCC 1686	1342225225	324.8 ± 1.3	${8.67}_{-0.00}^{+0.01}$	${51.8}_{-0.6}^{+0.6}$	${0.627}_{-0.008}^{+0.008}$
VCC 1725	1342225226	133.5 ± 0.8	${8.54}_{-0.00}^{+0.00}$	${38.4}_{-0.0}^{+0.0}$	${0.348}_{-0.002}^{+0.002}$

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Spectral Energy Distributions (SEDs) were created using FIR data from the Herschel Virgo Cluster Survey (HeVICS; Auld et al. 2013), as reanalyzed by Grossi et al. (2015), mid-infrared data from AllWISE (Wright et al. 2010; Cutri et al. 2013), and optical data from the SDSS/Extended Virgo Cluster Catalogue (EVCC; Kim et al. 2014), supplemented with our own analysis of near-infrared data from 2MASS (Skrutskie et al. 2006) and UV data from GALEX (Morrissey et al. 2007). For VCC 1686, which has a foreground star superposed on the galaxy, we used our own measurement from the AllWISE data and the SDSS fluxes, masking out the star and patching the region with a similar region from elsewhere in the galaxy. We also used our own measurement of the SDSS z band flux of VCC 693, as the literature value from Kim et al. (2014) was highly discrepant from the other fluxes (and our measurement) for unknown reasons. Errors were calculated following the prescriptions given in the documentation in the archives, or following Kim et al. (2014) for the SDSS. Absorption corrections to the UV, optical, and NIR fluxes were made using the dust measurements of Schlafly & Finkbeiner (2011), via the IRSA Galactic Dust Reddening and Extinction service. ⁶ As the galaxies in this sample are similar in size to or smaller than the beam (i.e., are point sources) for the crucial Herschel measurements of the FIR fluxes, it is not reliable to use a single aperture to measure the SED. We therefore use the integrated whole-galaxy flux at all wavelengths rather than attempting to make measurements within a defined aperture.

SED fitting was carried out using MAGPHYS (Multi-wavelength Analysis of Galaxy Physical Properties; da Cunha et al. 2008) which returns both an overall best-fit SED and marginalized probability distributions for the individual parameters. Fluxes and errors, both converted to Jy, were supplied as inputs to the fitting, either from the literature or based on our own measurements as described above (see Table A1). The plots in Figure 2 show the overall best-fit SED output by MAGPHYS; for our analysis, we use the marginalized probabilities for L_d ^tot (the MAGPHYS parameter that gives the TIR) using the 50% point of the probability distribution as the central estimator and the 16% and 84% points as the estimators for the 1σ error. The difference between the value of L_d ^tot returned for the best-fit model and the 50% point of the probability distribution of L_d ^tot is less than 1σ (or less than 1% in the cases where the error estimate is zero) except for VCC 562 (best-fit L_d ^tot 0.05 dex higher) and VCC 841 (best-fit L_d ^tot 0.21 dex higher).

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The results of the [C II] observations and the SED fitting are given in Table 2. Figure 3 shows how the [C II]/TIR ratio varies with angular distance from the giant elliptical M87 (taken to represent the cluster center), while Figure 4 shows the positions of the sources within the cluster. The values of L_TIR are calculated from the MAGPHYS output of L_d ^tot assuming a distance of 17 Mpc except for three galaxies that, as given in Table 1, are assigned to more distant subclusters in Virgo at 23 Mpc (VCC 699) and 32 Mpc (VCC 144 and VCC 340); no error on the distance is assumed.

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For the purpose of our analysis, we divide the sample into two groups: the central galaxies (VCC 213, VCC 334, VCC 562, VCC 841, and VCC 1686), which lie around M87 and are all north of decl. +12°, and the southern galaxies (VCC 144, VCC 324, VCC 340, VCC 693, VCC 699, VCC 737, VCC 1437, VCC 1575, and VCC 1725), which are all south of decl. +95 and lie around or south of M49. The central galaxies thus correspond to subcluster A and the southern galaxies to subcluster B, with the exception of VCC 699 (W${}^{{\prime} }$ cloud) and VCC 144 and 340 (W cloud), according to the standard subdivision of Virgo (e.g., Boselli et al. 2014). The southern galaxies thus combine three different environments, but as these form our control sample of galaxies outside of the center of the Virgo cluster this is not expected to affect our analysis.

The "central" galaxies are those closest to the center of the Virgo cluster while the "southern" galaxies form a control sample away from the cluster core, thus if there is any environmental effect it should manifest as a difference between these two samples. Comparator galaxies from the Herschel Dwarf Galaxies Survey (DGS; Cormier et al. 2015) are also used as to form a second control sample. The TIR for the DGS galaxies is, as with our galaxies, defined using the modeled dust SED and their measurement over 1–1000 μm can be expected to give an equivalent measurement of the total infrared flux to the MAGPHYS measurement over 3–1000 μm (da Cunha et al. 2008; Rémy-Ruyer et al. 2015, particularly Section 4.2 and Table 3). As the [C II]/TIR ratio is influenced by both metallicity and FIR luminosity (e.g., Cormier et al. (2015), Figure 5), we take a subsample of DGS galaxies that have similar metallicities (12 + log(O/H) = 8.0–8.8) and luminosities (L ${}_{\mathrm{TIR}}/{{\boldsymbol{ \mathcal L }}}_{\odot }^{{\bf{N}}}=\,5\times {10}^{7}$–2.5 × 10⁹) to those of the Virgo dwarfs.

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Four of the five central galaxies have [C II]/TIR > 0.5% compared to two out of nine in the southern region and two out of eight in the DGS comparator subsample. We compare the various samples statistically both by looking at their averages and by using the Kolmogorov–Smirnov test; as we are testing the hypothesis that the central galaxies have a higher [C II]/TIR than those in the control samples we use the single-sided two-sample Kolmogorov–Smirnov test. The distribution of the samples in [C II]/TIR, L_TIR and metallicity is shown in Figure 5. Including the full DGS sample (shown in Figure 5 by red squares), with a mean [C II]/TIR of 0.29 ± 0.02, leads to a very significant difference between our central sample and the DGS (3.5σ; p = 0.001 from the Kolmogorov–Smirnov test), but also to a significant difference appearing between our southern sample and the DGS (1.9σ; p = 0.02 from the Komogorov–Smirnov test) due to the inclusion of a large number of galaxies that are not similar to the Virgo dwarfs, motivating us to use only a subsample of DGS galaxies with similar luminosities and metallicities for our comparison sample.

The central galaxies have a mean [C II]/TIR of 0.62% ± 0.09% and the southern galaxies have a mean [C II]/TIR of 0.45% ± 0.08%, indicating a 1.4σ difference between these two samples (errors on the means in both cases estimated by propagating the errors on the individual measurements in the samples and combining these in quadrature with the uncertainty in the estimates of the means due to the scatter in the samples). This is confirmed by the one-sided Kolmogorov–Smirnov test, which, despite the small sizes of the samples, gives a likelihood of getting this distribution if both were drawn from the same parent population of p = 0.086. Similarly, the DGS subsample has a mean [C II]/TIR of 0.37% ± 0.05%, indicating a 2.4σ difference from the central sample, and the Kolmogorov–Smirnov test gives p = 0.040. The difference between the means of the DGS subsample and the southern sample is 0.8σ and the Kolmogorov–Smirnov test gives p = 0.57, both consistent with the DGS subsample and the southern Virgo dwarfs being drawn from the same parent population. We therefore combine these two control samples, getting a mean of 0.41 ± 0.05, giving a difference of 2.0σ from the mean of the central sample, and a result from the Kolmogorov–Smirnov test of p = 0.032. We conclude that both parametric and nonparametric tests show that there is a statistically significant difference between [C II]/TIR in the central sample and the control samples, with the caveat that the numbers in the samples remain small. The results of these statistical tests for difference are summarized in Table 3.

We compare our sample to the work of Köppen et al. (2018) to see whether galaxies with high values of [C II]/TIR correspond to those identified there as likely to be undergoing ram-pressure stripping. Köppen et al. (2018) categorize galaxies as "active strippers" (likely to be currently undergoing ram-pressure stripping) and "past strippers" (showing evidence of past gas loss, but not currently undergoing ram-pressure stripping) based on an analysis of how tightly bound their H i disk is and whether the local ram pressure would be sufficient to strip this. The local ram pressure, p_loc, is expected to decrease with increasing distance from the cluster center (see Figure 14 in Köppen et al. 2018). This pressure is fundamentally related to two parameters: the density of the ICM and the speed with which a galaxy is moving through that ICM, with ${p}_{\mathrm{loc}}\,=\,{\rho }_{\mathrm{ICM}}{v}_{\mathrm{gal}}^{2}$.

Köppen et al. (2018) use a β model of the cluster (Cavaliere & Fusco-Femiano 1976; Schindler et al. 1999; Vollmer et al. 2001) to estimate the ICM density and the cluster mass distribution:

$$\begin{eqnarray}&&\rho ={\rho }_{0}{\left(1+\displaystyle \frac{{r}^{2}}{{r}_{c}^{2}}\right)}^{-(3/2)\beta }.\end{eqnarray} \tag{ 1 }$$

The values adopted for the β model parameters in Köppen et al. (2018) are β = 0.47, r_c = 13.4 kpc, and ρ₀ = 4 ×10⁻² cm⁻³ for the ICM, and β = 1, r_c = 0.32 Mpc, and ρ₀ = 3.8 × 10⁻⁴ ${{\boldsymbol{ \mathcal M }}}_{\odot }^{{\bf{N}}}$ pc⁻³ for the dark matter cluster halo. The velocities of the galaxies at a given radius are estimated as being the local escape velocity using the β model with the parameters for the dark matter halo given above to estimate the mass distribution. There are limitations to this approach: the β model assumes a smooth, symmetrical relationship with radius, not taking into account structures and variations in the density, and subclusters, while the assumption that galaxies are moving at their local escape velocity may not be true for individual galaxies; however, it provides a reasonable description overall. They also define p_def as the pressure required to strip the galaxy to its current H i deficiency, with the ratio p_loc/p_def then giving whether a galaxy is currently being stripped (an "active stripper," with p_loc/p_def > 0.5) or was stripped in the past (a "past stripper," with p_loc/p_def < 0.5). We look here first at how their analysis compares to our data and then at how p_loc, calculated for our galaxies, corresponds to [C II]/TIR.

A comparison of their analysis with our data does not reveal any firm correlation between high values of [C II]/TIR and whether a galaxy is identified as likely to be undergoing ram-pressure stripping: of the galaxies in the central sample with values for [C II]/TIR > 0.5%, only VCC 1686 is an "active stripper" while VCC 213 and VCC 334 are both given as "past strippers" and VCC 841 is not listed in their sample. The only galaxy in the central sample with a value for [C II]/TIR < 0.5%, VCC 562, is also not listed. For the southern sample, of the two galaxies with values for [C II]/TIR > 0.5%, VCC 737 is a "past stripper" while VCC 340 is not listed; while among the galaxies with values for [C II]/TIR < 0.5%, VCC 1437 is an "active stripper," VCC 324, 693, 699, 1575, and 1725 are "past strippers," and VCC 144 is not listed. Figure 6 gives the distribution of [C II]/TIR for the sources in our sample assigned to each category by Köppen et al. (2018) with the x-axis showing their p_loc/p_def, with their break of p_loc/p_def = 0.5 as the demarcation between the active and past strippers marked. It can be seen that there is no significant difference between the mean [C II]/TIR for the "active strippers" and the "past strippers." Further to this, we find a Spearman's ρ = 0.38 giving, with 10 pairs, a significance of p = 0.28; while if the outlier VCC 737 is ignored we find ρ = 0.40 which, with 9 pairs, gives a significance of p = 0.29. This implies there is no significant correlation between [C II]/TIR and p_loc/p_def.

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This lack of a correlation may be partly explained by the effect of stripping on star formation. Grossi et al. (2015), from where the sample here is ultimately drawn, reported that the dwarfs in their sample that were undetected in HeViCS far-infrared continuum observations had a larger fraction of object with higher H i deficiencies than their detected dwarfs. While this is not a very strong effect, if stripped galaxies are less likely to be forming stars, then they are less likely to have been observed in this sample. This does not, however, look to be sufficient to explain the lack of correlation we see here, particularly as we do have both "active strippers" and "past strippers" in both samples.

A second effect that may lend an explanation, which is clear from Köppen et al. (2018), is that whether a galaxy is undergoing ram-pressure stripping depends both on the pressure it is feeling from the ICM (their p_loc) and the pressure needed to strip its neutral hydrogen (their p_def). A galaxy where p_def is substantially higher than p_loc will not currently be undergoing ram-pressure stripping, even if the value of p_loc is higher than for "active strippers." For example, VCC 1437, identified as an "active stripper," has p_loc = 230 cm⁻³ (km s⁻¹)² and p_def = 280 cm⁻³ (km s⁻¹)² while VCC 334, identified as a "past stripper" at almost the same distance from M87, has p_loc = 240 cm⁻³ (km s⁻¹)² (that is, marginally higher than for VCC 1437) but p_def = 550 cm⁻³ (km s⁻¹)². The difference between these two galaxies is not the ram pressure they feel but the effect that that ram pressure has on their (current) H i disk. ⁷

Following Köppen et al. (2018), we use the same β model of the cluster and associated parameters to estimate the ICM density and the cluster mass distribution. The velocities of the galaxies at a given radius, still following Köppen et al. (2018), are estimated as being the local escape velocity, derived using the β model for the cluster mass distribution. From this, we calculate the local ram pressure for those galaxies in our sample at the 17 Mpc distance of the main cluster (where the β model is applicable), assuming an average deprojected distance from the cluster center of $\sqrt{4/3}$ times their projected distance from the cluster center (i.e., spherical symmetry). The results are plotted in Figure 7, which shows [C II]/TIR versus the local ram pressure. The error bars on p_loc indicate fractional differences in the cluster-centric distances of $\sqrt{4/3}-1$, i.e., the difference between the uncorrected and deprojected cluster-centric distances.

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If the outlier VCC 737 (top left of Figure 7) is excluded, we find a Spearman's ρ = 0.75 which, with 10 pairs, gives a significance of p = 0.013, indicating that there is likely to be a correlation between [C II]/TIR and the local ram pressure. We fit this correlation with a power-law model as $\mathrm{log}([{\rm{C}}\,{\rm\small{II}}]/\mathrm{TIR})=(0.49\pm 0.13)\times \mathrm{log}({p}_{\mathrm{loc}})+(0.05\pm 0.12)$, where [C II]/TIR is given as a percentage and p_loc in units of 1000 cm⁻³(km/s)². The local ram pressure calculated here could be affected by projection effects, where the three-dimensional position of a source lies in front of or behind the plane of the cluster center, result in the projected distance to the cluster center being lower than the actual three-dimensional distance. As the ram pressure a galaxy feels is due to its actual distance from the cluster center but our calculation here is based on their projected distance, our estimates of the ram pressure will be high for galaxies that are projected onto the cluster center, moving them to the right on Figure 7. This provides a possible explanation for why VCC 562 falls clearly (including the uncertainties on its measurement) outside of the 3σ scatter around the best-fit line: it may lie either in front of or behind the cluster rather than near where it is seen in projection, so that its local ram pressure is lower than that calculated based on projected separation from the cluster center.

The correlation we see in Figure 7 is unexpectedly strong, continuing as it does into the outer parts of the cluster where [C II]/TIR is similar to that seen in our control sample from the DGS, which was expected to be free of environmental effects (although we do not know the local environments of the DGS galaxies), implying that ram pressure could have an effect on [C II]/TIR well outside the central region of the cluster and possibly even in galaxy groups (see Roberts et al. 2021).

Our finding that the central sample galaxies are more likely to have high values of [C II]/TIR than those in the southern sample thus appears to be linked to the higher values of p_loc felt by the central sample, i.e., their ram-pressure interaction with the ICM, without being necessarily linked to whether they are actively undergoing ram-pressure stripping of their gas. It seems likely, therefore, that the excess we see in [C II]/TIR is due to [C II] formation in shocks in the ISM of these galaxies caused by the ram pressure they are feeling.

We use the calibration of Hao et al. (2011) to derive a star-formation rate (SFR) from the GALEX far ultraviolet (FUV) luminosities combined with our TIR luminosities. We also use the [C II] calibration of De Looze et al. (2014) for low-metallicity dwarfs to derive an SFR from the [C II] luminosities. This gives us a measure of excess [C II] based on star formation, SFR_{[C II]}/SFR_FUV+TIR. ⁸

In Figure 8, we plot this against [C II]/TIR for the dwarf galaxies in the main cluster. It can be seen that this shows an increase in [C II] excess measures via SFR indicators as [C II]/TIR increases, as expected if the [C II]/TIR increase is due to the creation of [C II] via processes other than star formation. If, on the other hand, the excess in [C II]/TIR was caused by a rise in unobscured star formation, i.e., the [C II] seen here is being created by star formation that is not reflected in the TIR emission, we would not expect to see any increase in SFR_{[C II]}/SFR_FUV+TIR. Two galaxies fall below this trend: VCC 737, already identified as having an anomalously high value of [C II]/TIR for its position in the cluster, and VCC 1686 which, as described in Section 3.4.3 below, has strong UV emission that is not correlated spatially with either the [C II] or the dust and is probably due to recent star formation; this galaxy would thus be expected to show a deficit of [C II] relative to the FUV+TIR SFR, which translates into a deficit relative to the trend here.

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The trend seen here is much tighter than the scatter on the SFR indicators (shown by the error bar on the right side of the plot), which is dominated by the scatter on the [C II]–SFR calibration. One possibility for our trend being tighter than the scatter is the relatively narrow range of luminosities and metallicities covered by our sample, while another is that De Looze et al. (2014) use the 24 μm flux as a proxy for the total infrared flux in their measurement of SFR that they compare to the [C II], whereas we measure TIR based on the whole SED here.

As can be seen in Figure 8, the SFR calibrations used give an excess of [C II] for all of the galaxies in our sample, although, for four of the 11 galaxies, this is within the 1σ scatter around zero and, for all but VCC 841, it is within the 2σ scatter. However, applying the [C II]–SFR calibration of Herrera-Camus et al. (2015) gives (after correcting the SFR from the Salpeter (1955) initial mass function used by Herrera-Camus et al. (2015) to the Kroupa & Weidner (2003) initial mass function used by De Looze et al. (2014) and Hao et al. (2011) using the ratio of 0.67 found by Madau & Dickinson (2014)) a similar shape while showing a deficit of SFR_{[C II]} relative to SFR_FUV+TIR for all but four galaxies in the sample. Thus this appears to be an issue of calibration of the zero point, which is of secondary importance for the relationship being examined here: the clear increase in SFR_{[C II]}/SFR_FUV+TIR with increasing [C II]/TIR.

We present here notes on three of the galaxies with the highest [C II]/TIR in our sample: VCC 737, VCC 841, and VCC 1686. Two of these are in the central region of the cluster, while the third (VCC 737) is on the cluster outskirts and appears anomalous in terms of its [C II]/TIR compared to its cluster position.

3.4.1. VCC 737

VCC 737 is far from the cluster core, at a projected distance of 2.5 Mpc, and is estimated to have a low local ram pressure, but has the highest [C II]/TIR in our sample, making it an outlier in these relationships.

Figure 9 shows the SDSS (York et al. 2000) color image of VCC 737 from Data Release 13 (Albareti et al. 2017) and contour maps of the [C II], mid-IR and near-UV regions. There are two SDSS spectra on this galaxy, shown in Figure 10. The eastern part of this galaxy, including the location of the central spectrum, contains coincident peaks in [C II], dust, and UV. However, the western part has a stronger [C II] peak that is not matched by peaks in either dust or UV, giving it a clear excess of [C II]. The western SDSS spectrum shows a much stronger Hα line than the central spectrum (a flux of 615.8 ×10⁻¹⁷ erg cm⁻² s⁻¹ in the SDSS catalog versus 200 ×10⁻¹⁷ erg cm⁻² s⁻¹ for the central region), ⁹ and the Hα/Hβ ratios of 3.6 for the western region and 3.4 for the central region (again from the SDSS catalog) imply that the two regions have similar internal absorption. However, although there is a local peak in the near-UV emission near the location of the western spectrum, it is around half the strength of the near-UV emission in the central region: the local UV peaks at 0.088 cnt s⁻¹ pix⁻¹ in the pixel on the northern edge of the western SDSS spectrum versus 0.164 cnt s⁻¹ pix⁻¹ in the central pixel of the central SDSS spectrum. The eastern UV (which does not have an associated SDSS spectrum, but corresponds to the bluer eastern region in the SDSS image) peaks at 0.163 cnt s⁻¹ pix⁻¹.

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This pattern of [C II] enhancement in an area with little warm dust emission is quite distinct from that expected from ram-pressure shocks, where triggered star formation is expected to occur in the shocks (e.g., Kapferer et al. 2009), producing enhanced dust, [C II], and UV from star formation alongside the additional [C II] that may be formed directly from the shock (as seen in VCC 841; Figure 11). Based on its optical diameters in GOLDMine (Gavazzi et al. 2014) and its H i mass from ALFALFA (Haynes et al. 2018), we estimate an H i deficiency (Haynes & Giovanelli 1984) of −0.17 for VCC 737, within the range of normal (unstripped) galaxies.

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Clearly, the high [C II]/TIR measured in VCC 737 is due to the [C II] peak in the western part of this galaxy, but exploring the details of what might be exciting strong [C II] and Hα emission without the expected enhancement in the dust luminosity or UV in this galaxy is beyond the scope of this paper.

3.4.2. VCC 841

Figure 11 shows the SDSS color image of VCC 841 and contour maps of the [C II], mid-IR, and near-UV. The SDSS image shows an off-center nucleus, which is coincident with the [C II], dust, and UV centers, with the rest of the galaxy to the northwest of this nucleus. The SDSS spectrum of the nucleus (Figure 12) shows a strong Hα line. Comparison with Figure 4 shows that the nucleus is on the side of the galaxy closest to M87. This is consistent with this galaxy undergoing ram-pressure effects that are triggering star formation in the nucleus, which would naturally be expected to enhance both [C II] and TIR, with the excess [C II]/TIR being due to ram-pressure shocks. It is not listed in Köppen et al. (2018), but based on its optical diameters in GOLDMine and its H i mass from ALFALFA we estimate an H i deficiency of 0.67, consistent with it being ram-pressure stripped.

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3.4.3. VCC 1686

Figure 13 shows the SDSS color image of VCC 1686 and contour maps of the [C II], mid-IR, and near-UV regions. The [C II] and dust are well aligned, with the UV being enhanced in the areas to the northwest of the galaxy that also show very blue colors in the SDSS image and are probably the site of recent star formation as there is no dust or [C II] enhancement seen in this region. It is also likely that some [C II] flux is lost off the edge of the PACS footprint. This galaxy is considered an "active stripper" by Köppen et al. (2018), who calculate that it has an H i deficiency of 0.45. Based on its optical diameters in GOLDMine and an updated H i mass from ALFALFA, we estimate a slightly lower H i deficiency of 0.39, still consistent with it undergoing stripping.

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