The Evolution of NGC 7465 as Revealed by Its Molecular Gas Properties

The gas in early-type galaxies (ETGs) offers interesting clues into their evolution, as there can be a much stronger disconnect between the stars and the gas than there is in spiral galaxies. In spirals, the continuous star formation activity implies that there has been a continuous supply of cold gas over a Hubble time, and the stars and the gas have co-evolved in an uninterrupted symbiotic relationship. In contrast, in early-type galaxies, much of the cold and warm gas is kinematically misaligned (even retrograde) with respect to the stars, such that there is little possibility that the stars formed from that gas or the gas was expelled from those stars.

We expect irregular gas kinematics in the outer parts of galaxies where the cold gas (especially H i) is more vulnerable to interactions and will retain signatures of disruption for many Gyr. But such misalignments are also present in the inner few kiloparsecs of early-type galaxies, where we find that 20% of the regular, relaxed CO disks have polar or retrograde rotation with respect to the stars in the host galaxy. Thus, as much as 30%–50% of the molecular gas in early-type galaxies has been acquired recently from some external source such as an accreted dwarf galaxy (Davis et al. 2011). Similar results are found for ionized gas (Jin et al. 2016; Bryant et al. 2019). Of the remainder, some of the relaxed, prograde molecular gas may have been resident in its host early-type galaxy for a long time, through the galaxy's transition to the red sequence (e.g., Davis & Bureau 2016). Some of the molecular gas probably also originated in its current host but made an extended detour through a hot phase in the galactic halo before condensing back into a cold phase (e.g., Davis et al. 2019; Russell et al. 2019).

These varied histories also have implications for the physical properties of the gas. If the cold gas in an early-type galaxy was accreted in a minor merger or from a relatively pristine cold flow, the metallicity and isotopic abundance patterns in the molecular gas could reflect a significantly different star formation history and stellar initial mass function (IMF) from those that produced the current stellar populations in the galaxy (Davis & Young 2019). On the other hand, if the gas has been resident in the galaxy for a long time, gradually mixing with the mass-loss material from evolved stars in the galaxy, we would expect a much tighter correspondence between the metallicity and isotopic abundances of the stars and the gas.

In this context, we have been undertaking detailed studies of the chemical and physical properties of the molecular gas in early-type galaxies; these chemical and physical properties might also serve as indicators of the galaxies' histories in a complementary manner to the information revealed by kinematics. Here we present Atacama Large Millimeter/submillimeter Array (ALMA) observations of the early-type galaxy NGC 7465, which is known to have recently accreted its cold gas (Section 2). We present ¹²CO (1–0) data at 08 (110 pc) resolution and ¹²CO, ¹³CO, C¹⁸O, and several other higher-density molecular tracers at 2'' (280 pc) resolution (Section 3). We describe the millimeter continuum emission, compare it to the optical nebular emission-line ratios, and estimate the ionized gas metallicity (Section 4). We also make comparisons of CO to stellar and ionized gas kinematics at matched resolution (Section 5). Quantitative analysis of the molecular line ratios and their spatial variations and comparisons to other galaxy types (Sections 6–11) reveal clues to the evolution of NGC 7465 and the broader context of galaxy evolution.

NGC 7465 was originally selected because it is a member of the ATLAS^3D sample of early-type galaxies (Cappellari et al. 2011), so there is a wealth of information about its stellar and gas content, including two-dimensional maps of its stellar kinematics, ionized gas distribution, and kinematics, stellar populations, star formation history, atomic gas, and molecular gas (Krajnović et al. 2011; Young et al. 2011; Serra et al. 2012; McDermid et al. 2015, and references therein). It is one of the more CO-bright members of the ATLAS^3D sample, so there is extensive information on its ¹²CO J = 1–0 emission. Its stellar mass is $\mathrm{log}({M}_{\star }/{M}_{\odot })=10.4$, and its adopted distance is 29.3 Mpc (Cappellari et al. 2011).

Though it is an early-type galaxy, NGC 7465 shows moderate levels of current star formation. Davis et al. (2014) estimated its total star formation rate from its 22 μm Wide-field Infrared Survey Explorer flux, giving 1.37 ± 0.5 M_⊙ yr⁻¹ and a specific star formation rate SFR/M_⋆ ≈ (5.5 ± 2) × 10⁻¹¹ yr⁻¹. These rates are low when compared to nearby spirals, placing NGC 7465 below the "star formation main sequence" or SFR–M_⋆ relation (e.g., Cluver et al. 2014) but high when compared to other early-type galaxies (Davis et al. 2014). Its depletion time (SFR/M_gas) is relatively long, at 7 Gyr. Its radio and far-IR continuum emission imply a radio/FIR q ratio that is also consistent with star formation activity (Nyland et al. 2017).

In addition to being CO-bright, NGC 7465 is notable for signatures of interactions with nearby neighbors NGC 7463 and NGC 7464 at projected separations <20 kpc. Though it is an early-type galaxy (an S0 galaxy and a fast rotator), it has faint blue outer arms at radii of 8–10 kpc (Figure 1). Its H i emission is highly disturbed and strongly misaligned with respect to the stellar body of the galaxy (Li & Seaquist 1994; Serra et al. 2012). This kinematic evidence strongly suggests that the galaxy's cold gas was recently acquired from some external source, and this accretion/interaction event probably drove the formation of the blue outer arms. The stellar kinematics show a kinematically distinct core (Krajnović et al. 2011) which, as we will show, is probably not related to the most recent gas-transfer interaction. A deep u-band image from Duc et al. (2015) also shows recent star formation activity in an irregular spiral structure at radii ≈3''–20'' (≈0.4–3 kpc).

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NGC 7465 is classified as a barred S0 galaxy by some (e.g., de Vaucouleurs et al. 1991), and indeed, Figure 1 might be interpreted as a nearly face-on galaxy with a bar ≈1' (8.5 kpc) in length. An alternative interpretation arising from the ATLAS^3D dynamical analysis (Cappellari et al. 2013) is that the inner r ≲ 30'' of the galaxy is an edge-on axisymmetric fast rotator, and the outer blue arms are transient extraplanar structures. The primary reason for this alternative interpretation is that the inner part of NGC 7465 does not show typical kinematic features of strongly barred early-type galaxies, which are (1) large misalignments between the photometric major axis of the bar and the kinematic major axis of the stars in that region, and (2) cylindrical rotation in the stellar kinematics, yielding parallel isovelocity contours (Krajnović et al. 2011; Lablanche et al. 2012). Additional stellar kinematic data covering a larger field of view would be helpful in distinguishing between these alternatives.

Finally, IRAM 30 m observations of molecular lines in the ATLAS^3D galaxies have shown that NGC 7465 has some uncommon integrated molecular line ratios (Crocker et al. 2012). Relative to ¹²CO, NGC 7465 has unusually faint ¹³CO and HCN, and it has a low HCN/HCO⁺ ratio. At higher resolution, these line ratios might reflect the influence of an active galactic nucleus (AGN) on its surrounding medium. The galaxy contains a Seyfert or LINER nucleus (e.g., Gonçalves et al. 1999), a 5 GHz synchrotron point source (Nyland et al. 2016), and a Swift hard X-ray source (Baumgartner et al. 2013). Our new ALMA data, with additional lines and higher spatial resolution, provide a better perspective on how to interpret the molecular line ratios and connect them to the physical properties of the gas and to galaxy evolution.

NGC 7465 has been observed by ALMA in several projects. Here we used the Band 3 observations of ¹³CO, C¹⁸O, and CS in project 2018.1.01253.S (2018 December); ¹²CO, HCN, C₂H, and HCO⁺ in 2016.1.01119.S (2016 November); and ¹²CO and CN in 2018.1.01599.S (2018 October). We used the standard pipeline-calibrated raw data and carried out continuum subtraction, imaging, and cleaning ourselves. An additional round of phase self-calibration was not found to offer any improvement in the images because of the relatively modest signal-to-noise ratios. For continuum subtraction, we used zero-order or first-order fits to the line-free channels in the visibility domain; for imaging, we made a wide variety of images at varying channel widths and resolutions as necessary to optimize the resolution or to match resolutions for resolved line ratios. The times on source, along with fiducial ("natural" weighting) beam sizes and rms noise levels, are indicated in Table 1 for the detected spectral lines.

Line emission in the data cubes was cleaned down to about the 1σ level and primary beam corrections were applied. The ¹²CO integrated intensity images (e.g., Figure 1) were created by modestly smoothing the data cube in spatial and velocity dimensions and clipping on the smoothed cubes to create a velocity-dependent mask defining the volume with emission. These ¹²CO masks were then applied to the fainter lines for creating their integrated intensity images.

Line fluxes reported in this paper include uncertainties that are only based on the thermal fluctuations in the data, representing signal-to-noise considerations; they do not include uncertainties in the absolute flux calibration scale, such as those due to secular variability in the calibrator sources. The flux calibration of ALMA data that have been processed with standard calibration procedures is usually accurate to ≈5% to 10% (e.g., Andrews et al. 2018; Martín et al. 2019; Remijan et al. 2019). Line ratios between data taken in different observing setups and different tunings, such as ¹²CO/¹³CO (see Table 1), have an additional uncertainty due to this absolute flux calibration. However, line ratios between data taken simultaneously using the same setup (e.g., ¹³CO/C¹⁸O or HCN/HCO⁺) do not have this additional uncertainty.

In addition to the spectral lines mentioned above, we also have upper limits on CH₃OH (2_k −1_k ), SiO(2–1; v = 0), and HNCO (4_0,4−3_0,3). Furthermore, there is an unidentified line source in the vicinity of NGC 7465. It seems to be a galaxy at a much higher redshift, and it is described in more detail in Appendix A.

As we have observations at similar angular resolutions covering a large range of frequencies from 86 to 115 GHz (with gaps), the data are suitable for imaging the continuum intensity and estimating the spectral index of any emission. NGC 7465 has two continuum point sources, one in the nucleus and one 5'' south of the nucleus (Figure 2). The nuclear source has a falling spectral index; its flux densities measured at 86.15 and 107.9 GHz are 742 ± 12 μJy and 616 ± 18 μJy, respectively. Parameterizing the flux density as S_ν ∝ ν^α therefore gives α = −0.83 ± 0.13. Similarly, using all of the available line-free frequencies in CASA's multiterm, multifrequency synthesis deconvolver (Rau & Cornwell 2011), we find α = −0.59 ± 0.12. These estimates are consistent with each other, and both clearly indicate synchrotron emission in the nucleus of the galaxy, which is not surprising given the other AGN indicators mentioned in Section 2. For comparison, Domínguez-Fernández et al. (2020) find that at 230 GHz, the nuclear continuum source has a flux density of 900 ± 200 μJy, which is broadly consistent with the flux densities measured here and suggests that in this millimeter regime, the nuclear source's spectrum is flattening as dust emission begins to dominate.

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The fainter continuum source has flux densities of 155 ± 12 μJy and 245 ± 18 μJy at 86.15 and 107.9 GHz, suggesting instead a rising spectral index consistent with dust emission. It appears to be associated with a region of recent star formation activity, as indicated by a bright blue source in the u image, peaks in [O iii] and Hα emission (Ferruit et al. 2000), and a small peak in the molecular surface brightness.

Moustakas & Kennicutt (2006) presented optical spectroscopy and measurements of the nebular emission-line fluxes in NGC 7465. The nebular spectrum (from a region 25 square) has log([N ii]/Hα) = −0.28 ± 0.03 and log([O iii]/Hβ) = 0.44 ± 0.03, suggesting the ionization there is dominated by the AGN (Moustakas et al. 2010). The spectrum integrated over a much larger 50'' × 60'' rectangular region has log([N ii]/Hα) = −0.39 ± 0.03 and log([O iii]/Hβ) = 0.06 ± 0.03, consistent with star formation activity. The ATLAS^3D data provide more spatial information at higher resolution, though only on the [O iii]/Hβ ratio; they show an even higher ratio of log([O iii]/Hβ) = 0.62 in the nucleus, on the location of the nuclear radio synchrotron and hard X-ray sources (Figure 2). Lower ratios of log([O iii]/Hβ) from −0.25 to −0.3 are seen in patches at radii of 3''–10'', and while this single ratio cannot conclusively identify the source of ionization (e.g., Sarzi et al. 2010), all of the data are consistent with the interpretation that the current star formation activity in the interior of the galaxy is particularly concentrated in this annular region.

The nebular emission-line fluxes in Moustakas & Kennicutt (2006) also enable an estimate of the gas metallicity in NGC 7465. We use line fluxes integrated over the large 50'' × 60'' region; following the prescriptions outlined in Moustakas et al. (2010), the emission-line ratios suggest a metallicity consistent with solar. We find 12 + log(O/H) = 8.38 ± 0.06 using the calibration of Pilyugin & Thuan (2005) and ≈9.0 using the calibration of Kobulnicky & Kewley (2004); each of these is consistent with its respective median of the SINGS galaxies (mostly spirals) in Moustakas et al. (2010).

5.1. Large-scale Disk

The ¹²CO emission from NGC 7465 (Figure 1) shows a disky structure consisting of irregular, flocculent spiral arms extending to radii of at least 20''. Some emission is also detected beyond the half-power point of the ALMA primary beam, so it seems likely that there is significant molecular gas beyond the region that we are able to image in these data sets. This irregular disk has an axis ratio near 1, suggesting a low inclination, but its kinematic major axis is strongly twisted and asymmetric as might be expected of recently accreted gas (Figure 3). Although there is a large mismatch between the angular resolution of the H i and CO data, the CO velocity field on large scales is consistent with that of the H i (Li & Seaquist 1994), suggesting that those two gas phases probably have a common origin.

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Section 2 discusses the fact that it is not clear whether NGC 7465 is a face-on barred galaxy or an edge-on axisymmetric galaxy with extraplanar arm/ring structures. In either case, steady-state models are likely to be misleading when trying to interpret this galaxy's kinematics. The interactions with its neighbors mean that its potential is rapidly time-varying. Either these interactions, or the perturbations caused by a bar (if one is present), might explain the noncircular motions and possible gas inflows discussed in this section. In addition, we note that dense molecular gas is found over most of the extent of the elongated structure. In a bar model, this would imply that either dense gas is present beyond the inner Lindblad resonance (ILR), contrary to expectation (e.g., Athanassoula 1992), or alternatively that the radius of the ILR is much larger than is usual in early-type disk galaxies, and it extends to almost the end of the bar and thus corotation. Overall, we therefore argue that it is problematic to interpret the gas distribution and kinematics in NGC 7465 in terms of a relaxed/steady-state early-type barred disk galaxy potential.

5.2. Inner Kiloparsec and Decoupled Core

5.3. Other Molecular Species in the Inner Kiloparsec

The integrated intensity maps of the other molecular lines are presented in Figure 4. To avoid biases related to the different signal-to-noise ratios, a velocity-dependent mask tracing the ¹²CO emission is used for all of the other spectral lines. Their distributions can be broadly classified into two major groups. ¹²CO, HCN, HCO⁺, CN, and C₂H are centrally concentrated and peaked on the nucleus of the galaxy, whereas ¹³CO and C¹⁸O show peaks a couple of arcseconds (≈1 beam) north and east of the nucleus rather than on the nucleus itself. In ¹³CO, for example, the integrated intensity at the position of the nucleus is a factor of 2 lower than farther out along the molecular ridge. CS emission is relatively weak and while it appears to have a peak off the nucleus, the spatially resolved ratios (Section 6) suggest that this appearance may be due to noise in the integrated intensity. Finally, HNCO, CH₃OH, and SiO emission are not detected in these data.

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Position–velocity slices through the data cubes (Figure 5) are consistent with those from Domínguez-Fernández et al. (2020), showing the presence of two kinematic structures in the central molecular gas of NGC 7465. At radii < 13, we find a nuclear ring or bar (or possibly an outflow) that is most prominent in HCO⁺ and CN emission, also visible in HCN and ¹²CO emission, and extremely weak or absent in ¹³CO emission. Exterior to 13 the primary structure is the warped disk, where the ¹³CO emission is at its brightest, and there the ¹³CO emission follows the ¹²CO emission. The appearance of a Keplerian decline in the ¹²CO emission in Figure 5 is an artifact of the twist in the kinematic position angle, so that at large radii, this slice is closer to the kinematic minor axis than the major axis.

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For the bright lines of NGC 7465, we can create line ratio maps from the integrated intensity images, as in Figure 6 for ¹²CO/¹³CO. For the fainter lines, we measure integrated line fluxes and ratios by defining spatial regions (Figure 7), summing the cubes within the spatial regions to produce integrated spectra (e.g., Figure 8), and then summing the spectra over velocity ranges defined by the ¹²CO emission. This procedure ensures that the same data volume is used for both lines of a ratio, even if their signal-to-noise ratios are very different. Spatially resolved line ratios measured in this manner are presented in Figures 9 and 10. Reference column densities for the nuclear spectrum in the optically thin and local thermodynamic equilibrium (LTE) approximations are listed in Table 2 and line fluxes for all of our defined regions are presented in Appendix C.

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Table 2. Line Fluxes and Reference Column Densities in the Nucleus of NGC 7465

Species	Line Flux	$\mathrm{log}N$ (20 K)	$\mathrm{log}N$ (90 K)
	(Jy km s−1)	(cm−2)	(cm−2)
12CO	3.52 (0.04)	17.14	17.69
13CO	0.096 (0.014)	15.65	16.20
C18O	<0.039	<15.26	15.81
HCN	0.132 (0.012)	12.80	13.37
HCO+	0.303 (0.013)	13.39	13.96
CN	0.226 (0.006)	13.61	14.16
CS	0.041 (0.009)	13.02	13.54
C2H	0.091 (0.014)	14.70	15.27
CH3OH	<0.035	<13.95	<14.57
SiO	<0.040	<12.79	<13.33
HNCO	<0.037	<13.49	<14.19

Note. Line fluxes are measured in a circular region 2'' (280 pc) in diameter, centered on the nuclear 3 mm continuum source; column densities are averaged over this region. Statistical uncertainties of the line fluxes are listed in brackets, or the upper limits are quoted as three times the statistical uncertainty of the sum over the adopted velocity range. For ¹³CO, the peak intensity is offset from the nucleus and a 2'' region centered on the peak has a line flux of 0.015 Jy km s⁻¹. Column densities are calculated for two representative temperatures in the optically thin LTE assumption. Current data do not constrain the optical depths of the HCN and HCO⁺ transitions, but CN and C₂H are optically thin (Section 8), and even ¹²CO might be at this position (Section 11.1).

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For an alternate method of exploring spatial variations in line ratios, we also implemented a "shift and stack" technique. In this method, all of the spectra within a given annulus (about the nuclear 3 mm continuum peak) can be aligned in velocity, using the ¹²CO velocity field at each position to determine the velocity shift. Thus, all of the expected signals can be concentrated in a narrow range of frequencies. However, because of the departures from azimuthal symmetry in the center of NGC 7465, ratios produced by this technique have significantly larger uncertainties than those derived from individual regions.

NGC 7465 has a relatively weak ¹³CO emission. Measured ¹²CO(1–0)/¹³CO(1–0) ratios are highest at 39 ± 9 in the nucleus, or 33 ± 5 when averaged over a circle of 1'' radius, decreasing to ≈15 ± 2 at a radius of 1 kpc (see Figures 6 and 9). ⁷ The large ratios in the center of NGC 7465 are higher than those typically found in nearby spirals, which tend to be in the range of 8–20 (especially on scales > 1 kpc; e.g., Crocker et al. 2012; Cao et al. 2017; Cormier et al. 2018; Israel 2020). For comparison, Figure 11 shows typical ¹²CO/¹³CO line ratios for galaxies of various types and illustrates that the high ratios in the nucleus of NGC 7465 are similar to those found in ULIRGs and advanced mergers such as Arp 220 and NGC 2623 (Brown & Wilson 2019). Figure 11 also shows that the range of ¹²CO/¹³CO line ratios exhibited by early-type galaxies is a factor of 10, broader than the range exhibited by any other galaxy type. Other early-type galaxies known to have high ¹²CO/¹³CO ratios include NGC 1266 and UGC 09519 (Crocker et al. 2012); NGC 1266 is remarkable for having a strong AGN-powered molecular outflow (Alatalo et al. 2014), while UGC 09519 has a kinematically misaligned H i disk that suggests it was recently acquired (Serra et al. 2014). NGC 7465 is thus consistent with the pattern that high ¹²CO/¹³CO ratios tend to be associated with major disturbances to the ISM.

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NGC 7465's strong radial gradient in ¹²CO/¹³CO is also uncommon. It amounts to a factor of 2 over an unresolved length scale, such that the ratios at radii <140 pc are at least a factor of 2 higher than those outside and possibly more. For context, the spirals in Paglione et al. (2001), Cao et al. (2017), and Cormier et al. (2018) sometimes have enhancements of ¹³CO in their nuclei and sometimes deficits, but none of them are as extreme as in NGC 7465. Two barred lenticular galaxies are in fact similar to spirals in this respect (Topal et al. 2016), with typical ¹²CO/¹³CO ratios about 5–15, and with the higher ratios in the central kiloparsec or so; they only exhibit modest gradients of a factor of 2 over kiloparsec scales. Measurements at sub-kiloparsec resolution sometimes show spatial gradients of the same magnitude as those in NGC 7465. Meier & Turner (2004) found variations of factors of 5 over ≈200 pc in the center of NGC 6946, but unlike NGC 7465, the spiral shows an enhancement of ¹³CO in its center. The strong deficit of ¹³CO in the center of NGC 7465 is more reminiscent of the nearby early-type galaxy Cen A, where McCoy et al. (2017) find ¹²CO/¹³CO > 20 in the circumnuclear disk at radii <200 pc and ¹²CO/¹³CO reaching nearly to unity farther out in the arms.

Measurements of ¹³CO/C¹⁸O in NGC 7465 are limited to a few spatial regions with detections of C¹⁸O (e.g., Figure 12). In a large spatial region defined by all of the detected ¹²CO emission, we have a tentative detection of C¹⁸O yielding ¹³CO/C¹⁸O = 3.0 ± 1.0. The bright molecular ridge has a ratio of 4.0 ± 1.2. Figure 13 shows that these ratios are similar to those of galaxies of comparable IR luminosity (e.g., Jiménez-Donaire et al. 2017). Further interpretation of the CO isotopologue line ratios is in Section 11.

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Figures 9 and 10 show that, as expected, the emission from HCN, HCO⁺, and C₂H is enhanced relative to ¹²CO and ¹³CO in the center of NGC 7465. This pattern is common in nearby galaxies and is expected to be driven by higher gas densities in the centers (e.g., Jiménez-Donaire et al. 2019). We also observe radial trends in C₂H/HCN and C₂H/HCO⁺, in the sense that C₂H is more strongly enhanced just outside of the nucleus than in the nucleus itself. HCN/HCO⁺ and ¹²CO/CS are effectively constant wherever we can measure them, but CS/HCN increases with radius. All of these results are discussed in greater detail below.

7.1. HCN and HCO⁺

7.2. HCN/HCO⁺

7.3. Physical Properties of the Dense Molecular Gas

The C₂H molecule is regarded as a tracer of photon-dominated or photodissociation regions (PDRs), as its formation is driven by the presence of C⁺, and it has been shown to be brighter on the illuminated sides of molecular clouds (e.g., Meier & Turner 2005, 2012; Meier et al. 2015; García-Burillo et al. 2017). The two main fine-structure blends in our data are N = 1 − 0, $J=\tfrac{3}{2}-\tfrac{1}{2}$, with a rest frequency of 87.3169 GHz, and N = 1 − 0, $J=\tfrac{1}{2}-\tfrac{1}{2}$, at 87.40199 GHz. They are separated by 292 km s⁻¹, which is just greater than the velocity range covered by ¹²CO in NGC 7465, so they are not overlapping. The low-frequency component is detected in NGC 7465 but the high-frequency component is not; as the high-frequency component is a factor of 2.3 fainter in LTE in the optically thin limit, its nondetection is not surprising, but we can rule out line ratios ≈1 (the optically thick limit). Thus, we cannot measure the C₂H optical depth directly, but the data are consistent with the transitions being optically thin. Further, the C₂H line fluxes quoted in this paper refer only to the blend at 87.32 GHz.

The C₂H emission in NGC 7465 is relatively bright, when compared to HCN. We find C₂H/HCN ratios ranging from 0.71 ± 0.13 in the nucleus to 1.0 ± 0.2 in the outer parts of the molecular ridge. These are significantly higher than the corresponding ratios in the central kpc of NGC 253, which range from 0.17 to 0.47 (±15%; Meier et al. 2015). The C₂H/HCN ratios in NGC 7465 are also on the high side when compared to the set compiled by Martín et al. (2014) for active and starbursting galaxies. On the other hand, ratios of C₂H/HCO⁺ in NGC 7465 are very similar to those in NGC 253 and other galaxies. We find 0.31 ± 0.05 to 0.50 ± 0.11 in NGC 7465, whereas Meier et al. find 0.20–0.57 (±15%) in NGC 253 and most of the sample galaxies in Martín et al. (2014) show a similar range. These results suggest that HCN emission in NGC 7465 is relatively faint because of moderate densities in the molecular gas; both C₂H and HCO⁺ are relatively prominent because their critical densities are a factor of 10 lower than that of HCN.

In addition to being bright, the C₂H emission in NGC 7465 is also notable in that the radial trends of C₂H/HCN and C₂H/HCO⁺ in NGC 7465 are opposite to the trends in NGC 253. In the spiral, those ratios peak at the nucleus; in other words, C₂H is more strongly concentrated than HCN and HCO⁺ are. In NGC 7465 it is the opposite—C₂H is less centrally concentrated than HCN and HCO⁺. The radial trend in NGC 7465 thus suggests enhanced C₂H emission just outside of the nucleus, at radii 2''–5'' or 280–700 pc (as far out as we can detect C₂H). If the C⁺ ionization is indeed dominated by star formation activity, this radial trend might be consistent with the ionized gas emission-line ratios in Figure 2 (Section 4): the lowest [O iii]/Hβ ratios (most suggestive of star formation activity) occur slightly outside the nucleus at radii ≈3''–10''. Similarly, from the stellar population analysis of Krajnović et al. (2020), there is a suggestion that the smallest stellar ages occur at a radius of 25 rather than at larger or smaller radii.

As another tracer of C⁺, we note that NGC 7465 is also detected in [C ii] 158 μm emission (Lapham et al. 2017), though the spatial resolution is not good enough to compare to the radial trends in our ALMA data. Many spiral galaxies are known to have significant central deficits of [C ii] emission at radii <1 kpc, and many potential explanations have been proposed (e.g., Smith et al. 2017). Possibly, in some cases, the central [C ii] deficit arises because an AGN's hard radiation field drives the ionization balance toward C²⁺; in other cases, a softer radiation field (caused by old stellar populations) may be driving the ionization balance toward neutral carbon. At present, we simply comment that the central C₂H deficit in NGC 7465 (relative to HCN and HCO⁺) might be consistent with what is known about the ionization states of C in other galaxy nuclei.

The CN(1–0) transition is also a PDR tracer (Boger & Sternberg 2005), though it is expected to be even more strongly enhanced in X-ray-dominated regions (XDRs) and may thus help to distinguish between PDRs and XDRs (Meijerink et al. 2007). It has fine-structure components that appear in these data as blends with rest frequencies of about 113.17 GHz and 113.49 GHz; they are clearly separated in our data and both are clearly detected. The high-frequency blend is the brighter one and there is no evidence that their line ratio in NGC 7465 ever deviates from the theoretical optically thin limit of 2 (e.g., Meier et al. 2015). Thus, the CN emission in NGC 7465 is optically thin. For simplicity, we show only the brighter component in the figures, and the line fluxes we quote refer only to it.

The range of ¹²CO/CN ratios we measure in NGC 7465 is large, from 15.0 ± 0.4 in the nucleus to 93 ± 14 when averaged over the full ¹²CO emission region, and it is consistent with the wide range of ratios measured in other nearby galaxies (Wilson 2018). CN/HCN ratios are consistent with 1 aside from one region, and this again is very typical of local galaxies (Ueda et al. 2017; Cicone et al. 2020).

There is some evidence that CN is even more centrally concentrated in NGC 7465 than the other high-density tracers like HCN. The radial variations of ¹²CO/CN exceed those of all other ¹²CO ratios (Figure 9). The transition in line profile shapes, from double-peaked ¹²CO through flat-topped HCO⁺, to centrally peaked CN (Figures 8 and 5), thus might suggest the presence of a high-density circumnuclear disk at r ≲ 100 pc. CN might be particularly enhanced close to the AGN because of its X-ray sensitivity (Meijerink et al. 2007). Higher-resolution data, especially for CN, would be valuable for testing this scenario. As a caveat, we note that the current data do not show a measurable radial trend in CN/HCN (Figure 10), but such a trend could be obscured by the resolution and sensitivity of our data. We do find a systematic difference in the spatial distribution of the two PDR tracers, C₂H and CN; C₂H is less centrally concentrated than HCN whereas CN is equally or possibly more centrally concentrated than HCN. The differences between these two species might be consistent with an interpretation that C₂H is a better tracer of star formation activity whereas CN is a better tracer of XDRs.

The CS molecule is also a high-density tracer, whose J = 2 − 1 transition has a critical or effective density intermediate between those of HCN and HCO⁺ J = 1−0 (e.g., Leroy et al. 2017). There is also some evidence that CS emission is enhanced in PDRs, where the radiation field is stronger (Lintott et al. 2005; Meier & Turner 2005), which has motivated suggestions that CS emission traces massive star formation even better than HCN emission does (e.g., Bayet et al. 2009; Davis et al. 2013).

NGC 7465 has ¹²CO/CS ratios that are fairly typical compared to those of other nearby galaxies. We find ¹²CO/CS = 62 ± 13 in the nucleus, with variations of no more than 25% in all of the regions where we detect CS. For comparison, the spirals in Gallagher et al. (2018) have typical ratios of ¹²CO/CS ≈ 90 in their central kiloparsec, so NGC 7465 is brighter in CS than these. However, it is faint in CS compared to the other early-type galaxies in Davis et al. (2013), which have ¹²CO/CS in the range of 9–44. The spatially unresolved (IRAM 30 m) ratios in Davis et al. (2013) might also be biased high if the beam-filling factor of ¹²CO is higher than that of CS, so that resolved measurements of CS would probably increase the discrepancy between them and NGC 7465.

CS/HCN, which should provide better insight (than ¹²CO/CS) into the properties of the dense molecular phase, is very similar in NGC 7465 to other nearby galaxies. We find 0.26 ± 0.06 in the nucleus of NGC 7465 and 0.57 ± 0.14 for the larger regions that extend to 0.8 kpc. These ratios are consistent with those in the spirals Maffei 2 and NGC 253 (0.2–1; Meier & Turner 2012; Meier et al. 2015). The spirals in Gallagher et al. (2018) also typically have CS/HCN ≈ 0.1 to 0.3, with localized ratios up to 0.5 in the central 200 pc of NGC 3627 or the arms of NGC 4321. CS/HCN in NGC 7465 is also consistent with the single-dish measurements of some other early-type galaxies in Davis et al. (2013). It is notable that other early-type galaxies in that sample have highly unusual ratios of CS/HCN as large as 3, so that bright CS emission might be a feature of some early-type galaxies.

In terms of radial trends, it is striking that the nucleus of NGC 7465 does not differ from the off-nuclear regions in terms of ¹²CO/CS; in contrast, there are strong radial trends of factors of 2–4 in ¹²CO/HCN, ¹²CO/HCO⁺, ¹²CO/C₂H, and ¹²CO/CN. We also find no radial variation of HCN/HCO⁺ but a factor of 2 in CS/HCN. If the density of the molecular gas were the only factor driving changes in those line ratios, we would expect HCN, HCO⁺, and CS to behave in qualitatively the same fashion as the critical density of CS is intermediate between those of HCO⁺ and HCN. Evidently, then, we require more than simple density variations to explain the behavior of these high-density tracers in NGC 7465.

Davis et al. (2013) have also studied unresolved CS emission in a small sample of early-type galaxies. They compared it to [O iii]/Hβ, which can serve as an indicator of whether the ionization in the optically emitting ionized gas is dominated by AGN activity or star formation (see also Section 4). In their early-type galaxies, the global [O iii]/Hβ ratio is correlated with CS/HCN in the sense that galaxies whose ionization is more dominated by star formation (smaller [O iii]/Hβ; softer radiation fields) have higher CS/HCN. NGC 7465 is consistent with this trend, both in terms of its integrated line ratios and its internal spatially resolved behavior, for which we find higher CS/HCN and lower [O iii]/Hβ (and bluer optical colors) at r ≈ 0.5–1 kpc than toward the nucleus. NGC 7465 thus provides circumstantial evidence supporting the interpretation that CS emission is enhanced by PDR conditions.

For the ionized gas metallicity estimates in Section 4 and the column density estimates in Table 2, NGC 7465 is also consistent with the suggestions of Bayet et al. (2012) and Davis et al. (2013) that CS/HCN could be used as an indicator of the metallicity of the molecular gas. Future work on spatially resolved nebular emission-line data might also be valuable to test whether the trend in Davis et al. (2013) also applies within individual galaxies.

Emission from SiO, CH₃OH, and HNCO is commonly interpreted as tracing shocks in the ISM (e.g., Meier & Turner 2012; Meier et al. 2015), as significant energy input in the form of shocks is required to liberate them from grain surfaces. These molecules are not detected in NGC 7465, with the most stringent limits being ¹²CO/(SiO, CH₃OH, or HNCO) > 83 near the center of NGC 7465. Ratios relative to ¹²CO are not particularly significant in terms of a physical interpretation, but here merely serve as an indication of the relative faintness of the lines compared to detections in other galaxies. For context, in NGC 253, Meier et al. (2015) found spatially resolved ratios of ¹²CO/SiO and ¹²CO/HNCO ≈ 20 to 80; NGC 253 thus has brighter SiO and HNCO than NGC 7465, by at least a factor of 4 relative to ¹²CO. The circumnuclear disk of NGC 1068 has ¹²CO/SiO = 12.5 ± 1.5 (García-Burillo et al. 2010), which again is at least a factor of 7 brighter in SiO than NGC 7465. Finally, Topal et al. (2016) also measured ¹²CO/HNCO = 66 ± 5 and 78 ± 18 in the centers of the lenticulars NGC 4710 and NGC 5866. Thus, these two also have modestly brighter HNCO than NGC 7465.

For CH₃OH, Davis et al. (2013) detected a small sample of early-type galaxies with unresolved measurements of ¹²CO/CH₃OH in the range 14–70. As we measure ¹²CO/CH₃OH > 83, we conclude that NGC 7465 is also relatively faint in CH₃OH emission. Similarly, unresolved measurements of CH₃OH/HCN reach as high as 1.5–2.2 in the early-type galaxies NGC 6014 and NGC 5866 (Davis et al. 2013), whereas the resolved upper limits in NGC 7465 are 0.2–0.8.

In short, SiO, CH₃OH, and HNCO are relatively weak in NGC 7465 compared to other galaxies that have been studied to date. The relatively weak emission from these shock tracers is a bit surprising, given the obvious disturbances in the galaxy (Sections 2 and 5). The molecular and H i disks are strongly warped and their kinematics are significantly misaligned with respect to those of the stars, so it is clear that the gas was recently acquired from an external source. We might have expected stronger emission from shock tracers as the gas disk is still in the process of settling; possibly, the shocks are primarily occurring farther out in the galaxy, where our sensitivity is not currently good enough to detect the shock tracers.

11.1. CO Isotopic Ratios in NGC 7465

Figure 11 shows that the center of NGC 7465 boasts an unusually high ¹²CO/¹³CO line ratio; it is reminiscent of the ratios commonly observed in LIRGs and ULIRGs rather than those in spirals and other early-type galaxies. It also constrains the [¹²CO/¹³CO] abundance in the nucleus of NGC 7465 to be ≥39 ± 9, which is consistent with most measurements in the disk of the Milky Way, in other nearby spirals, and in part of the LMC (e.g., Johansson et al. 1994; Meier et al. 2008; Romano et al. 2019, and references therein). However, it is incompatible with the very low abundance ratios of [¹²C/¹³C] = 9 (±2), 21 (±6), and 24 (±1) estimated in the centers of NGC 4945, NGC 253, and the Milky Way (Langer & Penzias 1990; Martín et al. 2019; Tang et al. 2019). Thus, the chemical enrichment pattern in the central molecular gas of NGC 7465 differs from the pattern in these nearby spirals.

The smaller ¹³C abundance in the center of NGC 7465 is plausibly connected with the clear signs of gas accretion, as the gas currently in the center of NGC 7465 was recently in a different galaxy and possibly in the outskirts of that galaxy. For context, the stellar dynamical analysis of Cappellari et al. (2013) implies a maximum circular rotation speed of 163 km s⁻¹, so the orbital time at the outer edge of the molecular arms in our data (15'' or 2.1 kpc) is 80 Myr. It may have taken a few orbital times for the gas to make its way inward to the center of the galaxy, and this timescale is comparable to or shorter than the timescale for significant ¹³C production (e.g., Romano et al. 2019).

Low inferred optical depths in ¹²CO(1–0) (see below) also imply that both ¹³CO and C¹⁸O are optically thin, and thus, the measurements of [¹³CO/C¹⁸O] in NGC 7465 are 2.98 ± 0.99 and 4.0 ± 1.2. These measurements are made in overlapping regions so they are not independent.

The optical depths of the CO transitions also convey information about the physical properties of the molecular gas, and if the [¹²CO/¹³CO] abundance ratio in the center of NGC 7465 is in the typical range of 40–60, then the ¹²CO emission is unusually optically thin. We thus consider several possible explanations for the unusually high ¹²CO/¹³CO line ratio in the center of NGC 7465 and its strong radial gradient.

(1) Unusual isotopic abundances due to a burst of star formation? Away from the nucleus, the ¹²CO/¹³CO and ¹³CO/C¹⁸O line ratios in NGC 7465 are similar to those in nearby spirals. But a very recent burst of star formation in the center of the galaxy would affect the isotopic abundances, as ¹³C, in particular, comes from low-mass stars with long lifetimes, and a region undergoing a starburst might therefore be deficient in ¹³C. Indeed, this effect probably contributes to the exceptionally high ratios of ¹²CO/¹³CO ≈ 90 in the advanced merger NGC 2623 (Brown & Wilson 2019). In NGC 7465, although there is some current star formation activity in the central kiloparsec (Sections 2 and 4), the star formation rate is modest compared to what we usually think of as starbursts. The luminosity-weighted mean stellar ages are younger in the center of the galaxy (Krajnović et al. 2020) but neither the ages themselves nor the radial gradient in stellar ages are unusual even for early-type galaxies. Ultimately, a full multilevel isotopic abundance study would be necessary to resolve the question, but with current data, the motivation for assuming unusual isotopic abundances in NGC 7465 is not compelling.

(2) Unusual molecular abundances due to fractionation or photodissociation of the rarer isotopes? The photodissociation of CO and H₂ molecules usually proceeds through absorption-driven excitation, meaning that the molecules can shield themselves from photodissociation if they have sufficient column densities to make the relevant UV transitions optically thick. ¹³CO and C¹⁸O molecules should then be found deep within the dark interior of a molecular cloud, in the same way (but more so) that ¹²CO molecules should be found deeper than H₂ and there may be a skin of CO-dark H₂ around a UV-illuminated cloud. However, this isotope-specific photodissociation is probably not the explanation for the high ¹²CO/¹³CO ratio in the center of NGC 7465. The C¹⁸O species should be even less abundant than ¹³CO (e.g., Meier et al. 2008; Romano et al. 2019). Thus, photodissociation effects predict that unusually large ¹²CO/¹³CO line ratios should also be accompanied by unusually large ¹³CO/C¹⁸O ratios, which we do not find in NGC 7465 (Figure 13). We further note that the center of NGC 7465 does not exhibit particularly unusual levels of photodissociation in general, as its [C ii]/CO line ratio is typical for nearby spiral galaxies (Figure 14). Finally, Viti et al. (2020) argue that fractionation is unlikely to be a significant effect for the CO isotopologues in typical conditions, though it may be more important for other molecular species.

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(3) Variations in the physical properties of the gas? The spatial variation of ¹²CO/¹³CO, with higher ratios in the central 100 pc of the galaxy (Figure 6), suggests that the optical depth of both molecular species is lower in the center. For typical abundances and conditions, the optical depth of ¹²CO (1–0) must change by a factor of a few such that it is >1 at 1 kpc and <1, possibly as low as 0.1, at 100 pc. Thus, while the ¹²CO column density estimates in Table 2 are underestimates, they are not as far off as usually assumed, and they are probably less than a factor of 2 too low. Further, the inferred variations of optical depth could be plausibly explained by radial variations of the temperature and/or density of the molecular gas.

Bayet et al. (2013) used the J = 1−0 and 2−1 transitions of ¹²CO and ¹³CO to constrain the densities and temperatures of the molecular gas in 18 early-type galaxies, including NGC 7465. They found the most probable conditions for NGC 7465 to be in the range 10^4.5–10^5.6 cm⁻³ and 80–150 K. Notably, these are the highest temperatures inferred for the sample of 18. These estimates are based on single-dish spectra, so they are spatially unresolved and dominated by the conditions at small radii where the emission is brightest. However, for these densities and temperatures, calculations using the RADEX code (van der Tak et al. 2007) suggest that increases in the temperature of a factor of 3 (e.g., from 30 to 90 K) could account for the larger ¹²CO/¹³CO 1–0 ratio in the central 100 pc of NGC 7465. Density changes could have a similar effect on ¹²CO/¹³CO but would then also produce a factor of at least 2 variation in HCN/HCO⁺, which we have ruled out. Therefore, unless the CO-emitting gas is completely disconnected from the HCN-emitting gas, temperature variations are more likely than density variations to explain the ¹²CO/¹³CO gradient in NGC 7465. On the other hand, Bayet et al. (2013) did not make use of resolved kinematic information or the fact that the molecular line widths all increase dramatically toward the nucleus of NGC 7465, which can also affect the line ratios and is discussed in more detail below.

(4) Lower opacity due to higher line width? It is striking that the highest ¹²CO/¹³CO ratios in NGC 7465 are coincident with the largest line widths and strongest local velocity gradients in the galaxy (Figure 5; Appendix B). The ¹²CO line profiles in the disk at radii of 5''–20'' (0.7–2.8 kpc) have FWHMs of 23.5 ± 5 km s⁻¹, whereas the line profiles in the nucleus are much broader and double peaked (Figure 8). At 08 (110 pc) resolution, the line profile toward the nucleus has an FWHM of 230 km s⁻¹, which is 10 times larger than in the disk of the galaxy. And because the optical depth of a transition depends on the ratio of the total column density to the line width (e.g., Paglione et al. 2001; van der Tak et al. 2007), such a dramatic increase in the line width could produce a corresponding factor of 10 decrease in the optical depth and a rise in the ¹²CO/¹³CO ratio.

With the present data, it is not possible to distinguish whether the ¹²CO optical depths in NGC 7465 are more significantly affected by temperature changes or increases of the line widths. Some of the line profiles in the centers of the lenticulars NGC 4710 and NGC 5866 have similarly large line widths to NGC 7465, but they do not have unusually large ¹²CO/¹³CO ratios (Topal et al. 2016). Thus, it is not yet clear what the difference is between these cases and NGC 7465. Additional data on higher-energy transitions would help distinguish between the possibilities.

For context, some authors have suggested that mechanical feedback from star formation increases local velocity dispersions in molecular gas, produces low-density, diffuse gas, and contributes to increases in ¹²CO/¹³CO ratios near regions of active star formation (e.g., Tan et al. 2011). But in most spirals, the variations of the line widths and ¹²CO/¹³CO have a smaller magnitude than what we find. Conversely, other studies of early-type galaxies have found that dynamically regular, relaxed disks in high-density environments like the Virgo Cluster tend to have low ¹²CO/¹³CO ratios, and those could be due to a combination of relaxed kinematics and ram pressure stripping of low-density gas (e.g., Crocker et al. 2012; Alatalo et al. 2015).

AGN-driven outflows might also disturb their surrounding molecular gas, increasing its line widths and its ¹²CO/¹³CO line ratios (e.g., NGC 1266, Crocker et al. 2012). Further inspection of the ¹²CO/¹³CO ratios in Figure 6 shows that the ratios are high (the optical depths are low) not just toward the AGN but also in a band stretching along the minor axis of the molecular ridge. In combination with the unusually misaligned gas kinematics in the center of the galaxy (Figure 3; Appendix B), this pattern in the ¹²CO/¹³CO ratio image suggests that there might be a modest-velocity ionized gas outflow from the AGN stirring up the gas along the minor axis of the molecular ridge.

11.2. Implications of the CO Isotopic Abundances for Galaxy Evolution

We present spatially resolved molecular line observations of NGC 7465, an unusually gas-rich early-type galaxy that recently acquired ≈10¹⁰ M_⊙ of atomic and molecular gas in an interaction with another galaxy. Its disturbed kinematics indicate that the gas is still in the process of settling and migrating inward. We analyze ALMA observations of ¹²CO (1–0) at 08 resolution (110 pc) plus the 3 mm lines of ¹³CO, C¹⁸O, HCO⁺, HCN, CN, C₂H, and CS at ≈2'' resolution (280 pc) and the continuum emission. Besides NGC 7465, the data reveal an unidentified line source with a relatively bright peak line flux density of 2.5 mJy; it is probably a galaxy at z = 0.2 or 1.4.

We find two 3 mm continuum sources in NGC 7465; the brighter one is a nuclear synchrotron source associated with the AGN that is also detected in low-frequency radio and hard X-ray emission. The fainter one is associated with molecular clouds and star formation activity, traced by local peaks in ¹²CO, ¹³CO, HCN, HCO⁺, and CS (though not CN).

The ¹²CO (1–0) distribution at 08 resolution (110 pc) shows a low-inclination flocculent disk with a strong kinematic position angle twist spiraling inwards to a linear ridge or bar-like feature where the highest velocities are found. In the inner 300 pc of the galaxy, the molecular gas kinematics are misaligned by ≈120° with respect to the large-scale stellar rotation, by ≈100° (in the other direction) with respect to the stellar kinematically decoupled core, and by about 45° with respect to the ionized gas kinematics. We conclude that the prominent stellar kinematically decoupled core did not form out of the molecular gas present now; it must have had its origin in a previous event. Furthermore, the complex misalignments may be signatures of outflows or other noncircular kinematics in ionized gas. Despite the dramatic kinematic misalignments, there is no evidence of enhanced emission from the shock tracers SiO, CH₃OH, or HNCO.

All of the detected molecules are centrally concentrated except for ¹³CO (and possibly C¹⁸O and CS). The distribution of ¹³CO has a prominent central dip of almost a factor of 2 in integrated line intensity, and it peaks at about 300 pc from the nucleus. The central ¹³CO dip produces an unusually large nuclear ¹²CO/¹³CO line ratio, 39 ± 9, at this resolution. This ratio is higher than those found in typical spirals and early-type galaxies; it is comparable to those measured in ULIRGs and z ≈ 2–3 submillimeter galaxies. It also constrains the abundance ratio [¹²CO/¹³CO] ≥ 39 ± 9, which is typical of spiral disks but is higher than sometimes found in spiral nuclei. The isotopic ratios are thus consistent with the gas having been accreted from another spiral galaxy and transported rapidly (faster than the ¹³C enrichment timescale) to the nucleus of NGC 7465. The observed ¹³CO/C¹⁸O line ratio suggests [¹³CO/C¹⁸O] = 4.0 ± 1.2, which is also consistent with a spiral galaxy origin but is significantly higher than corresponding ratios estimated in ULIRGs/submillimeter galaxies and lower than ratios in Local Group dwarfs.

Modest star formation activity is occurring in the center of NGC 7465, but there is no compelling reason to assume the intrinsic [¹²CO/¹³CO] abundance ratio is very different from the range of 40–60 that is usually found in the disks of spirals. In this case, the ¹²CO (1–0) emission from the nucleus of NGC 7465 must be unusually optically thin, perhaps even having τ < 1. Such low optical depths are plausible because of (1) the high CO temperatures, ≈100 K, inferred from single-dish data on multiple J-level transitions, and (2) the very large line width in the center of the galaxy. The nuclear spectrum (r < 140 pc) has an FWHM of 250 km s⁻¹, a factor of 10 larger than elsewhere in the galaxy, due to the strong velocity gradient in the central misaligned molecular structure (possibly an edge-on circumnuclear ring).

HCN emission is relatively faint in NGC 7465, and HCO⁺ is relatively bright, yielding HCN/HCO⁺ ratios in the range of 0.4 to 0.6 everywhere we can measure—these ratios are a factor of 2 lower than typical for spiral galaxies. An assumption of LTE requires an intrinsic abundance ratio [HCN/HCO⁺] < 0.25 but even for densities too low to approach LTE, the data still require [HCN/HCO⁺] < 3, which is smaller than is found in some nearby spirals. We also find (based on previously published nebular line fluxes) roughly solar metallicity in the ionized gas outside of the nucleus, where ionization is not dominated by the AGN. Thus, even though the galaxy's gas-phase metallicity does not suggest an intrinsically low N abundance, the observed HCN/HCO⁺ line ratios do still seem to require relatively low densities in the dense molecular phase (e.g., ${n}_{{{\rm{H}}}_{2}}\leqslant {10}^{4.3}$ cm⁻³) and/or low HCN abundances.

We find no measurable gradient in the HCN/HCO⁺ line ratio on scales of 100 pc to 1 kpc. Thus, even though there is an AGN in NGC 7465, it is not affecting the J = 1–0 line ratios of the dense molecular tracers on those scales (in marked contrast to some other AGNs). On the other hand, the proportion of dense relative to diffuse molecular gas clearly changes with radius, as reflected by ratios like ¹²CO/HCN, ¹²CO/HCO⁺, and ¹²CO/CN. Thus, the physical properties of the densest phase of the molecular gas do not appear to change with radius, even though the properties of the more diffuse phase must be changing to reproduce the ¹²CO/¹³CO variation.

The CN emission from NGC 7465 is optically thin and unremarkable in its intensity. C₂H is relatively bright, with C₂H/HCN ≈ 1. As C₂H and HCO⁺ have quite similar critical densities, we infer that the relative brightness of C₂H is also an indicator of relatively low densities in the "high-density" molecular phase. The intensity and spatial distribution of CS emission in NGC 7465 are consistent with previous suggestions that CS can be enhanced in regions of star formation activity, here traced by low [O iii]/Hβ ratios.

All of the gas-phase data gathered about NGC 7465 to date—the gas content, kinematics, metallicity, and molecular and isotopic abundance patterns—are consistent with the interpretation that it acquired its gas recently from a large spiral galaxy rather than a dwarf galaxy. In the broader context, however, more work is needed on isotopic abundance ratios in early-type galaxies. Some of them are believed to have accreted their gas recently from a dwarf galaxy, while others may have retained small quantities of molecular gas from their previous lives as submillimeter galaxies at z ≈ 2–3, through the universe's peak star formation epoch, all the way down to the present day. More quantitative work is also required on modeling the isotopic abundance evolution of early-type galaxies to test whether the current-day properties of these galaxies can be reproduced and what constraints they may impose on the evolutionary models.

We thank Davor Krajnović for helpful discussions on the stellar kinematics of barred galaxies. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2018.1.01253.S, ADS/JAO.ALMA#2016.1.01119.S, and ADS/JAO.ALMA#2018.1.01599.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Facility: ALMA. -

Software: CASA (Emonts et al. 2019), Astropy (Astropy Collaboration et al. 2013, 2018).

We find strong line emission and 3 mm continuum emission from an unidentified source near (in projection) to NGC 7465. The line is centered at 97.67 GHz, and its width is 0.14 GHz. The source is modestly resolved in these data, and its position is 23^h 02^m 02857, +15° 58' 178 (ICRS). Figure 15 shows its location on a deep optical image from the MATLAS survey (Duc et al. 2015), and Figure 16 shows its spectrum. It also has a 3 mm continuum flux density of (0.11 ± 0.01) mJy/beam and a slight suggestion of rotation along a northwest—southeast axis, but the distance between the image centroids in the extreme channels is only 035, so the rotation is not well resolved in these data.

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Although the source is superposed on the outer parts of NGC 7465, where there are bright blue patches associated with recent star formation activity, there is no optical source coincident with the 3 mm emission. It certainly is not a part of NGC 7465 due to its large line width and the fact that its frequency does not match any known bright line. But as we only have one detected spectral line in the frequencies covered by these data, it is difficult to identify the line. If it is ¹²CO (1–0), it would be at z = 0.18. In this case, ¹³CO(1–0) would be around 93.4 GHz, where we have no coverage, and HCN(1–0) would be beyond the low-frequency end of ALMA Band 3. If the line is ¹²CO(2–1), it would be at z = 1.3604, ¹³CO(2–1) would be at 93.3 GHz, and HCN(3–2) would be at 112.6 GHz. No line is apparent at 112.6 GHz but the data do not rule out typical HCN/¹²CO ratios like those seen in local galaxies. Similarly, if the line is ¹²CO(3–2) at z = 2.5404, HCN(4–3) would be at 100.1 GHz but the limits on a nondetection there are not yet useful. Firm identifications of the redshift in this case will probably require searches for the corresponding higher-J transitions of ¹²CO in the higher ALMA bands or the lower-J transitions at the Jansky Very Large Array.

Table 3 lists estimated line fluxes, luminosities, and masses for the most probable line identifications. Assuming the object is in dynamical equilibrium, so that we can compare its inferred dynamical and molecular masses, it seems more likely that the line is ¹²CO(1–0) or ¹²CO(2–1) than any higher-J transition. The higher-J levels would require the source to be quite close to face-on in order to make the inferred dynamical mass larger than the molecular mass. The line profile is consistent with a rectangular or double-horned shape, suggesting it comes from a rotating disk with gas extending to the flat part of the rotation curve, so the dynamical equilibrium assumption is plausible by this measure.

Table 3. Flux, Luminosity, and Mass Estimates for the Unidentified Source

Line ID	z	Diameter	Line Width	Mdyn	Flux	${L}_{\mathrm{line}}^{{\prime} }$	Mmol	Mmol/Mdyn
		(kpc)	(km s−1)	(M⊙)	(Jy km s−1)	(K km s−1 pc2)	(M⊙)
12CO(1–0)	0.1802	1.06	409	5.2 × 109/${\sin }^{2}i$	0.87	1.4 × 109	4.9 × 109	0.95 ${\sin }^{2}i$
12CO(2–1)	1.3604	2.94	217	4.0 × 109/${\sin }^{2}i$	0.46	1.1 × 1010	4.0 × 1010	10.0 ${\sin }^{2}i$
12CO(3–2)	2.5404	2.81	115	1.1 × 109/${\sin }^{2}i$	0.24	8.1 × 109	2.9 × 1010	27 ${\sin }^{2}i$

Note. Calculations are made assuming a flat universe with H₀ = 70 km s⁻¹ Mpc⁻¹, Ω_m = 0.3, and Ω_Λ = 0.7. The linear diameter is estimated from the emission centroids in the outermost channels. The line luminosity ${L}_{\mathrm{line}}^{{\prime} }$ is calculated as in Carilli & Walter (2013). Estimated luminosity conversions from J = 2–1 or J = 3–2 to J = 1–0 are made using assumed excitations as in Boogaard et al. (2019) and the molecular mass (with He) is then estimated from the inferred ¹²CO(1–0), luminosity using a conversion factor α = 3.6 M_⊙ (K km s⁻¹ pc²)⁻¹ (Boogaard et al. 2019).

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The source is very similar in integrated line flux and width to the typical 3 mm ¹²CO lines detected by ALMA in a blind survey of the Hubble Ultra-Deep Field (HUDF; Walter et al. 2016; González-López et al. 2019). Its continuum is brighter than is typical, as the brightest 3 mm continuum source in the HUDF has a flux density of 46 ± 7 μJy whereas this one has 110 ± 10 μJy. But overall, this source is compatible with being a similar object to those. Most of them are identified as ¹²CO(2–1) at redshifts of 1.0–1.5, based on associations with optical counterparts and occasionally detections of higher-J lines in higher ALMA bands. In fact, all of the ¹²CO detections in the HUDF have optical counterparts (González-López et al. 2019), and they are typically 0.01–1 μJy at an observed wavelength of 1 μm (Boogaard et al. 2019). The ground-based optical data shown in Figure 15 cannot rule out the faintest of those typical optical counterparts.

Figure 17 shows a side-by-side comparison of the inner structure of NGC 7465, in some indicators that might reveal a small AGN-driven ionized outflow. Ferruit et al. (2000) presented narrowband HST [O iii] and Hα imaging, which showed enhanced [O iii]/Hα in an r ≲ 2'' region with a southeast–northwest elongation, consistent with the structure in the ATLAS^3D [O iii]/Hβ data. This region of more strongly ionized gas is expected to trace enhanced shocks and/or harder radiation fields. The velocity of the ionized gas shows a gradient along a similar kinematic axis of roughly −35°, as indicated by the ellipse in Figure 17, and the velocity dispersion in [O iii] is also enhanced near the ends of the major axis of this ellipse. However, the velocity resolution of the [O iii] kinematic data is not good enough to show any signatures of double-peaked line profiles.

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The ¹²CO kinematic and photometric position angles in this region show that the molecular gas predominantly traces a disk or a ring perpendicular to the suggested outflow axis. Thus, enhanced [O iii]/Hβ ratios along that axis might simply reflect lower opacities on that axis and higher opacities in the dusty molecular disk, rather than actual outflow. The enhanced ¹²CO velocity dispersion along the proposed outflow axis is probably then related to the kinematics of the molecular disk rather than the outflow, and enhanced ¹²CO/¹³CO line ratios along the proposed outflow axis might reflect higher temperatures or larger line widths (Section 11.1). In short, the features observed here could be caused by an ionized outflow but are not definitive evidence; higher-resolution spectroscopy targeting the optical nebular lines would be useful in testing for the existence of an outflow.

Table 4 presents all of the line fluxes measured in the regions used for the analysis of spatial variations in the molecular properties of NGC 7465.