# The Evolution of NGC 7465 as Revealed by its Molecular Gas Properties

Lisa M. Young Physics Department, New Mexico Institute of Mining and
Technology, 801 Leroy Place, Socorro, NM 87801, USA  Adjunct Astronomer,
National Radio Astronomy Observatory, Socorro, NM 87801, USA  David S. Meier
Physics Department, New Mexico Institute of Mining and Technology, 801 Leroy
Place, Socorro, NM 87801, USA  Adjunct Astronomer, National Radio Astronomy
Observatory, Socorro, NM 87801, USA  Martin Bureau Sub-department of
Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson
Building, Keble Road, Oxford, OX1 3RH, UK  Alison Crocker Department of
Physics, Reed College, Portland, OR 97202, USA  Timothy A. Davis School of
Physics & Astronomy, Cardiff University, Queens Buildings, The Parade,
Cardiff, CF24 3AA, UK  Selçuk Topal Department of Physics, Van Yüzüncü Yıl
University, Van 65080, Turkey

(Received 26 November 2020; Revised 25 January 2021; Accepted 27 January 2021)

###### Abstract

We present ALMA observations of CO isotopologues and high-density molecular
tracers (HCN, HCO+, CN, etc.) in NGC 7465, an unusually gas-rich early-type
galaxy that acquired its cold gas recently. In the inner 300 pc, the molecular
gas kinematics are misaligned with respect to all other galaxy components; as
the gas works its way inward it is torqued into polar orbits about the stellar
kinematically-decoupled core (KDC), indicating that the stellar KDC is not
related to the current gas accretion event. The galaxy also exhibits unusually
high 12CO/13CO line ratios in its nucleus but typical 13CO/C18O ratios. Our
calculations show that this result does not necessarily indicate an unusual
[12CO/13CO] abundance ratio but rather that 12CO (1-0) is optically thin due
to high temperatures and/or large linewidths associated with the inner
decoupled, misaligned molecular structure. Line ratios of the higher-density
tracers suggest that the densest phase of molecular gas in NGC 7465 has a
lower density than is typical for nearby galaxies, possibly as a result of the
recent gas accretion. All of the observed molecular properties of NGC 7465 are
consistent with it having acquired its molecular (and atomic) gas from a
spiral galaxy. Further detailed studies of the CO isotopologues in other
early-type galaxies would be valuable for investigating the histories of those
that may have acquired their gas from dwarfs. Finally, these ALMA data also
show an unidentified line source that is probably a background galaxy similar
to those found at $z=1-3$ in blind CO surveys.

Early-type galaxies (429) – Interstellar molecules (849) – CO line emission
(262) – Molecular gas (1073) – Galaxy evolution (594)

††facilities: ALMA††software: CASA (Emonts et al., 2019), Astropy (Astropy
Collaboration et al., 2013, 2018)

##  1 Introduction

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 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 HI) is more vulnerable to interactions and will retain
signatures of disruption for many Gyr. But such misalignments are also present
in the inner few kpc 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% to 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. Russell et al., 2019; Davis 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
0.′′8 (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,
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.

##  2 About NGC 7465

NGC 7465 was originally selected because it is a member of the ATLAS3D 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
ATLAS3D sample, so there is extensive information on its 12CO J=1-0 emission.
Its stellar mass is $\log(M_{\star}/M_{\odot})=10.4$ 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 22µm WISE flux, giving 1.37 $\pm\ 0.5$ M⊙ yr-1 and a specific star
formation rate SFR/M⋆ $\approx(5.5\pm 2)\times 10^{-11}$ 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/Mgas) 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 near 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 to 10 kpc (Figure
1). Its HI 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 $\approx$
3′′ to 20′′ ($\approx$ 0.4 to 3 kpc).

Figure 1: Top: color composite of the MATLAS $u,g,$ and $i$ images (Duc et
al., 2015). Two neighboring galaxies are visible off the northwest corner of
the image. Bottom: 12CO contours superposed on a $u-i$ color image (bluer
colors are white). Contour levels are 0.0164 $\times$ (1, 3, 9, 27, 81) Jy
bm-1 km s-1, where the lowest contour is the nominal column density
sensitivity equivalent to a 5$\sigma$ signal in one channel of width 15 km
s-1. The synthesized beam (0.′′8 $\times$ 0.′′7) is shown as a blue ellipse in
the bottom left corner. A primary beam correction has been applied; in both
panels the dotted circle indicates the half power point of the ALMA primary
beam at 114 GHz.

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 $\approx$ 1′ (8.5 kpc) in length. An alternative
interpretation arising from the ATLAS3D dynamical analysis (Cappellari et al.,
2013) is that the inner $r\lesssim 30$′′ 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 iso-velocity 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 30m observations of molecular lines in the ATLAS3D 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 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.

##  3 Observations

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 (December
2018); 12CO, HCN, C2H, and HCO+ in 2016.1.01119.S (November 2016); and 12CO
and CN in 2018.1.01599.S (October 2018). 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 1$\sigma$ 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 which 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 $\approx$ 5% to 10%
(e.g. Martín et al., 2019; Andrews et al., 2018; 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.

Table 1: Fiducial data cube parameters for NGC 7465

Line | Time | Beam size | rms | $\Delta v$
---|---|---|---|---
| (sec) | (′′) | (mJy bm-1) | (km s-1)
12CO(1-0) | 1590 | 1.60$\times$1.18 | 1.59 | 5.1
| 11733 | 0.81$\times$0.74 | 0.53 | 2.6
CN(1-0) | 11733 | 0.84$\times$0.76 | 0.066 | 45.0
13CO(1-0) | 3810 | 1.89$\times$1.81 | 0.38 | 13.3
C18O(1-0) | 3810 | 1.90$\times$1.82 | 0.35 | 13.3
CS(2-1) | 3810 | 2.12$\times$2.04 | 0.29 | 14.9
CH3OH($2_{k}-1_{k}$) | 3810 | 2.18$\times$2.06 | 0.35 | 15.1
HCO+(1-0) | 8890 | 1.70$\times$1.37 | 0.26 | 13.1
HCN(1-0) | 8890 | 1.71$\times$1.38 | 0.26 | 13.2
HNCO $(4_{0,4}-3_{0,3})$ | 8890 | 1.72$\times$1.39 | 0.28 | 13.3
C2H(N=1-0) | 8890 | 1.72$\times$1.38 | 0.29 | 13.4
SiO(2-1, $v=0$) | 8890 | 1.73$\times$1.39 | 0.28 | 13.5

Note. — Beam sizes and rms noise levels refer to images made with “natural”
uv-weighting and the listed channel widths. In the case of the 12CO and CN
observations, the channel widths in the last column are close to the best
velocity resolutions allowed by the data. For the other lines, the listed
channel widths are representative but not the best possible. At a distance of
29.3 Mpc, 1′′ corresponds to 140 pc.

##  4 Radio continuum and ionized gas

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 $\pm$ 12
$\mu$Jy and 616 $\pm$ 18 $\mu$Jy, respectively. Parametrizing the flux density
as $S_{\nu}\propto\nu^{\alpha}$ therefore gives $\alpha=-0.83\pm 0.13.$
Similarly, using all of the available line-free frequencies in CASA’s multi-
term, multi-frequency synthesis deconvolver (Rau & Cornwell, 2011), we find
$\alpha=-0.59\pm 0.12.$ 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 active galactic nucleus (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 $\pm$
200 $\mu$Jy, which is broadly consistent with the flux densities measured here
and suggests that in this mm regime the nuclear source’s spectrum is
flattening as dust emission begins to dominate.

The fainter continuum source has flux densities of 155 $\pm$ 12 $\mu$Jy and
245 $\pm$ 18 $\mu$Jy 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 H$\alpha$ 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 2.′′5 square) has log([N II]/H$\alpha$) = -0.28 $\pm$ 0.03 and log([O
III]/H$\beta$) = 0.44 $\pm$ 0.03, suggesting the ionization there is dominated
by the AGN (Moustakas et al., 2010). The spectrum integrated over a much
larger 50′′ $\times$ 60′′ rectangular region has log([N II]/H$\alpha$) = -0.39
$\pm$ 0.03 and log([O III]/H$\beta$) = 0.06 $\pm$ 0.03, consistent with star
formation activity. The ATLAS3D data provide more spatial information at
higher resolution, though only on the [O III]/H$\beta$ ratio; they show an
even higher ratio of log([O III]/H$\beta$) = 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]/H$\beta$) from $-0.25$ to $-0.3$ are seen in
patches at radii of 3′′ to 10′′, and while this single ratio cannot
conclusively identify the source of ionization (e.g. Sarzi et al., 2010), all
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′′ $\times$ 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 $\pm$ 0.06 using the
calibration of Pilyugin & Thuan (2005) and $\approx$ 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).

Figure 2: The [O III]/H$\beta$ ratio from the ATLAS3D project (Cappellari et
al., 2011). Contours are the 3mm continuum emission, showing the two point
sources. Contour levels are ($-3$, 3, 10, and 25) times the rms noise, which
is 11 $\mu$Jy bm-1; the peak intensity is 590 $\mu$Jy bm-1. The beam size of
the continuum emission is shown as the solid blue ellipse in the lower left
corner.

##  5 Molecular Gas Distribution and Kinematics

###  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 datasets. 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 HI and CO data, the CO velocity field on large scales is
consistent with that of the HI (Li & Seaquist, 1994), suggesting that those
two gas phases probably have a common origin.

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 non-circular 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 kpc and decoupled core

At radii $\lesssim$ 5′′ (where we have most of our molecular line detections)
the ATLAS3D stellar isophotes are flattened and their major axis is aligned to
the kinematically-decoupled stellar core (KDC; Figure 3, center panel). The
KDC is both photometrically and kinematically misaligned by 150° with respect
to stellar structures farther out. In this region the molecular gas forms a
bright ridge about 10′′ long (1.4 kpc), oriented along a northeast-southwest
axis. The ridge is misaligned by about 100° with respect to the stellar KDC
and 120° with respect to the stellar kinematic and photometric axes at larger
radii. The molecular gas shows a strong velocity gradient along the ridge, so
that the most extreme CO velocities are found along the ridge, about 1′′ from
the nucleus as defined by the radio continuum position. 12CO (2-1) emission at
higher resolution (Domínguez-Fernández et al., 2020) shows a patchy spiral
feature that can be followed inward to a radius of 2′′ and a smaller molecular
ridge 2′′ long, with a saddle at the location of the AGN. But the smaller
ridge observed in (2-1) emission maintains the same orientation as the more
extended ridge we observe in the present data; thus, on scales of 1′′ (140 pc)
the molecular ridge is still perpendicular to the stellar KDC.

Davis et al. (2011) discussed the kinematics of ionized and molecular gas in
early-type galaxies, and noted that the two phases agreed in all of the cases
they studied at moderate ($\approx$ 1 kpc) resolution. That is also still true
in NGC 7465 for radii $\gtrsim$ 2′′ (280 pc), but for smaller radii the
kinematics of the ionized and molecular gas diverge. The ionized gas continues
its inward twist such that at small radii the kinematic position angle of the
ionized gas nearly matches that of the stellar KDC but is 45° offset from the
molecular kinematic position angle. The molecular gas, on the other hand,
maintains its fixed orientation nearly perpendicular to the stellar KDC. This
divergence of the kinematics is suggestive of more complex noncircular motions
or perhaps multiple ionization sources in the galaxy’s nucleus, such that the
inner ionized gas is decoupled from the molecular gas. The ionized gas
kinematics in the center of NGC 7465 may even be tracing an AGN-driven ionized
outflow, and Appendix B presents a closer look at that evidence.

The mismatch between molecular and stellar kinematics in the center of NGC
7465 also means that the stellar KDC has not formed out of the molecular gas
currently present in the galaxy. It must represent a previous formation event,
or perhaps a much earlier episode of an extended interaction. More detailed
observations of the stellar populations in the KDC would be required to date
its formation (e.g. Sarzi et al., 2016).

Figure 3: CO, stellar, and ionized-gas velocity fields of NGC 7465. In the
12CO velocity field, the contours show the integrated CO intensity at
0.8′′$\times$ 0.7′′ resolution, with contour levels at 0.03, 0.3, and 1.0 Jy
bm-1 km s-1. The CO beam size is shown as a gray ellipse in the bottom left
corner. These CO data have slightly better angular resolution than the ground-
based optical data, which are from Cappellari et al. (2011). In the stellar
and ionized gas panels, the contours are stellar isophotes.

###  5.3 Other molecular species in the inner kpc

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 ($\approx$ 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 two 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.

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 $<$
1.′′3 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 1.′′3 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.

Figure 4: Integrated intensity images of multiple molecular species in the
inner few kpc of NGC 7465. The position of the nuclear 3mm continuum source is
marked with a cross, to guide the eye. These integrated intensities are
computed using a velocity-dependent mask that follows the rotation of the gas,
as defined by 12CO emission. Each line’s synthesized beam is indicated by the
ellipse in the lower right corner, and the colors are scaled to the minimum
and maximum of each image. Integrated line fluxes towards the nucleus are
given in Table 2. For the fainter lines, a dotted white contour indicates
pixels where the integrated intensity has a signal-to-noise ratio $>2.5.$ The
corresponding 12CO contour is mostly outside this field of view. Figure 5:
Position-velocity slices at a position angle of +42°, one beam wide. The
slices cut through the nucleus and follow the molecular ridge, tracking the
kinematic major axis at small radii (see Figure 3). Velocities are measured
with respect to the systemic velocity, 1961 km s-1. Dotted lines at $\pm$
1.′′3 demarcate the inner kinematic component with the strong velocity
gradient. 12CO is plotted both at the highest available resolution and at a
resolution matching the 13CO data.

##  6 Line strengths: CO isotopologues

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.

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 3mm 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 signal 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.

Figure 6: 12CO/13CO integrated intensity ratio (computed in units of
brightness temperature). Black contours show the 3mm continuum emission, and
the angular resolution of the 12CO/13CO ratio image is shown by the gray
ellipse in the lower left corner.

Figure 7: Integrated intensities of 12CO (top) and 13CO (bottom). The angular
resolution in each panel is shown with a gray ellipse in the lower left
corner. Black and white polygons show the regions used for summing spectra, to
investigate the spatial variations of the line ratios. The boundary of the
nuclear region is dotted. A few white pixels in the lower portion of the top
panel are locations where 12CO is not detected at this angular resolution.
Numbers in the lower panel identify the regions for reference to the table in
Appendix C.  Figure 8: Spectra extracted from the nuclear region, which is 2′′
(1 beam) in diameter and centered on the 3mm continuum point source. The grey
boxes mark the velocity range used for integrating at this position. In the
C2H panel, the two dotted lines mark the centers of the two main fine
structure blends and the velocity scale is calculated for the low frequency
($J=\frac{3}{2}-\frac{1}{2}$) blend. These nuclear spectra exhibit unusually
weak 13CO emission.  Figure 9: Line ratios are integrated over the regions
shown in Figure 7; the horizontal error bars indicate the range of radii
probed by each region. Ratios measured towards the nucleus are plotted as
stars, and those measured over the entire region with detected 12CO emission
are plotted at 10′′ and circled. The region with relatively low 12CO/HCN at
5.′′3 is centered on the fainter 3mm continuum source. For purposes of
clarity, only the detections ($>3\sigma$) are plotted. Figure 10: Similar to
Figure 9, for different ratios; symbols have the same meanings. The radial
coordinates for some ratios have been slightly shifted for better visibility.
Table 2: Line fluxes and reference column densities in the nucleus of NGC 7465

Species | Line flux |  $\log N$ (20 K) |  $\log N$ (90 K)
---|---|---|---
| (Jy km s-1) | (cm-2) | (cm-2)
12CO |  003.52 (0.04) | 17.14 | 17.69
13CO |  000.096 (0.014) | 15.65 | 16.20
C18O |  $<$ 0.039 |  $<$ 15.26 | 15.81
HCN |  000.132 (0.012) | 12.80 | 13.37
HCO+ |  000.303 (0.013) | 13.39 | 13.96
CN |  000.226 (0.006) | 13.61 | 14.16
CS |  000.041 (0.009) | 13.02 | 13.54
C2H |  000.091 (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 3mm 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 3 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).

NGC 7465 has relatively weak 13CO emission. Measured 12CO(1-0)/13CO(1-0)
ratios are highest at 39 $\pm$ 9 in the nucleus, or 33 $\pm$ 5 when averaged
over a circle of 1′′ radius, decreasing to $\approx$ 15 $\pm$ 2 at a radius of
1 kpc (see Figures 6 and 9).111All line ratios reported in this paper are
computed using integrated fluxes expressed in temperature units (K km s-1).
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 HI 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.

NGC 7465’s strong radial gradient in 12CO/13CO is also uncommon. It amounts to
a factor of two over an unresolved length scale, such that the ratios at radii
$<$ 140 pc are at least a factor of two 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 to 15, and with
the higher ratios in the central kpc or so; they only exhibit modest gradients
of a factor of two over kpc scales. Measurements at sub-kpc resolution
sometimes show spatial gradients of the same magnitude as those in NGC 7465.
Meier & Turner (2004) found variations of factors of 5 over $\approx$ 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.

Figure 11: A compilation of observed 12CO/13CO line ratios, in the same spirit
as Figure 1 of Zhang et al. (2018). Symbols in grey are dwarf galaxies,
spirals, and ULIRGs; at the highest luminosities they are sub-mm galaxies at
$z\approx 2-3.$ Data are drawn from Davis (2014), Jiménez-Donaire et al.
(2017), Brown & Wilson (2019), Cormier et al. (2018), Henkel et al. (2014),
Braine et al. (2017), and Heikkilä et al. (1999); see also the citations in
Zhang et al. (2018). The large light error bars for the SMC and the LMC show
the full range of ratios measured by Israel et al. (2003) at 12 pc resolution.
Open grey circles are stacks of multiple galaxies (Méndez-Hernández et al.,
2020). We use an improved FIR luminosity for IC 10 from Rémy-Ruyer et al.
(2015). Line ratios of early-type galaxies (orange symbols) are compiled from
Crocker et al. (2012) and Alatalo et al. (2015); Cen A is from McCoy et al.
(2017). For NGC 7465 and Cen A (green and purple, respectively), the error
bars show the full range of ratios measured in the galaxy.

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 $\pm$ 1.0. The bright molecular ridge has a ratio of
4.0 $\pm$ 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.

Figure 12: 13CO and C18O in NGC 7465. These spectra are from a region at the
northeast end of the molecular ridge, where the 13CO emission is brightest.
Gaussian fits are overlaid, though our analysis uses line ratios derived from
simple sums rather than from the fits. Since the C18O line is so faint, its
width and center velocity have been constrained to match those of 13CO in the
fits.  Figure 13: A compilation of observed 13CO/C18O line ratios, after
Figure 1 of Zhang et al. (2018). For NGC 4526 (Young et al. in prep) and Cen A
(McCoy et al., 2017), the error bars show the full range of ratios measured in
the galaxy, with the symbol at the median ratio. In addition to the citations
noted in Figure 11 and Zhang et al. (2018), this figure includes measurements
by Johansson et al. (1994) and Wang et al. (2009).

##  7 High-Density tracers

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+

Relative to 12CO, NGC 7465 has fairly typical HCN brightness compared to other
nearby galaxies. Specifically, 12CO/HCN in Figure 9 ranges from 16 $\pm$ 1 to
50 $\pm$ 15\. These ratios are very similar to their analogs measured in the
nearby spirals of the EMPIRE survey by Jiménez-Donaire et al. (2019) and in
two other lenticulars by Topal et al. (2016). The EMPIRE data have somewhat
lower linear resolution (1 to 2 kpc) than our measurements in NGC 7465 (240
pc) but in all of these cases the disks are well resolved.

In contrast, the HCN in NGC 7465 is relatively faint compared to that in a
larger sample of early-type galaxies; in the unresolved single-dish
measurements of Crocker et al. (2012), NGC 7465 has the highest 12CO/HCN ratio
of the sample. The unresolved measurements undoubtedly fold in some radial
gradients, so it is difficult to make detailed comparisons at different scales
and it would be worthwhile to obtain additional resolved 12CO/HCN measurements
in early-type galaxies.

The large difference in critical or effective density between CO and HCN
implies that observed ratios like 12CO/HCN encode information about the
density distribution of the molecular gas, or equivalently the relative
proportions of low-density and high-density gas (e.g. $10^{2}$ – $10^{3}$ cm-3
vs. $10^{5}$ – 106 cm-3; Meier & Turner, 2012; Leroy et al., 2017). Thus, NGC
7465 has a mix of low/high-density gas which is typical of most nearby
spirals, but it is relatively deficient in high-density gas compared to many
other local early-type galaxies. The proportion of high-density gas is
maximized in the nucleus but is also high towards the southern continuum
source, where we find the second-lowest 12CO/HCN and 12CO/HCO+ ratios in the
galaxy as well as some of the lowest reliably-measured 12CO/13CO ratios. This
region must contain a concentration of dense gas with relatively high CO
optical depth.

Relative to 12CO, NGC 7465 is exceptionally bright in HCO+ emission when
compared to nearby galaxies. We measure local 12CO/HCO+ ratios from $6.9\pm
0.3$ in the nucleus to $20.7\pm 2.9$ in the disk. In comparison, the spiral
galaxies of the EMPIRE survey usually have 12CO/HCO+ $>25$ even in their
nuclei (Jiménez-Donaire et al., 2019). Thus, the regions of NGC 7465 with the
faintest HCO+, at radii $\approx 1$ kpc, still have brighter HCO+ than is
common even in the nuclei of the spirals. It is also notable that the HCO+
emission in the nucleus of NGC 7465 has a dramatically different line shape
than 12CO (Figure 8), which probably reflects differing spatial distributions.

###  7.2 HCN/HCO+

Because HCO+ is so bright, the HCN/HCO+ ratios in NGC 7465 are low; they are
significantly $<1$ everywhere we are able to measure them, and this is unusual
for nearby galaxies. We measure HCN/HCO+ = 0.44 $\pm$ 0.04 in the nucleus,
with ratios elsewhere ranging up to 0.60 $\pm$ 0.11. In nearby galaxies it is
much more common to find HCN/HCO+ $>$ 1 (e.g. Jiménez-Donaire et al., 2019;
Topal et al., 2016). Ratios $<$ 1 are seen in restricted parts of a few
spirals (e.g. parts of NGC 4254 and Maffei 2; Meier & Turner, 2012; Gallagher
et al., 2018). Low ratios of HCN/HCO+ $\approx$ 0.5 are occasionally seen in
other early-type galaxies (Crocker et al., 2012) and more commonly in ULIRGs
and starburst galaxies (e.g. Baan et al., 2008; Krips et al., 2008; Privon et
al., 2015; Sliwa & Downes, 2017). Extremely low HCN/HCO+ ratios $\lesssim$ 0.2
have been measured in dwarf galaxies like the LMC and IC 10 (Seale et al.,
2012; Braine et al., 2017; Kepley et al., 2018; Anderson et al., 2014). In
this respect, NGC 7465 has more in common with starburst or dwarf galaxies
than with nearby spirals. The low HCN/HCO+ and HCN/CO ratios in NGC 7465 also
place it firmly amongst the nuclear starburst galaxies and “composite”
Seyferts in the HCN(1-0) diagnostic diagram of Kohno et al. (2001).

Spatial variations in the HCN/HCO+ ratios of NGC 7465 do not significantly
exceed the ratios’ mutual statistical uncertainties, so there is no compelling
evidence for a radial gradient (Figure 10). Even though there is an AGN in NGC
7465, there is definitely no enhancement in HCN/HCO+ towards the nucleus,
unless it is on length scales of tens of pc where it would be obscured by our
spatial resolution. There is certainly nothing like the factor of 2 rise in
HCN/HCO+ seen in the central 0.5 kpc of NGC 1068 (Viti et al., 2014).

###  7.3 Physical properties of the dense molecular gas

Due to the difference in critical densities, the HCN/HCO+ line ratio is most
commonly interpreted as an indicator of density in the high-density ($n\sim
10^{4}-10^{6}$ cm-3) portion of the molecular gas. However, for the lowest
HCN/HCO+ line ratios in the LMC, Anderson et al. (2014) have appealed to a
combination of low density and low gas-phase metallicity, with low metallicity
contributing in two ways. First, lower heavy element abundances will produce
lower column densities of these species and smaller optical depths in their
transitions, which is necessary for the line ratio to deviate from 1. On top
of that, lower metallicities are associated with lower [N/O] abundance ratios
(van Zee & Haynes, 2006), which might manifest as lower abundances of HCN
relative to HCO+ (Braine et al., 2017).

NGC 7465 does not have a low metallicity (Section 4), so there is no reason to
assume a low [N/O] ratio a priori. However, the observed line ratios in NGC
7465 do constrain the abundance ratio [HCN/HCO+] to be significantly smaller
than that inferred for NGC 253 by Meier et al. (2015), which was [HCN/HCO+]
$\approx$ 5\. In LTE, reproducing our observed line ratios of HCN/HCO+ = 0.44
$\pm$ 0.04 requires an abundance ratio [HCN/HCO+] $\leq 0.25$ (Table 2).222We
adopt the convention that ratios written without brackets (e.g. HCN/HCO+)
refer to measured line ratios whereas ratios inside square brackets (e.g.
[HCN/HCO+]) refer to estimated molecular abundances after correcting for
optical depth and excitation effects. In the more likely situation that LTE
does not apply, we make calculations using the RADEX large velocity gradient
code (van der Tak et al., 2007) to account for optical depth and excitation
effects on the HCN/HCO+ line ratio; broad ranges of parameters will reproduce
the data from NGC 7465 but they always require an abundance ratio [HCN/HCO+]
$\lesssim 3,$ and a ratio of 5 is ruled out as it cannot reproduce the line
ratios in NGC 7465. For an assumed [HCN/HCO+] = 3, relatively low densities
$n_{\rm H_{2}}\leq 10^{4.3}$ cm-3 are required to reproduce the observed line
ratios. Thus, even though the metallicity of NGC 7465 is not low enough to
suggest unusually low N abundance, the observed HCN/HCO+ line ratios in NGC
7465 are best explained with a combination of relatively low-density gas
and/or a relatively low HCN abundance (or enhanced HCO+). More specific
estimates of the density and temperature will require additional transitions
or isotopologues, but the inference of a low HCN abundance is robust to other
variations in physical properties.

Viewing the molecular gas as two “phases” – a more diffuse phase with
densities $\sim 10^{2}$ to $10^{3}$ cm-3 traced by CO, and a denser phase
$\gtrsim 10^{5}$ cm-3 traced by HCN and similar species – the strong spatial
trends in diffuse/dense gas tracers suggest that the relative proportions of
the two phases change with radius in NGC 7465. However, the internal
properties of the denser phase do not seem to change, even though the
properties in the more diffuse phase are dramatically varying (Section 11.1).

##  8 C2H and CN: photodissociation regions

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
$N=1-0$, $J=\frac{3}{2}-\frac{1}{2}$, with a rest frequency of 87.3169 GHz,
and $N=1-0$, $J=\frac{1}{2}-\frac{1}{2}$, 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
$\approx$ 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 $\pm$ 0.13 in the nucleus to 1.0 $\pm$
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 ($\pm$ 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 $\pm$ 0.05 to 0.50 $\pm$ 0.11 in NGC 7465, where Meier et al.
find 0.20 to 0.57 ($\pm$ 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′′ to 5′′ or 280 to 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]/H$\beta$ ratios (most
suggestive of star formation activity) occur slightly outside the nucleus at
radii $\approx$ 3′′ to 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 2.′′5 rather than at larger or smaller radii.

As another tracer of C+, we note that NGC 7465 is also detected in [C II]
158µm 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
towards C2+; in other cases, a softer radiation field (caused by old stellar
populations) may be driving the ionization balance towards 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 $15.0\pm
0.4$ in the nucleus to $93\pm 14$ 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), might thus suggest the presence of a high-density
circumnuclear disk at $r\lesssim 100$ 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.

##  9 CS

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 $\pm$ 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
$\approx$ 90 in their central kpc, 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 to 44. The spatially-
unresolved (IRAM 30m) ratios in Davis et al. (2013) might also be biased high
if the beam-filling factor of 12CO is higher than 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 $\pm$ 0.06 in the nucleus of NGC 7465 and 0.57 $\pm$
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 to 1; Meier &
Turner, 2012; Meier et al., 2015). The spirals in Gallagher et al. (2018) also
typically have CS/HCN $\approx$ 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 others of the 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 two 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]/H$\beta$, 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]/H$\beta$ ratio is correlated
with CS/HCN in the sense that galaxies whose ionization is more dominated by
star formation (smaller [O III]/H$\beta$; 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]/H$\beta$ (and bluer optical colors) at
$r\approx$ 0.5 to 1 kpc than towards 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.

##  10 Shock tracers

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 $\approx$ 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 $\pm$ 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 $\pm$ 5 and 78
$\pm$ 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 to 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 to 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 to 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 HI 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 Discussion: constraints on abundances and galaxy evolution

###  11.1 CO isotopic ratios in NGC 7465

Figure 11 shows that the center of NGC 7465 boasts an unsually 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 $\geq$
39 $\pm$ 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 ($\pm$ 2), 21 ($\pm$ 6), and 24 ($\pm$ 1) estimated in the centers of NGC
4945, NGC 253, and the Milky Way (Langer & Penzias, 1990; Tang et al., 2019;
Martín 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 $\pm$ 0.99 and 4.0 $\pm$ 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
$\approx$ 90 in the advanced merger NGC 2623 (Brown & Wilson, 2019). In NGC
7465, although there is some current star formation activity in the central
kpc (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 multi-level 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.

Figure 14: Matched-resolution [C II] and 12CO intensities for early-type
galaxies (circles; Lapham et al., 2017; Werner et al., 2014; Wilson et al.,
2013; Mittal et al., 2011, 2012). For comparison, the greyscale shows a two-
dimensional histogram of the same lines in the spiral galaxies of the KINGFISH
and HERACLES surveys (Kennicutt et al., 2011; Leroy et al., 2009). The angular
resolution of all these data is 12′′, which is $\lesssim$ 500 pc in the
spirals but $\approx$ 2 kpc in the early-type galaxies.

(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 $\mathrm{J}=1-0$ 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 $10^{4.5}$ to $10^{5.6}$ cm-3 and
80 to 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 K 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 towards 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 linewidths and
strongest local velocity gradients in the galaxy (Figure 5; Appendix B). The
12CO line profiles in the disk at radii of 5′′ to 20′′ (0.7 to 2.8 kpc) have
full widths at half maximum (FWHM) of 23.5 $\pm$ 5 km s-1, whereas the line
profiles in the nucleus are much broader and double-peaked (Figure 8). At
0.′′8 (110 pc) resolution, the line profile toward the nucleus has a FWHM of
230 km s-1; that is 10 times larger than in the disk of the galaxy. And since
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
linewidths to NGC 7465 but they do not have unusually large 12CO/13CO ratios
(Topal et al., 2016). Thus, it’s 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 linewidths and 12CO/13CO have smaller magnitude than 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 linewidths 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 towards
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

Comparisons of the stellar and gas kinematics in early-type galaxies provide
clear evidence that much – perhaps as much as half – of the gas in early-type
galaxies has been acquired from some outside source, after the main stellar
body was in place (Sarzi et al., 2006; Davis et al., 2011; Bryant et al.,
2019). Additional detailed work on the gas/dust ratios and associations with
morphological disturbances suggests that most of the “outside sources” are
minor mergers with mass ratios $\gtrsim 10:1$ (Kaviraj et al., 2012; Davis et
al., 2015). In some cases there are also additional clues about the origin of
the gas from similarities or differences between the metallicities of the gas
and the stars; for gas accreted in a minor merger, one might expect the
metallicity of the accreted gas to be lower than the metallicity of the bulk
of the stars (Griffith et al., 2019; Davis & Young, 2019).

NGC 7465 clearly did acquire its cold gas from an external source, given the
kinematic mismatches between the CO, HI, and the stars. But the gas probably
did not come from a dwarf galaxy in a minor merger. The large HI content
(9.5$\times 10^{9}$ M⊙; Serra et al., 2012) is most consistent with the gas
coming from a large spiral galaxy. Indeed, there are several other similarly
gas-rich galaxies still in its group (Li & Seaquist, 1994; Serra et al.,
2012). Furthermore, the metallicity of the ionized gas in NGC 7465 is
consistent with that of a spiral rather than a dwarf (Section 4). And finally,
the inferred CO isotopic ratios in NGC 7465 are also consistent with it having
captured its molecular gas from a spiral. Its 13CO/C18O ratios are entirely
unremarkable for spirals; its nuclear 12CO/13CO line ratio is a factor of 2 to
3 higher than is common in spirals, but the measurement can be plausibly
attributed to high temperatures and/or large linewidths rather than to
abundance variations.

For other early-type galaxies with smaller gas masses, CO isotopic ratios may
serve as an important complement to metallicity for identifying the original
host of their external-source cold gas. Or, indeed, for early-type galaxies
with relaxed prograde gas, these indicators should help identify externally-
sourced cold gas even when the kinematics are inconclusive. As 13C is a slow-
release element with significant production in low- to intermediate-mass
stars, it may retain signatures of an external origin longer than 12C or 18O,
and possibly also longer than kinematic disturbances (e.g. if the gas is now
located in the interior of the galaxy where dynamical timescales are short).

Figures 11 and 13, which are motivated by the discussions in Taniguchi &
Ohyama (1998), Zhang et al. (2018), and Méndez-Hernández et al. (2020),
illustrate that ULIRGs and sub-mm galaxies tend to have high 12CO/13CO line
ratios $(\gtrsim 20)$ and low 13CO/C18O ratios $(\lesssim 2)$. In fact, there
also appears to be a strong correlation between these line ratios and the
total FIR luminosity of a galaxy. The cause of such a correlation is not
obvious; presumably the total FIR luminosity of a galaxy is a proxy for other
properties that are more fundamental. Zhang et al. (2018) argued it is a proxy
for the IMF, though Romano et al. (2019) cast doubt on that interpretation
given the highly uncertain production rates of 18O.

In any case, a thorough understanding of the chemical evolution of galaxies
requires that we should be able to connect the sub-mm galaxies at $z\approx
2-3,$ at the highest luminosities in Figures 11 and 13, to their descendants
in the local universe. It is usually assumed that the sub-mm galaxies have
actual abundance ratios [13CO/C18O] $\approx$ 1 to 2, just like their observed
line ratios, because their high 12CO/13CO line ratios indicate that 13CO and
C18O should be optically thin (e.g. Danielson et al., 2013; Sliwa et al.,
2017). Over time the 13C abundance should increase due to its production in
low- to intermediate-mass stars. And as the sub-mm galaxies already have
stellar masses typically $10^{10}$ to $10^{11.5}$ M⊙ at $z\approx 2-3$
(Boogaard et al., 2019), their modern-day descendants will be amongst the most
massive local galaxies. But large galaxies in the local universe tend to have
higher ratios of [13CO/C18O] or [13C/12C][16O/18O] around 5 to 6 (e.g. Martín
et al., 2019; Harada et al., 2018; Romano et al., 2019). Whether the chemical
evolution models can reproduce these kinds of variations on the appropriate
timescales depends a great deal on their assumptions, particularly involving
the production of the rare isotopes (Romano et al., 2019). Further isotopic
observations of massive early-type galaxies, particularly those that might
have retained their molecular gas through the cosmic star formation peak and
through their transition to the red sequence, would be useful for testing the
chemical evolution models.

On a related note, Bayet et al. (2012) and Davis et al. (2013) have argued
that the [HCN/CO] abundance ratio might be a useful indicator of
$\alpha$-element enhancement in molecular gas. As this $\alpha$ enhancement is
common in the stellar populations of early-type galaxies, studies of [HCN/CO]
and [HCN/CS] abundance ratios (Section 9) could provide additional chemical
clues to the evolution of early-type galaxies and the origin of their gas.

##  12 Summary

We present spatially-resolved molecular line observations of NGC 7465, an
unusually gas-rich early-type galaxy that recently acquired $\approx 10^{10}$
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 0.′′8
resolution (110 pc) plus the 3mm lines of 13CO, C18O, HCO+, HCN, CN, C2H, and
CS at $\approx$ 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 3mm 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 0.′′8 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 $\approx$ 120° with respect to the large-scale
stellar rotation, by $\approx$ 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 non-circular 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 two 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, $39\pm 9$ 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 $z\approx 2-3$ sub-mm galaxies. It also
constrains the abundance ratio [12CO/13CO] $\geq 39\pm 9,$ 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 $\pm$ 1.2, which is also consistent with a spiral
galaxy origin but is significantly higher than corresponding ratios estimated
in ULIRGs/sub-mm 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 $\tau<1.$ Such low
optical depths are plausible because of (1) the high CO temperatures $\approx
100$ K inferred from single-dish data on multiple J-level transitions, and (2)
the very large linewidth in the center of the galaxy. The nuclear spectrum
($r<140$ pc) has a 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 two 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. $n_{\rm H_{2}}\leq 10^{4.3}$ 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 AGN). On the other hand, the proportion of dense
relative to diffuse molecular gas is clearly changing 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 $\approx$ 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]/H$\beta$ 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 sub-mm galaxies at $z\approx 2-3,$ 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.

##  Appendix A A distant, unidentified source near NGC 7465

We find strong line emission and 3mm 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 23h 02m 02.s857, $+$15° 58′ 17.′′8 (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 3mm continuum flux density of
(0.11 $\pm$ 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 0.′′35 so the rotation is not well resolved in
these data.

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 3mm emission. It certainly is not a
part of NGC 7465 due to its large linewidth 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: Flux, luminosity and mass estimates for the unidentified source

Line ID | $z$ | Diameter | Line width | Mdyn | Flux | $L^{\prime}_{\mathrm{line}}$ | 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$\times 10^{9}$/$\sin^{2}i$ | 0.87 | 1.4$\times 10^{9}$ | 4.9$\times 10^{9}$ | 0.95 $\sin^{2}i$
12CO(2-1) | 1.3604 | 2.94 | 217 | 4.0$\times 10^{9}$/$\sin^{2}i$ | 0.46 | 1.1$\times 10^{10}$ | 4.0$\times 10^{10}$ | 10.0 $\sin^{2}i$
12CO(3-2) | 2.5404 | 2.81 | 115 | 1.1$\times 10^{9}$/$\sin^{2}i$ | 0.24 | 8.1$\times 10^{9}$ | 2.9$\times 10^{10}$ | 27 $\sin^{2}i$

Note. — Calculations are made assuming a flat universe with H0 = 70 km s-1
Mpc-1, $\Omega_{m}=0.3$, and $\Omega_{\Lambda}=0.7$. The linear diameter is
estimated from the emission centroids in the outermost channels. The line
luminosity $L^{\prime}_{\mathrm{line}}$ 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 $\alpha=3.6$ M⊙ (K km s-1 pc2)-1 (Boogaard et al., 2019).

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.

The source is very similar in integrated line flux and width to the typical
3mm 12CO lines detected by ALMA in a blind survey of the Hubble Ultra-Deep
Field (HUDF; González-López et al., 2019; Walter et al., 2016). Its continuum
is brighter than is typical, as the brightest 3mm continuum source in the HUDF
has a flux density of 46 $\pm$ 7 $\mu$Jy whereas this one has 110 $\pm$ 10
$\mu$Jy. 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 to 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 to 1 $\mu$Jy at an observed wavelength of 1 $\mu$m (Boogaard et
al., 2019). The ground-based optical data shown in Figure 15 cannot rule out
the faintest of those typical optical counterparts.

Figure 15: Contours show the line emission from the unidentified source;
levels are $(0.2,0.5,0.8)\times 3.12\times 10^{5}$ (Jy bm-1) Hz. The
background is the color composite of the MATLAS $u$, $g,$ and $i$ images (Duc
et al., 2015).  Figure 16: 3mm spectrum of the unidentified source. Primary
beam corrections have been applied.

##  Appendix B Possible evidence for an AGN-driven ionized outflow

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 H$\alpha$ imaging
which showed enhanced [O III]/H$\alpha$ in a $r\lesssim 2$′′ region with a
southeast-northwest elongation, consistent with the structure in the ATLAS3D
[O III]/H$\beta$ 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.

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]/H$\beta$ 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 linewidths (Section 11.1). In short, the features
observed here could be caused by an ionized outflow but are not definitive
evidence; higher resolution spectrosocopy targeting the optical nebular lines
would be useful in testing for the existence of an outflow.

Figure 17: The inner structure of NGC 7465. The velocities and line ratios are
the same data previously shown in Figures 2, 3, and 6; the panels on the right
side also show the velocity dispersions in 12CO and [O III]. Beam sizes for
the ALMA data are shown as ellipses in the lower right corners. The cross
marks the location of the nuclear 3mm continuum source, and the dashed ellipse
(6′′ $\times$ 4′′, 850 $\times$ 570 pc) roughly indicates the region where
possible outflow signatures can be seen in the optical data.

##  Appendix C Line fluxes in NGC 7465.

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

=5.5cm

Table 4: Line fluxes in the defined regions of NGC 7465

Rgn | Dist. | Area |  12CO |  13CO | C18O | HCN | HCO+ | CN | CS | C2H | CH3OH | SiO | HNCO
---|---|---|---|---|---|---|---|---|---|---|---|---|---
ID | (′′) | ($\square$′′) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1) | (Jy km s-1)
1 | 0.5 | 3.1 | 3.515 (0.040) | 0.096 (0.013) |  $<$ 0.039 | 0.132 (0.012) | 0.303 (0.013) | 0.226 (0.006) | 0.041 (0.009) | 0.091 (0.014) |  $<$ 0.035 |  $<$ 0.040 |  $<$ 0.037
2 | 2.3 | 8.8 | 6.877 (0.069) | 0.325 (0.023) |  $<$ 0.061 | 0.128 (0.022) | 0.219 (0.018) | 0.122 (0.015) | 0.087 (0.018) | 0.104 (0.022) |  $<$ 0.057 |  $<$ 0.066 |  $<$ 0.061
3 | 1.9 | 5.9 | 4.852 (0.062) | 0.186 (0.020) |  $<$ 0.050 | 0.092 (0.019) | 0.222 (0.018) | 0.162 (0.008) |  $<$ 0.041 |  $<$ 0.065 |  $<$ 0.057 |  $<$ 0.053 |  $<$ 0.058
4 | 3.4 | 7.4 | 3.787 (0.053) | 0.182 (0.018) |  $<$ 0.054 | 0.045 (0.014) | 0.109 (0.015) | 0.058 (0.007) |  $<$ 0.043 | 0.048 (0.016) |  $<$ 0.042 |  $<$ 0.047 |  $<$ 0.050
5 | 2.9 | 57.2 | 30.888 (0.277) | 1.266 (0.093) | 0.311 (0.091) | 0.477 (0.070) | 1.122 (0.070) | 0.814 (0.035) | 0.337 (0.068) | 0.468 (0.086) |  $<$ 0.260 |  $<$ 0.234 |  $<$ 0.222
6 | 5.8 | 10.0 | 2.163 (0.047) | 0.130 (0.014) |  $<$ 0.041 |  $<$ 0.041 |  $<$ 0.034 |  $<$ 0.018 |  $<$ 0.034 |  $<$ 0.036 |  $<$ 0.027 |  $<$ 0.039 |  $<$ 0.032
7 | 7.9 | 15.6 | 4.312 (0.091) | 0.201 (0.029) |  $<$ 0.082 |  $<$ 0.057 |  $<$ 0.067 |  $<$ 0.037 |  $<$ 0.053 |  $<$ 0.069 |  $<$ 0.060 |  $<$ 0.064 |  $<$ 0.073
8 | 5.3 | 3.1 | 0.598 (0.016) | 0.039 (0.005) |  $<$ 0.013 | 0.019 (0.005) | 0.040 (0.005) |  $<$ 0.007 |  $<$ 0.013 |  $<$ 0.014 |  $<$ 0.015 |  $<$ 0.017 |  $<$ 0.018
9 | 10.0 | 759.8 | 80.792 (0.866) | 2.540 (0.279) | 0.845 (0.265) |  $<$ 0.668 | 1.328 (0.212) | 0.843 (0.131) |  $<$ 0.643 |  $<$ 0.602 |  $<$ 0.659 |  $<$ 0.631 |  $<$ 0.745

Note. — Regions are identified by number in Figure 7. The distance in column 2
refers to the typical distance between the nuclear continuum source and points
within the region, and it indicates the radial coordinate used for each region
in Figures 9 and 10. Region 9 is a much larger irregular area roughly 38′′ by
32′′ in diameter, encompassing virtually all of the detected 12CO emission in
these data, and its typical radial coordinate is arbitrarily set to 10′′ for
ease in plotting. Line ratios measured in K units can be calculated from these
table entries by multiplying the ratio in Jy units by the square of the ratio
of the lines’ wavelengths.

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