# Star Formation Suppression by Tidal Removal of Cold Molecular Gas
from an Intermediate-Redshift Massive Post-starburst Galaxy

Justin S. Spilker Department of Physics and Astronomy and George P. and
Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas
A&M University, 4242 TAMU, College Station, TX 77843-4242, US  Katherine A.
Suess Department of Astronomy and Astrophysics, University of California,
Santa Cruz, CA, 95064, USA  Kavli Institute for Particle Astrophysics and
Cosmology and Department of Physics, Stanford University, Stanford, CA 94305,
USA  David J. Setton Department of Physics and Astronomy and PITT PACC,
University of Pittsburgh, Pittsburgh, PA 15260, USA  Rachel Bezanson
Department of Physics and Astronomy and PITT PACC, University of Pittsburgh,
Pittsburgh, PA 15260, USA  Robert Feldmann Institute for Computational
Science, University of Zurich, CH-8057, Zurich, Switzerland  Jenny E. Greene
Department of Astrophysical Sciences, Princeton University, Princeton, NJ
08544, USA  Mariska Kriek Leiden Observatory, Leiden University, NL-2300 RA
Leiden, Netherlands  Sidney Lower Department of Astronomy, University of
Florida, Gainesville, FL 32611, USA  Desika Narayanan Department of Astronomy,
University of Florida, Gainesville, FL 32611, USA  Margaret Verrico Department
of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh,
PA 15260, USA

###### Abstract

Observations and simulations have demonstrated that star formation in galaxies
must be actively suppressed to prevent the formation of over-massive galaxies.
Galactic outflows driven by stellar feedback or supermassive black hole
accretion are often invoked to regulate the amount of cold molecular gas
available for future star formation, but may not be the only relevant
quenching processes in all galaxies. We present the discovery of vast
molecular tidal features extending up to 64 kpc outside of a massive $z=0.646$
post-starburst galaxy that recently concluded its primary star-forming
episode. The tidal tails contain $(1.2\pm 0.1)\times 10^{10}$ $\rm{M}_{\odot}$
of molecular gas, $47\pm 5$% of the total cold gas reservoir of the system.
Both the scale and magnitude of the molecular tidal features are unprecedented
compared to all known nearby or high-redshift merging systems. We infer that
the cold gas was stripped from the host galaxies during the merger, which is
most likely responsible for triggering the initial burst phase and the
subsequent suppression of star formation. While only a single example, this
result shows that galaxy mergers can regulate the cold gas contents in distant
galaxies by directly removing a large fraction of the molecular gas fuel, and
plausibly suppress star formation directly, a qualitatively different physical
mechanism than feedback-driven outflows.

††software:  CASA (McMullin et al., 2007), DRAGONS (Labrie et al., 2019),
prospector (Leja et al., 2017; Johnson et al., 2020), astropy (Astropy
Collaboration et al., 2018), matplotlib (Hunter, 2007)

##  1 Introduction

The regulation of galactic reservoirs of cold molecular gas – the fuel for
star formation – over cosmic history is widely viewed as a critical but
poorly-understood component of galaxy formation and evolution. A wide range of
energetic ‘feedback’ processes are required in modern galaxy formation
simulations to moderate galaxy growth and produce galaxy populations that
agree with even the most basic observational characteristics. Perhaps most
strikingly, massive galaxy-scale outflows of cold molecular gas have been
observed in galaxies and AGN at low and high redshifts with energetics capable
of rapidly suppressing (or ‘quenching’) ongoing star formation (Fluetsch et
al. 2019; Spilker et al. 2020a, and see Veilleux et al. 2020 for a recent
review of cold gas outflows). Despite their undeniable importance in galaxy
evolution, however, outflows driven by star formation and/or AGN need not be
the only physical mechanisms relevant for all galaxy populations at all cosmic
epochs.

Post-starburst galaxies, whose A-star dominated optical spectra indicate that
an intense period of star formation was followed by rapid quenching
$\approx$0.1–1 Gyr ago, are valuable probes of star formation suppression as
the direct and immediate products of quenching (see French, 2021, for a recent
review). While rare locally ($\sim$0.1% of all galaxies), post-starbursts are
50$\times$ more common by $z\sim 2$ (e.g. Wild et al., 2016) and represent
about half of the growth of the quiescent population (e.g. Belli et al.,
2019). The burst in high-redshift post-starbursts also typically represents
the primary formation phase of those galaxies, forming $\sim$70% of the
stellar mass during the burst compared to $\sim$10% locally (French et al.,
2018; Wild et al., 2020). The SQuIGG$\vec{L}$E survey (Suess et al., 2022)
selected $\sim$1300 massive post-starburst galaxies from the SDSS at
intermediate redshifts $z\sim 0.7$, intended to be distant enough to be
representative of the high-redshift post-starburst population yet nearby
enough to allow detailed studies. Sample galaxies show no radial age
gradients, implying quenching took place uniformly throughout the galaxies
(Setton et al., 2020). Previous ALMA observations of a sample of
SQuIGG$\vec{L}$E post-starbursts found that the quenching of star formation
surprisingly does not require the total removal/destruction of the cold
molecular gas reservoir, but this cold gas does rapidly disappear over the
ensuing $\sim$100 Myr post-quenching (Suess et al., 2017; Bezanson et al.,
2022).

Here we present the discovery of vast molecular tidal gas features reaching
out from SDSS J144845.91+101010.5 (SDSS J1448+1010), a $z=0.646$ post-
starburst galaxy from the SQuIGG$\vec{L}$E survey. The cold gas contained in
the tidal tails represents about half the cold gas in the entire system, and
the nearly-concluded merger and accompanying tidal removal of the star-forming
fuel is plausibly the proximate cause of the suppressed star formation in this
galaxy. Section 2 describes the target galaxy and observational data, Section
3 presents our main findings, and we summarize and conclude in Section 4. We
assume a Chabrier (2003) initial mass function and a concordance flat
$\Lambda$CDM cosmology with $\Omega_{m}=0.3$ and $h=0.7$.

##  2 Data and Methods

Figure 1:  Near-IR and CO emission of SDSS J1448+1010. CO is detected within
the prominent stellar tidal tails seen in the HST image, reaching projected
distances up to 64 kpc from the central galaxy. The northern (southern) tidal
features are blue- (red-)shifted from the systemic velocity by $\approx$150
$\rm{km\,s}^{-1}$. The gas in the central galaxy is highly turbulent, while
the dispersion is $<$50 $\rm{km\,s}^{-1}$ at all locations within the tidal
features. Contours of the integrated CO emission begin at 3$\sigma$ and
increase in powers of 2, and are repeated in all panels. The ALMA synthesized
beam is indicated at lower-left. The dotted circle in the second panel
indicates the emission we consider to belong to the central galaxy; all else
constitutes the tails.

###  2.1 ALMA Observations and Analysis

SDSS J1448+1010 was selected for CO(2–1) observations as part of a survey of
13 SQuIGG$\vec{L}$E post-starbursts presented in Bezanson et al. (2022). The
targets in Bezanson et al. (2022) sample the full range of spectral diversity
of the parent sample (e.g. $D_{n}4000$, $H\delta_{A}$), but are preferentially
brighter in the $i$ band due to a combination of lower than average redshift
and higher than average stellar mass than the full sample. Initial ALMA Band 4
observations of SDSS J1448+1010 were carried out in program 2017.1.01109.S
targeting redshifted CO(2–1) at 140.06 GHz. The total on-source time was 100
min, sampling 15–780 m baselines; see Bezanson et al. (2022) for further
details. These data detected CO(2–1) emission extending well beyond the
optical extent of the galaxy at $\approx 5\sigma$ significance.

Concerned that large-scale low surface brightness emission may have been
resolved out, we acquired deeper low-resolution data using the same observing
setup in project 2018.1.01264.S. The new data sample 15–460 m baselines, and
the on-source integration time was 150 min. We jointly imaged the data from
both projects in CASA (McMullin et al., 2007) using natural weighting to
create both continuum images and CO(2–1) spectral cubes. We perform our
subsequent analysis on a cube that averages 4 native channels with spectral
resolution $\approx$33 $\rm{km\,s}^{-1}$. The final images reach a resolution
of 1.3”$\times$1.6”, continuum sensitivity 4.5 $\mu\rm{Jy}$/beam, and spectral
line sensitivity $\approx$97 $\mu\rm{Jy}$/beam in 33 $\rm{km\,s}^{-1}$
channels.

We use a slightly non-standard procedure to create CO(2–1) moment maps.
Because the emission in the tidal features is much narrower than the central
galaxy, simply integrating the CO cube over the spectral dimension would
overwhelm the tidal features with many signal-free channels of noise. Instead
we create moment maps by first identifying $\geq 4\sigma$ peaks in the cube,
then dilating outwards to the 2$\sigma$ contour in each spectral channel. The
4$\sigma$ threshold was chosen such that we expect $<1$ false peak within a
30”$\times$30”$\times$25 channel region of the cube given the synthesized beam
size. Maps of the integrated CO emission, velocity field, and velocity
dispersion are shown in Figure 1. We extract CO spectra from within the
dilated region, shown in Figure 2. We separate the emission from the galaxy
from the northern and southern tidal tails by cutting perpendicular to the
minimum level of the ‘valley’ between the components in the integrated flux
map, indicated by the dashed ellipse in Fig. 1. We verified that this
procedure produces integrated fluxes consistent with traditional aperture-
based and image-plane fitting methods.

We detect weak continuum emission at the position of the central galaxy,
$S_{\mathrm{147GHz}}=19.3\pm 4.5$ $\mu\rm{Jy}$. The flux ratio between the
upper and lower sidebands (sky frequencies 153 and 141 GHz), 0.9$\pm$0.3, is
marginally inconsistent with that expected for dust emission on the Rayleigh-
Jeans tail ($\approx$1.3–1.4 assuming a dust spectral index $\beta=1.5-2$).
This suggests that some of the 2 mm continuum emission is due to non-thermal
synchrotron emission instead of dust. Assuming a synchrotron spectral index
$\alpha=-0.7$, we infer that $\gtrsim$25% of the 2 mm continuum emission is
due to synchrotron emission.

Figure 2:  ALMA CO(2–1) spectra of SDSS J1448+1010 extracted from, in order,
the central galaxy and the northern and southern tidal tails. Combined, the
tidal tails contain 47$\pm$5% of the total CO luminosity in the system.

###  2.2 Near-Infrared Observations and Analysis

We observed SDSS J1448+1010 in H-band using the FLAMINGOS-2 camera (Eikenberry
et al., 2004) on Gemini-South in program GS-2020A-FT-103 (PI Spilker). The
instrument was used in imaging mode, and observations consisted of a repeating
9-point dither pattern with 8 s exposures totaling 1440 s on-source. The data
were reduced using the Gemini DRAGONS software (Labrie et al., 2019),
consisting of standard flat-field correction, dark and sky subtraction,
alignment and stacking. The photometric calibration, checked against 2MASS
stars in the field of view, is accurate to 0.04 mag, and the seeing conditions
average 0.5” in the final image. The FLAMINGOS-2 image is shown in Figure 3.

We also observed SDSS J1448+1010 in a single orbit of HST WFC3/IR F110W
imaging in program PID 16201 (PI Spilker). The total exposure time was 2.45 ks
divided into a 4$\times$500 s dither pattern and a 450 s exposure at the final
dither position. We fit a two-dimensional Sérsic profile to the galaxy using
galfit (Peng et al., 2010) after masking nearby sources and the tidal
features, finding a best-fit half-light radius 0.54” (3.8 kpc). The size of
the central galaxy agrees well with expectations for quiescent galaxies at
this mass and epoch (van der Wel et al., 2014) and other SQuIGG$\vec{L}$E
post-starbursts (Setton et al., 2022).

We identify a secondary peak near the center of the galaxy in the residual
image (Fig. 3); it is also visible by eye in the unsubtracted image with
suitable stretch, and is not an artifact of the fitting procedure. We re-fit
the image using a two-component model and found that the fainter component has
very little impact on the best-fit parameters of the main galaxy. The peak is
separated from the galaxy center by 0.6” ($\approx$5 resolution elements), but
more detailed properties are hampered by degeneracies due to the much brighter
central peak. This secondary peak could be interpreted as a clump within the
merger, a knot near the base of the northern tidal tail, or the remnant of the
merging companion galaxy itself. In any case, the lack of an obvious merging
companion implies that the merger is near final coalescence.

Figure 3:  Left: HST WFC3/F110W image of SDSS J1448+1010, as in Fig. 1, shown
on log-stretch. Center: After subtraction of the best-fit single Sérsic
profile, a second peak $\approx$0.6” northwest of the main galaxy is more
easily visible. Right: Gemini FLAMINGOS-2 H-band image with CO(2–1) contours.
The white circle shows the photometric aperture used for all imaging data,
matched to the SDSS catalog photometry. Note the object just off-frame to the
northwest is an unrelated foreground galaxy.

###  2.3 Spectrophotometric Fitting

We jointly fit the available imaging and spectroscopic data following Suess et
al. (2022) to measure the mass, SFR, and star formation history (SFH) of SDSS
J1448+1010. Briefly, we use the prospector SED fitting software (Leja et al.,
2017; Johnson et al., 2020) with a custom flexible SFH designed to be able to
capture both a recent burst and variable level of star formation after the
burst. This SFH has been extensively tested using mock data (Suess et al. in
prep.). The starburst is defined as the time between which the derivative of
the SFR rises above and declines below a threshold value (Suess et al., 2022).
In addition to the SDSS photometry and spectroscopy and WISE photometry, we
also include the photometric measurements from our new WFC3/F110W and
FLAMINGOS-2 H-band imaging measured in a 5.2” radius aperture matched to the
SDSS catalogs. From the WFC3/F110W image, we estimate that an additional 6% of
the total light of the system is excluded by this aperture; this has
negligible impact on our results.

The data, best-fit model, and reconstructed SFH are shown in Figure 4. We find
a stellar mass $\log M_{\rm{*}}/\rm{M}_{\odot}=11.4\pm 0.1$, metallicity $\log
Z_{\rm{*}}/\rm{Z}_{\odot}=0.2^{+0.2}_{-0.1}$, velocity dispersion
$\sigma_{\mathrm{*}}=210\pm 13$ $\rm{km\,s}^{-1}$, and dust extinction
$\mathrm{A}_{\mathrm{V}}=0.4$ mag. The SFH indicates that the galaxy had a
starburst that began 1.0${}^{+0.3}_{-0.4}$ Gyr before the time of observation,
ended 70${}^{+40}_{-30}$ Myr before observation, and produced 43$\pm$24% of
the galaxy’s total stellar mass. After the conclusion of the starburst, the
SFR dropped to $3^{+2}_{-1}$ $\rm{M}_{\odot}$/yr by the observed epoch, a
factor of 50–100 lower than the starburst peak.

It is possible this method has missed highly obscured star formation,
extincted enough to be undetectable in the rest-UV to mid-IR data. We place a
(loose) constraint on the obscured SFR using the ALMA 2 mm detection; this is
highly uncertain because we do not constrain the peak of the IR SED and the
total IR luminosity depends strongly on the dust temperature $T_{\rm{dust}}$.
Conservatively ignoring the possible $\gtrsim$25% contribution to the 2 mm
flux density from synchrotron emission, we assume a modified blackbody with
luminosity-weighted $T_{\rm{dust}}=35$ K and a standard conversion between
$L_{\rm{IR}}$ and SFR (Kennicutt & Evans, 2012). We find an obscured SFR $\sim
10$ $\rm{M}_{\odot}$/yr with large systematic uncertainty due to the unknown
$T_{\rm{dust}}$. We note that $L_{\rm{IR}}$ traces the SFR on timescales up to
$\sim$100 Myr (e.g. Kennicutt & Evans, 2012), similar to the inferred time
since quenching, which may explain why this estimate is higher than the
‘instantaneous’ estimate from the spectral fitting. From the limited data
available we find no evidence for substantial obscured star formation.

Figure 4:  The available multiwavelength imaging and SDSS spectroscopic data
(upper panels) allow detailed constraints on the star formation history of
SDSS J1448+1010. The optical spectrum shows prominent Balmer absorption
features, the hallmark of post-starburst galaxies. The reconstructed SFH
(lower panels) indicates that the starburst event began $1.0^{+0.3}_{-0.4}$
Gyr and ended $70^{+40}_{-30}$ Myr ago, and formed $\sim$40% of the total
stellar mass. The photometric data (black points) and best-fit model
photometry (red points) are shown in the upper left panel. The SDSS spectrum
(unbinned: gray; binned: black) and best-fit model spectrum (red) are shown in
the upper right. Vertical gray bands highlight wavelengths around [OII]
3727$\AA$ and [OIII] 4959,5007$\AA$, which are masked in our fitting. Red
shaded regions in the lower panels indicate the 16-84th percentile confidence
interval on the reconstructed SFH, and the vertical blue shaded regions
indicate the time period of the recent starburst.

##  3 Results and Discussion

###  3.1 Vast and Massive Molecular Tidal Tails

In addition to the central galaxy, we detect CO emission in two tail-like
features extending up to $\approx$9” east/northeast and south of the galaxy
(Fig. 1. At the redshift of SDSS J1448+1010, these features are separated from
the main galaxy by physical distances on the sky up to 64 kpc. The northern
features are mostly blueshifted by $\approx$100–150 $\rm{km\,s}^{-1}$ and the
southern feature redshifted by $\approx$100–150 $\rm{km\,s}^{-1}$. There is no
discernible coherent velocity gradient in the gas within the central
galaxy.111We also explored more aggressive visibility weighting schemes that
improve resolution at the expense of sensitivity and found no evidence of
ordered motion at $\approx$0.8” resolution. In contrast to the high velocity
dispersion observed in the gas located within the central galaxy,
$\sigma_{\mathrm{CO}}=290\pm 20$ $\rm{km\,s}^{-1}$, both extended features are
narrow, with dispersion $<$50 $\rm{km\,s}^{-1}$ at all locations. The gas
within the central galaxy appears to be genuinely dispersion dominated – it is
implausible that inclination or resolution effects would result in such a high
dispersion and lack of velocity gradient for a rotating system.

From the HST/WFC3 and Gemini/FLAMINGOS-2 imaging (Fig. 3), it is clear that
the extended CO features are embedded within prominent stellar tidal features.
Extended tidal tails such as these are produced in the late stages of galaxy-
galaxy mergers with near-equal mass ratio between the merging partners (e.g.
Toomre & Toomre, 1972; Cox et al., 2008). The spatial coincidence between the
stellar and gas features indicates that the latter are also tidal in origin.
In nearby galaxy mergers, tidal molecular gas is thought to have been stripped
from the host galaxies in some cases, while in others the molecular gas may
have formed in-situ from recycled atomic gas. In these latter cases, gas
streams along the tidal tails and collects at the tips, where the pile-up
increases the gas density and facilitates the conversion of some of the atomic
gas into molecular form (e.g. Braine et al., 2000; Bournaud et al., 2004).
Because the molecular gas in the tidal tails of SDSS J1448+1010 is not
concentrated at the tips of the tidal tails, the hallmark of in-situ
formation, we conclude that the cold gas was stripped from the pre-existing
reservoirs of the galaxies by tidal forces as the merger proceeded.

CO spectra of the central galaxy and extended features are shown in Fig. 2. We
fit the spectra with single Gaussian components; two Gaussians or directly
summing over the channels with significant emission yielded consistent line
fluxes. We find CO luminosities for the central galaxy and tails
$\rm{L}_{\rm{CO(2-1),gal}}^{\prime}=(3.2\pm 0.2)\times 10^{9}$ K
$\rm{km\,s}^{-1}$ pc2 and $\rm{L}_{\rm{CO(2-1),tails}}^{\prime}=(2.9\pm
0.2)\times 10^{9}$ K $\rm{km\,s}^{-1}$ pc2, respectively. We assume standard
literature conversions between CO luminosity and cold gas mass, namely
thermalized CO emission and a CO-H2 conversion factor $\alpha_{\rm{CO}}=4.0$
$\rm{M}_{\odot}$/(K $\rm{km\,s}^{-1}$ pc2), for both the central galaxy and
the extended features, appropriate for non-starbursting, high-metallicty
galaxies (e.g. Bolatto et al., 2013).

Under these assumptions we measure molecular masses $M_{\rm{mol,gal}}=(1.3\pm
0.1)\times 10^{10}$ $\rm{M}_{\odot}$ for the galaxy and
$M_{\rm{mol,tails}}=(1.2\pm 0.1)\times 10^{10}$ $\rm{M}_{\odot}$ in the tails.
Combined, the extended tidal tails contain 47$\pm$5% of the total molecular
gas in the system. We emphasize that this result derives from conservative
assumptions. Estimates of the absolute molecular masses of galaxy and tails
depend on the absolute values of the CO(2–1)/CO(1–0) line ratio $r_{21}$ and
$\alpha_{\rm{CO}}$, while the fraction of cold gas in the tails is only
dependent on relative differences between the galaxy and tails.

The CO excitation is a subdominant source of uncertainty due to the limited
plausible range of $r_{21}$. Our assumption of thermalized emission
($r_{21}=1$) is conservative, as lower values would imply higher gas masses.
The value of $\alpha_{\rm{CO}}$ is more uncertain. For the central galaxy, we
can cross-check our assumption using the 2 mm continuum detection (although
the tails are similarly massive, their larger spatial extent makes them
undetected at the depth of our data). Although there is evidence that $>$25%
of the 2 mm continuum arises from synchrotron emission (Sec. 2.1), we assume
it is entirely due to dust emission to be conservative. Under standard
assumptions for the dust emissivity and a mass-weighted dust
temperature222Distinct from the luminosity-weighted $T_{\rm{dust}}$ used to
estimate $L_{\rm{IR}}$ (Section 2.3); see discussion in Scoville et al.
(2016). of 25 K (e.g. Dunne et al., 2003; Scoville et al., 2016), we find a
dust-based gas mass $M_{\rm{mol,gal}}=(1.2\pm 0.3)\times 10^{10}$
$\rm{M}_{\odot}$, implying $\alpha_{\rm{CO}}\approx 3.8$, in excellent
agreement with our assumptions. A $>$25% contribution to the 2 mm continuum
from synchrotron emission would imply $\alpha_{\rm{CO}}<2.9$ for the central
galaxy. If this limit on $\alpha_{\rm{CO}}$ applies to the galaxy but not the
tails, the fraction of cold gas contained in the tidal tails would increase to
$>$56%.

###  3.2 Extreme Cold Gas Ejection by a Major Merger

The molecular gas properties of the tidal tails in SDSS J1448+1010 are
remarkable compared to all known galaxy mergers in the nearby or distant
universe. We compiled a sample of 18 low-redshift ($z\lesssim 0.3$) galaxy
mergers in which CO has been detected in the tidal tails (Combes et al., 1988;
Becker et al., 1989; Smith et al., 1999; Heithausen & Walter, 2000; Schinnerer
et al., 2000; Aalto et al., 2001; Braine et al., 2000, 2001, 2003, 2004; Duc
et al., 2007; Lisenfeld et al., 2008; Boquien et al., 2011; Villar-Martín et
al., 2013; Sun et al., 2014; Sliwa et al., 2017; Fotopoulou et al., 2019). We
also note the existence of two $z\sim 1.5$ systems in which some of the
detected CO emission was claimed to arise from tidally-dispersed material
(Silverman et al., 2018; Puglisi et al., 2021). We excluded these objects from
the comparison sample because the evidence that the emission is associated
with tidal tails is indirect, based on subtraction or decomposition of low-
resolution, low-S/N data. Consequently, the masses of host and tidal gas, the
division between them, and the maximum spatial extent of any tidal gas are
highly uncertain. The comparison sample is admittedly highly heterogeneous and
spans a wide range of merger stage, total system mass, and so on.

We systematized the literature sample to match our assumptions for the CO
excitation and $\alpha_{\rm{CO}}$. Compared to the original values, this
increases the fraction of the gas in tidal tails for some objects (if the CO
line opacity and therefore $\alpha_{\rm{CO}}$ was thought to be low in the
tails) but lowers it for others (if a ‘starburst’ $\alpha_{\rm{CO}}$ was
assumed for the central galaxies). Uncertainties on the gas properties of the
comparison sample, like for SDSS J1448+1010, are dominated by the systematic
uncertainty $\alpha_{\rm{CO}}$. When not quoted by the original authors, we
estimate the maximum projected separation of the tidal molecular features by
measuring the extent from the galaxy nucleus to the most distant detected CO
emission in the plane of the sky. These distances represent lower limits to
the true maximum extent for all galaxies (including SDSS J1448+1010) because
the line-of-sight extent is unmeasured. As for SDSS J1448+1010, we consider
the molecular gas to have formed in situ if CO is detected at the tip of the
tidal tail(s), or stripped otherwise (e.g. Braine et al., 2000; Bournaud et
al., 2004).

Figure 5 compares the molecular gas properties of the tidal tails in SDSS
J1448+1010 with the CO detected in the nearby merger comparison sample. In
terms of molecular gas contained in the tails, fraction of the total gas
within the tails, and maximum extent of detected CO emission, SDSS J1448+1010
is clearly extreme in comparison to nearby mergers. SDSS J1448+1010 would also
be highly unusual based on direct observables alone (i.e. CO luminosity). The
molecular tidal features of nearby mergers broadly cluster in two groups
depending on whether the molecular gas was formed in situ from atomic gas or
stripped during the merger. Molecular gas can form in situ at large distances
$\geq$80 kpc, but this gas constitutes only a small fraction of the total cold
gas contents of the system. Much larger amounts of molecular gas can be
removed by tidal stripping, up to $\approx$40% of the total, but in these
cases the molecular features are only detected to projected distances $<$20
kpc. SDSS J1448+1010 is unique in comparison to these nearby mergers in both
the scale and magnitude of the stripped molecular gas.

Figure 5:  The molecular gas in the SDSS J1448+1010 tidal tails are extreme in
both scale and magnitude in comparison to all known mergers at any redshift.
The low-redshift merger comparison sample is coded by whether or not the
molecular gas is thought to have been stripped from the host (solid) or
recycled / formed in situ (empty). All objects use the same assumptions for CO
excitation and $\alpha_{\rm{CO}}$; this figure would appear identical using
pure observable quantities.

###  3.3 Quenching by Tidal Cold Gas Removal

Having shown that the molecular gas tidal tail properties of SDSS J1448+1010
are extreme compared to all known examples in nearby galaxies, we finally
briefly speculate on the origins and implications of these features for
quenching the star formation in this massive post-starburst galaxy. We note up
front that there are of course many physical processes that may have
contributed to the quenching of star formation – notably, this galaxy is also
an optical AGN based on its [OIII]/H$\beta$ line ratio, as in $\sim$10% of the
youngest post-starbursts in SQuIGG$\vec{L}$E (Greene et al., 2020). It is
clear that the AGN did not produce the extended CO features, which perfectly
trace the stellar tidal tails and have very cold dynamics, but it is less
clear what role the AGN injection of thermal energy has contributed to the
abrupt quenching or low ongoing SFR in this galaxy. Our goal here instead is
to understand the potential consequences of the features thus far uniquely
identified in SDSS J1448+1010 – namely, vast extended tidal tails of molecular
gas.

While reconstructing the history of the merger is not possible with the
available data, extended tidal tails such as those in SDSS J1448+1010 are
typical of the late stages of galaxy mergers. Analysis of the HST/WFC3 image
reveals a second peak in the near-IR light profile, separated from the galaxy
center by $\sim$4.4 kpc (Fig. 3; Section 2.2). If interpreted as the remnant
of the second galaxy in the merging pair, the small separation between nuclei
also implies the merger is near final coalescence, $\sim$0.5–2 Gyr after the
interaction began (e.g. Lacey & Cole, 1993). This timescale is consistent with
the timing of the onset of the starburst found from the spectral modeling,
estimated to have begun $1.0^{+0.3}_{-0.4}$ Gyr prior to observation (Figure
4). Based on the well-constrained SFH of SDSS J1448+1010, the most plausible
physical scenario is that the initial stages of the merger also triggered the
active starburst phase.

The tidal removal of molecular gas and the associated tidal stirring in the
final mergers stages also may be directly responsible for the quenching of the
starburst that concluded $70^{+40}_{-30}$ Myr ago. By construction the
SQuIGG$\vec{L}$E survey selects galaxies that experienced a sharp decline in
SFR within the last billion years (Suess et al., 2022); a gradual decline in
SFR cannot produce a galaxy-integrated spectrum with the prominent Balmer
absorption features that are the hallmark of post-starburst galaxies.

Tidal tails form and disperse over timescales of several hundred Myr (e.g.
Toomre & Toomre, 1972; Hibbard & Mihos, 1995), seemingly in conflict with the
abrupt truncation of the starburst inferred from spectral modeling. However,
the majority of the molecular gas in the tidal tails ($\sim$80%,
$M_{\rm{mol}}=0.9\times 10^{10}$ $\rm{M}_{\odot}$) is located at projected
galactocentric distances of 15–30 kpc. From the CO velocity field (Fig. 1) we
see that this tidal material is traveling at a typical velocity $\approx$150
$\rm{km\,s}^{-1}$ away from the central galaxy; this is a lower limit to the
true gas velocity because motion in the plane of the sky is unobservable. At
this minimum velocity the molecular gas in the tidal tails could have reached
its present location in 100–200 Myr, in good agreement with the timing of the
end of the starburst. The cold gas not removed in the tidal interaction,
meanwhile, has most likely been unable to form stars because it has been
stirred into a highly turbulent state not conducive to star formation; the
velocity dispersion of the gas in the galaxy, $\sigma_{\rm{CO}}=290\pm 20$
$\rm{km\,s}^{-1}$, is $\sim$50% higher than the stellar velocity dispersion.
Based on a larger sample of SQuIGG$\vec{L}$E post-starburst galaxies, we
expect this cold gas to be destroyed, heated, and/or removed over the next
$\sim$100 Myr (Bezanson et al., 2022).

It is clear that the tidal removal of molecular gas from SDSS J1448+1010 has
had and will continue to have a large impact on the cold gas reservoir of this
galaxy over a $>$1 Gyr timescale, regardless of whether it is the proximate
cause of the suppressed star formation. Galactic outflows of cold gas driven
by supernovae and/or AGN are also often invoked to heat and disrupt the cold
gas supply of galaxies and prevent the formation of over-massive galaxies
(e.g. Hopkins et al., 2014; Hayward & Hopkins, 2017; Davé et al., 2019). In
comparison to molecular outflows detected at low and high redshifts, the tidal
interaction in SDSS J1448+1010 has removed a much larger fraction of the cold
gas than typical outflows, which generally contain 1-10% of the gas mass of
the galaxy (e.g. Fluetsch et al., 2019; Spilker et al., 2020a). In contrast,
the velocity of the tidally-removed gas is likely much lower than the gas
ejected in typical outflows, which reaches velocities of hundreds to $>$1000
$\rm{km\,s}^{-1}$ (e.g. Geach et al., 2014; Fluetsch et al., 2019; Spilker et
al., 2020b). Tidal removal offers a slower, gentler mechanism by which the
cold gas reservoir of galaxies is governed, played out over billion-year
timescales.

##  4 Synthesis and Conclusions

We have discovered a spectacular vast molecular gas reservoir extending up to
$>$60 kpc outside of SDSS J1448+1010, a $z=0.646$ massive post-starburst
galaxy from the SQuIGG$\vec{L}$E survey. Near-IR imaging makes clear that the
molecular gas features are co-located within extended tidal tails resulting
from a recent/ongoing major merger. The tidal tails contain at least $47\pm
5$% of the total molecular gas in the system, and are extreme in both scale
and magnitude compared to all known merging systems at any redshift. The
timing of the merger and well-constrained star formation history are
consistent with a scenario in which the initial phases of the merger triggered
the recent starburst, while the subsequent rapid quenching event coincided
with the ejection of most of the molecular gas into the tidal tails.

From the single example we have discovered alone, it is unclear how common
tidal ejection of cold gas from high-redshift galaxies may be. There are
reasons to expect that tidal cold gas removal may be a much more common
phenomenon at high redshift than today. High-redshift galaxies are far more
gas-rich than modern galaxies, with the molecular mass increasing as
$(1+z)^{a}$ with $a\approx 2.5-3$ (e.g. Tacconi et al., 2020), which may
explain why the tails in SDSS J1448+1010 are so gas-rich relative to low-
redshift mergers in absolute terms. The rate of galaxy mergers is also a
strong function of cosmic age, increasing as $(1+z)^{m}$ with $m\approx 2-3$
(e.g. Lotz et al., 2008; Conselice et al., 2009; Bridge et al., 2010; Duncan
et al., 2019; Ferreira et al., 2020), a consequence of the falling density of
galaxies with time due to cosmic expansion. Together, the increased gas supply
and higher merger rate suggest that mergers may play a far more important role
in regulating the molecular gas contents of high-redshift galaxies than they
do today.

It is clear that the merger event has played and will continue to play a large
role in the molecular gas contents of this system over long timescales. More
speculatively, the tidal removal of the cold gas supply may have played a
large role in directly quenching the star formation in this galaxy. Our
discovery of vast and massive molecular tidal features was unexpected; future
targeted observations will be able to determine how often this phenomenon
occurs and place it in the broader context of galaxy quenching. This result
opens up the possibility that tidal removal of the fuel for star formation may
be a viable quenching mechanism that should be considered alongside galactic
feedback and outflows.

JSS thanks Kathleen Labrie for assistance with the Gemini data reduction, and
was supported in part by NASA Hubble Fellowship grant #HF2-51446 awarded by
the Space Telescope Science Institute, which is operated by the Association of
Universities for Research in Astronomy, Inc., for NASA, under contract
NAS5-26555. This research used observations made with the NASA/ESA Hubble
Space Telescope operated under the same contract. DJS, RB, JEG, and DN
acknowledge support from NSF-AAG #1907697, #1907723, and #1908137. This paper
makes use of the following ALMA data: ADS/JAO.ALMA#2017.1.01109.S,
ADS/JAO.ALMA#2018.1.01264.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. Based on observations obtained at the international Gemini
Observatory, a program of NSF’s NOIRLab, which is managed by AURA under a
cooperative agreement with the NSF on behalf of the Gemini Observatory
partnership. ALMA, HST (WFC3), Gemini (FLAMINGOS-2), Sloan

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