# Molecular and Ionized Gas in Tidal Dwarf Galaxies: The Spatially Resolved
Star-Formation Relation

N. Kovakkuni1, F. Lelli2, P.-A. Duc3, M. Boquien1, J. Braine4, E. Brinks5, V.
Charmandaris6,7,8, F. Combes9, J. Fensch10, U. Lisenfeld11,12, S. S.
McGaugh13, J. C. Mihos13, M. S. Pawlowski14, Y. Revaz15, P. M. Weilbacher14
1Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos
601, Antofagasta, Chile
2INAF, Arcetri Astrophysical Observatory, Largo Enrico Fermi 5, 50125,
Florence, Italy
3Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg
(ObAS), UMR 7550, 67000 Strasbourg, France
4Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée
Geoffroy Saint-Hilaire, 33615 Pessac, France
5Centre for Astrophysics Research, University of Hertfordshire, College Lane,
Hatfield AL10 9AB, UK
6Department of Physics, University of Crete, Heraklion, 71003, Greece
7Institute of Astrophysics, Foundation for Research and Technology-Hellas
(FORTH), Heraklion, 70013, Greece
8School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516
Nicosia, Cyprus
9LERMA, Observatoire de Paris, PSL Research Université, CNRS, Sorbonne
Université, UPMC, Paris, France
10Univ. Lyon, ENS de Lyon, Univ. Lyon 1, CNRS, Centre de Recherche
Astrophysique de Lyon, UMR5574, 69007 Lyon, France
11Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071
Granada, Spain
12 Instituto Carlos I de Física Teórica y Computacional, Facultad de Ciencias,
18071 Granada, Spain
13Department of Astronomy, Case Western Reserve University, Cleveland OH
44106, USA
14Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482
Potsdam, Germany
15Institute of Physics, Laboratory of Astrophysics, École Polytechnique
Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
E-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>
###### Abstract

Tidal dwarf galaxies (TDGs) are low-mass objects that form within tidal and/or
collisional debris ejected from more massive interacting galaxies. We use
CO($1-0$) observations from ALMA and integral-field spectroscopy from MUSE to
study molecular and ionized gas in three TDGs: two around the collisional
galaxy NGC 5291 and one in the late-stage merger NGC 7252. The CO and
H$\alpha$ emission is more compact than the H I emission and displaced from
the H I dynamical center, so these gas phases cannot be used to study the
internal dynamics of TDGs. We use CO, H I, and H$\alpha$ data to measure the
surface densities of molecular gas ($\Sigma_{\rm mol}$), atomic gas
($\Sigma_{\rm atom}$) and star-formation rate ($\Sigma_{\rm SFR}$),
respectively. We confirm that TDGs follow the same spatially integrated
$\Sigma_{\rm SFR}-\Sigma_{\rm gas}$ relation of regular galaxies, where
$\Sigma_{\rm gas}=\Sigma_{\rm mol}+\Sigma_{\rm atom}$, even though they are H
I dominated. We find a more complex behaviour in terms of the spatially
resolved $\Sigma_{\rm SFR}-\Sigma_{\rm mol}$ relation on sub-kpc scales. The
majority ($\sim$60$\%$) of SF regions in TDGs lie on the same $\Sigma_{\rm
SFR}-\Sigma_{\rm mol}$ relation of normal spiral galaxies but show a higher
dispersion around the mean. The remaining fraction of SF regions
($\sim$40$\%$) lie in the starburst region and are associated with the
formation of massive super star clusters, as shown by Hubble Space Telescope
images. We conclude that the local SF activity in TDGs proceeds in a hybrid
fashion, with some regions comparable to normal spiral galaxies and others to
extreme starbursts.

###### keywords:

galaxies: dwarf – galaxies: evolution – galaxies: formation – galaxies:
interactions – galaxies: ISM — galaxies: star formation

††pagerange: Molecular and Ionized Gas in Tidal Dwarf Galaxies: The Spatially Resolved Star-Formation Relation–Molecular and Ionized Gas in Tidal Dwarf Galaxies: The Spatially Resolved Star-Formation Relation Table 1: TDG sample. Distances are adopted from Lelli et al. (2015). Redshifts and systemic velocities are computed from H$\alpha$ emission lines (Sect. 3.2). SFRs, molecular gas masses, and atomic gas masses are computed within the total CO emitting area, but the TDG size is significantly larger (see Sect. 3.3 for details). Galaxy | R.A. | Dec. | Dist. | $z$ | Vsys | SFR | Mmol | Matom | Area
---|---|---|---|---|---|---|---|---|---
| (J2000) | (J2000) | (Mpc) |  | (km s-1) | (M⊙ yr-1) | (107 M⊙) | (107 M⊙) | (kpc2)
NGC 5291N | 13 47 20.3 | -30 20 54 | 62 | 0.014 | 4228.8 | 0.6$\pm$0.1 | 5.5$\pm$1.1 | 163.6$\pm$16.7 | 25.0
NGC 5291S | 13 47 22.7 | -30 27 40 | 62 | 0.016 | 4780.4 | 0.2$\pm$0.1 | 2.6$\pm$0.4 | 124.1$\pm$16.1 | 16.0
NGC 7252NW | 22 20 33.7 | -24 37 24 | 66.5 | 0.016 | 4771.6 | 0.06$\pm$0.02 | 3.2$\pm$0.6 | 11.7$\pm$2.2 | 12.3

##  1 Introduction

The process of star formation (SF) plays a key role in the formation and
evolution of galaxies. Key insights into the SF process are given by empirical
relations that connect the star formation rate (SFR) of a galaxy to the
availability of gas in the interstellar medium (ISM). One such relation is the
Kennicutt-Schmidt (KS) relation. In its original form, the Schmidt (1959)
relation connected volume densities of SFR ($\rho_{{}_{\rm SFR}}$) and atomic
gas mass ($\rho_{{}_{\rm HI}}$) in star-forming regions of the Milky Way:

$\rho_{{}_{\rm SFR}}\propto\rho_{{}_{\rm HI}}^{n}.$ (1)

Subsequent studies (Kennicutt Jr, 1998b) of star-forming galaxies revealed a
tight relation between the disk-averaged SFR surface densities ($\Sigma_{\rm
SFR}$) and total (atomic plus molecular) gas mass surface densities
($\Sigma_{\rm gas}$):

$\Sigma_{\rm SFR}=A\,\Sigma_{\rm gas}^{N}.$ (2)

With the advent of multi-wavelength observations at high angular resolution,
it has become possible to study the KS relation in a spatially resolved
fashion on kpc-scales in a variety of environments, from spiral galaxies to
interacting objects (e.g., Bigiel et al., 2008; Leroy et al., 2008; Boquien et
al., 2011). These works suggested that the SFR surface density correlates more
strongly with molecular gas (H2) than atomic gas in the inner H2-dominated
regions of star-forming disks. The situation, however, remains unclear in the
H I-dominated regime, typical of dwarf galaxies (Roychowdhury et al., 2014,
2015) as well as the outermost parts of spiral galaxies (Bigiel et al., 2010).
In this regime, flares in the outer gaseous disks (i.e., a radial increase of
the disk thickness) may play an important role (Bacchini et al., 2019, 2020),
suggesting that the volume density of total gas (atomic plus molecular) best
correlates with the volume density of SFR, in analogy to the original form in
Eq. 1.

A key question is whether every galaxy follows the same KS relation, thus
whether the SF process is “universal” or not on sub-kpc scales. For example,
there is a clear link between SF activity and galaxy interactions (e.g.,
Ellison et al., 2013). Tidal interactions lead to gas inflows towards the
galaxy centers, which can temporarily enhance the SFRs, and move the resulting
systems above the mean KS relation in the so-called starburst regime (e.g.
Barnes & Hernquist, 1991; Renaud et al., 2014; Ellison et al., 2020). At the
same time, galaxy collisions expel gas and stars into intergalactic space,
leading to the formation of tails and bridges in which new stars can form. SF,
indeed, can occur within gas debris surrounding interacting systems, sometimes
even at 100 kpc away from the parent galaxies (Mirabel et al., 1992; Duc et
al., 2006; Boquien et al., 2007, 2011). Is the SF occurring in such extreme
environments proceeding in a similar way as in galaxy disks?

Tidal dwarf galaxies (TDGs) are self-gravitating objects found within tidal
debris, which show in-situ SF and have masses and sizes comparable to “normal”
dwarf galaxies (Duc et al. 2006). Zwicky (1956) was the first to suggest the
possible formation of TDGs around interacting galaxies; this hypothesis was
later confirmed by several observational and theoretical studies (e.g. Mirabel
et al. 1992; Barnes & Hernquist 1992; Elmegreen et al. 1993). Hereafter, for
simplicity, we will use the term TDG to also include newborn galaxies that
form within collisional debris (rather than tidal ones), such as the SF
complexes in the collisional ring of NGC 5291 (Bournaud et al., 2007). In
addition, we will refer to “bona-fide TDGs” to indicate systems that show
internal kinematics decoupled from the surrounding debris, pointing to self-
gravity within a local potential well (e.g., Lelli et al., 2015). Bona-fide
TDGs have been identified around several interacting systems (e.g., Duc et
al., 2006; Lelli et al., 2015).

TDGs form out of gas that has been pre-enriched in their parent massive
galaxies, giving them higher gas-phase metallicities (about
$0.3-0.5Z_{\odot}$) than “classical” dwarf galaxies (Duc & Mirabel, 1998).
Thus, contrarily to most dwarf galaxies, TDGs are easily detected in the CO
emission line, and it is sensible to use the Milky-Way $X_{\rm CO}$ factor to
convert the observed CO flux density into H2 column densities (Braine et al.,
2001; Lisenfeld et al., 2016; Querejeta et al., 2021a). Braine et al. (2001)
used single-dish CO observations of eight TDGs to conclude that they follow
the same galaxy-averaged KS relation of usual galaxies. Boquien et al. (2011)
reached the same conclusion for a TDG candidate in the interacting system Arp
158. On the other hand, Lisenfeld et al. (2016) presented a spatially resolved
study of a TDG in the Virgo Cluster and found that it lies below the mean KS
relation. Finally, Querejeta et al. (2021a) analysed high-resolution ALMA
observations of a TDG in the interacting system Arp 94, and found that it may
lie either on or off the KS relation, depending on whether one considers the
whole CO flux or only the one associated to giant molecular clouds. The
diverse outcome of these works may point to different behaviour in the galaxy-
averaged and spatially-resolved KS relation and/or intrinsic differences in
the SF properties of individual TDGs, potentially related to their different
formation and evolutionary histories. New studies are needed to clarify the
situation.

In this paper, we study the spatially resolved KS relation in a sample of
three TDGs: NGC 5291N, NGC 5291S, and NGC 7252NW. We probe their molecular gas
content using CO(1-0) observations from the Atacama Large
Millimeter/submillimeter Array (ALMA), and their ionized gas content
(H$\alpha$ and H$\beta$ emission) using integral-field spectroscopy (IFS) from
the Multi-Unit Spectroscopic Explorer (MUSE) mounted on the Very Large
Telescope (VLT). New observations and ancillary data are described in Sect. 2.
Results on the gas distribution and kinematics as well as on the KS relation
are presented in Sect. 3. Finally, we summarize our findings in Sect. 4

##  2 Observations

###  2.1 Galaxy sample

We obtained high-resolution ALMA observations for three TDGs that were
selected from the sample of Lelli et al. (2015), based on the availability of
single-dish CO fluxes and high-quality H I maps. Two of them are part of the
NGC 5291 system; the remaining one is part of the NGC 7252 merger. The
location of these TDGs within the overall structure of the parent system can
be appreciated in Figure 1 of Lelli et al. (2015). Table 1 lists the general
properties of these TDGs.

NGC 5291 is a perturbed early-type galaxy that is surrounded by a giant H I
ring (Malphrus et al., 1997), suggesting a past head-on collision (Bournaud et
al., 2007). The system is located in the outer region of the galaxy cluster
Abell 3574. Early studies by Longmore et al. (1979) in the collisional ring of
NGC 5291 revealed the presence of star-forming regions out to 100 kpc from the
central galaxy. Subsequently, Duc & Mirabel (1998) found that these star-
forming complexes have similar sizes and SFRs of dwarf galaxies but higher
metallicities, suggesting a TDG origin. Three star-forming complexes (NGC
5291N, NGC 5291S and NGC 5291SW) are associated with strong H I concentrations
that display a velocity gradient decoupled from the underlying collisional
material, pointing to rotation within a self-gravitating potential well
(Bournaud et al., 2007; Lelli et al., 2015). In this paper, we focus on NGC
5291N and NGC 5291S because no single-dish CO observations are available for
NGC 5291SW.

NGC 7252 (also known as “Atoms for Peace”) is a late-stage merger remnant with
two gas-rich tidal tails extending to the east and north-west. Numerical
simulations (Borne & Richstone, 1991; Hibbard & Mihos, 1995; Chien & Barnes,
2010) were able to reproduce the observed morphology and kinematics of NGC
7252 from the merging of two disk galaxies. Hibbard et al. (1994) identified
two TDG candidates in the North-Western and Eastern tails (NGC 7252NW and NGC
7252E). These star-forming complexes were confirmed as bona-fide TDGs based on
their H$\alpha$ kinematics (Bournaud et al., 2004), H I kinematics (Lelli et
al., 2015) and relatively high gas metallicities (Lelli et al., 2015). In this
paper, we focus on NGC 7252NW because no single-dish CO observations are
available for NGC 7252SE.

###  2.2 ALMA data

The three TDGs were observed by the ALMA 12m array in January 2016 (Project
2015.1.00645.S; PI: F. Lelli). The time on source was about 2.2 hrs for both
NGC 5291N and NGC 5291S, and about 4.5 hrs for NGC 7252NW. We used ALMA band 3
with a mixed spectral setup, using four spectral windows with a bandwidth of
1875 MHz each. A high-resolution spectral window was centered at the frequency
of the redshifted CO($1-0$) line and covered with 3480 channels, providing a
spectral resolution of 976.6 kHz ($\sim$2.6 km s-1). Three low-resolution
spectral windows were centered around 99, 100, and 110 GHz to target the mm
continuum; they were covered with 128 channels providing a spectral resolution
of 31.250 MHz (ranging from $\sim$84 to $\sim$94 km s-1). The observations
were pointed at the H I kinematic center of the TDGs and have a field of view
of $\sim$50′′, set by the full-width half-maximum (FWHM) of the primary beam.

Table 2: Properties of CO data cubes with a channel width of $\sim$5 km s-1. Galaxy | Beam | Beam PA | $\sigma_{\rm cube}$
---|---|---|---
| (arcsec$\times$arcsec) | (degrees) | (mJy beam-1)
NGC 5291N | $2.1\times 1.7$ | 75.5 | 0.5
NGC 5291S | $2.1\times 1.8$ | -4.6 | 0.5
NGC 7252NW | $2.7\times 1.6$ | -8.3 | 0.5

The data reduction was performed with the Common Astronomy Software
Applications (Casa) package (McMullin et al., 2007). The $uv$ data were
flagged and calibrated using the standard Casa pipeline. Both continuum and
line data were imaged using the tclean task with a Högbom deconvolver and
Briggs weighting with a robust parameter of 0.5. Continuum images were
constructed by combining all four spectral windows, excluding channels with
line emission. Continuum emission was detected only in NGC 5291NW and
subtracted from the CO($1-0$) line channels using the task uvcontsub. The
properties of the CO($1-0$) line cubes are summarized in Table 2. In
particular, the spatial resolution (FWHM of the synthesized beam) is about 2′′
which corresponds to about 600 pc and 650 pc at the distances of NGC 5291 and
NGC 7252, respectively.

Figure 1: Gas distribution and kinematics in NGC 5291N (top), NGC 5291S
(middle), and NGC 7252NW (bottom). In all panels, the CO($1-0$) data are from
this work, while the other data come from Lelli et al. (2015). Left panels:
optical R$-$band image overlaid with the H I map (blue contours) and the
CO($1-0$) map (red contours). H I contours are the same as in Lelli et al.
(2015). CO contours are at (3, 6, 12, 24) $\sigma_{\rm map}$, where
$\sigma_{\rm map}=0.02$ Jy beam-1 km s-1 for NGC 7252NW and $\sigma_{\rm
map}=0.03$ Jy beam-1 km s-1 for both NGC 5291N and NGC 5291S. The cross shows
the H I kinematic center which was chosen as the ALMA pointing center. The
ALMA beam is shown by the red ellipse to the bottom-right corner; the H I beam
(not shown) is about 4 times larger for NGC 5291 and 6 times for NGC 7252.
Middle panels: H I velocity field overlaid with the CO map. The dashed line
shows the slit used to extract the PV diagram. The physical scale of 1 kpc is
indicated by the bar in the bottom-right corner. Right panels: PV diagrams
from the H I cube (blue colorscale) overlaid with those from the CO cube (red
contours). CO contours range from 3$\sigma_{\rm cube}$ to 15$\sigma_{\rm
cube}$ in steps of 3$\sigma_{\rm cube}$ (see Table 2).

CO intensity (moment-zero) maps were constructed by summing channels with CO
emission and are shown in Fig. 1. They are discussed in Sect. 3.1 for
illustrative purpose only: CO fluxes are measured by extracting integrated
spectra in various spatial regions as described in Sect. 3.2. No corrections
for the primary beam are applied in the moment maps, instead they are applied
to the derived fluxes as described in Sect. 3.2. Since the ALMA primary beam
is significantly larger than the CO emitting area, the primary beam
attenuation has little effect on the moment-zero maps, with the exception of
NGC 5291S. For this TDG, the pointing center was chosen in-between two main SF
complexes so that the CO emission in the Northern complex (the Southern is not
detected) is $\sim$10′′ offset from the pointing center (see Fig. 1). It is
also possible that the two SF complexes are two distinct objects, whose
individual kinematics cannot be discerned with the available H I observations
(Lelli et al., 2015). Correction for primary beam attenuation is described in
Sect. 3.2.

The CO emission is very compact and confined to one or two major clumps that
display no appreciable velocity gradients (Fig. 1), so moment-one and moment-
two maps are not very useful. We will discuss the CO kinematics using
position-velocity (PV) diagrams that provide the most direct representation of
the 3D data (Sect. 3.1). Before extracting integrated spectra, to enhance the
signal-to-noise (S/N) ratio of the CO line, we performed Hanning smoothing
over three spectral channels, giving a final velocity resolution of $\sim$10
km/s.

To investigate whether the ALMA interferometric observations may be missing
diffuse flux on large scales, we compared spatially-integrated CO spectra from
ALMA with those from previous single-dish observations (Braine et al., 2001).
The low S/N of the previous single-dish observations do not allow us to
quantify the amount of diffuse molecular gas missed by the ALMA
interferometric observations. The comparison, therefore, was inconclusive, but
we note that a substantial amount of diffuse CO emission has been found in
another TDG (J1023+1952) using ALMA total power (TP) and Atacama Compact Array
(ACA) observations in addition to the 12m array (Querejeta et al., 2021b). It
is possible, therefore, that our high-resolution observations are probing only
the densest CO emission and may be missing some flux on larger scales. ACA and
TP observations (or deep IRAM-30 observations) are needed to check this
possibility.

###  2.3 MUSE data

NGC 5291N was observed on 26 June 2014 (Program ID: 60.A-9320; PI: P.-A. Duc)
without adaptive optics (AO) for a total exposure time of 1800 sec. These
observations are presented in Fensch et al. (2016), who provides an in-depth
study of line ratios and gas metallicity. NGC 5291N was re-observed on 19 June
2017 during a MUSE AO commissioning run (Program ID: 60.A-9100(G)), but the AO
system could not significantly improve on the external seeing, so we will not
use these data.

NGC 5291S was observed on 22 January 2018 as part of Program ID 097.B-0152
(PI: M. Boquien) without AO for a total exposure time of 1800 sec. NGC 7252NW
was observed on 16 July 2017 during a commissioning run of the MUSE AO wide-
field mode (Program ID: 60.A-9100(H)) for a total exposure time of 900 sec.
All data were reduced using the MUSE pipeline v2.4 and following standard
procedures (Weilbacher et al., 2020). For NGC 5291S, the sky was estimated
using an offset field, while for NGC 7252NW, it was estimated within the
science exposure. We refer to Fensch et al. (2016) for further details on the
data reduction.

Figure 2: Spatial distribution of H$\alpha$ (left), H$\beta$ (middle), and
CO($1-0$) velocity-integrated emission (right) in NGC 5291N (top), NGC 5291S
(middle) and NGC 7252NW (bottom). The elliptical area marked with blue dashed
line shows the integration region used to compute the total H$\alpha$,
H$\beta$, and CO fluxes of the TDGs.

Moment-zero maps were constructed by summing channels with H$\alpha$ and
H$\beta$ emission. These maps are intended to show the morphology of the
ionized gas; line fluxes will be measured by extracting integrated spectra and
subtracting the stellar continuum (Sect. 3.2).

Figure 3: Integration regions for spatially resolved flux measurements in NGC
5291N (left), NGC 5291S (middle), and NGC 7252NW (right). The CO($1-0$)
intensity maps (red colorscale) are overlaid with the areas (green ellipses)
within which integrated spectra are extracted. In NGC 5291N and NGC 5191S, the
cyan crosses indicate the location of the “secure” massive star clusters
identified by Fensch et al. (2019) using HST images. The size of the ellipse
shown in the bottom-left corner is equivalent to the CO($1-0$) beam. Red
contours are the same as in Figure 1. Figure 4: The CO, H$\alpha$+[N II], and
H$\beta$ spectra of the star-forming region 9 in NGC 5291N. Red vertical lines
show the range over which the spectra are integrated.

##  3 Results

###  3.1 Distribution and kinematics of multiphase gas

Figure 1 compares the distribution and kinematics of molecular (CO) and atomic
(H I) gas in our sampled TDGs. In all galaxies, the detected CO emission is
much more compact than the H I emission (left panels) and associated with
intense SF, as we describe later on. The CO extent is of the order of $1-2$
kpc, while the H I disks have diameters ranging from $\sim$10 to $\sim$15 kpc
(Lelli et al., 2015). The CO emission lies near the edges of the H I peaks, so
there is no direct correspondence between the atomic and molecular gas
distribution on kpc scales. Moreover, the CO emission is not at the dynamical
center of the TDGs, so it cannot be used to trace the underlying large-scale
gas rotation (middle panels). The same behaviour is often seen in typical
dwarfs and in the outskirts of spirals, where the H I emission traces the
object more globally, and there may be regions where atomic gas can cool,
condense into H2, and SF can commence (e.g., Hunt et al., 2023).

The atomic gas mass of TDGs outweighs the molecular gas mass by factors of
$3-30$ within the CO emitting area (see Table 1) and the stellar mass by
factors of $5-15$ (see Table 5 in Lelli et al., 2015), so the H I center
corresponds to the center of mass (in the case of negligible dark matter).
Interestingly, position-velocity (PV) diagrams along the CO distribution
(right panels) show that there is good agreement between CO and H I line-of-
sight velocities, suggesting that the two gas phases are dynamically coupled.
The most likely interpretation is that molecular gas is currently forming out
of atomic gas, keeping the same kinematics.

Figure 2 compares the distribution of molecular and ionized gas, specifically
the H$\alpha$ and H$\beta$ lines that will be used to measure the SFRs.
Molecular and ionized gas are roughly co-spatial on kpc scales but display
different morphologies, so the spatial relation between current star-formation
activity and molecular gas reservoir is complex. Importantly, both molecular
and ionized gas are much more compact than the atomic gas and similarly
displaced from the H I dynamical center. Thus, no firm statement on the large-
scale dynamics of TDGs can be inferred from H$\alpha$ emission. The same
conclusion was drawn by Lelli et al. (2015) for NGC 7252NW using H$\alpha$
data from GIRAFFE, which is in good agreement with the new MUSE data. On the
contrary, Flores et al. (2016) inferred strong conclusions on the dynamics of
the TDGs in NGC 5291 using H$\alpha$ data from GIRAFFE. Fig. 1 and Fig.2 show
that the bright H$\alpha$ emission cannot be used to trace the large-scale gas
kinematics of TDGs.

The H I emission appears to be the most extended and most promising tracer to
probe the internal dynamics in TDGs, provided that the kinematically decoupled
part can be discerned from the tidal tails and properly resolved. The H
I-to-H2 conversion and the most intense SF activity, however, do not occur
exactly at the dynamical center, possibly due to variable local conditions
such as gas pressure, temperature, and volume density. A similar situation
occurs in “regular” starburst dwarfs, such as blue compact dwarfs (BCDs),
which often show offsets between the peak SF activity and the dynamical center
of the galaxy (e.g., Lelli et al., 2014b).

###  3.2 Emission-Line Measurements

We combine $\Sigma_{\rm SFR}$ traced by H$\alpha$ emission with $\Sigma_{\rm
mol}$ traced by CO emission to investigate the spatially resolved KS relation.
We define a set of independent elliptical apertures with major and minor axes
matching the FWHM of the ALMA synthesized beam (see Figure 3). We then extract
integrated CO, H$\alpha$, and H$\beta$ spectra within each elliptical aperture
from the ALMA and MUSE cubes, respectively. This is nearly equivalent to
smoothing and/or re-binning the MUSE data to the lower spatial resolution of
the ALMA data. The key advantage of this procedure is to ensure independent
measurements of $\Sigma_{\rm SFR}$ and $\Sigma_{\rm mol}$ because the
elliptical regions are equal to or larger than the angular resolution. The
ellipses are chosen to cover the CO emission down to a contour with
$\rm{S/N=3}$; for simplicity, they are oriented in the North-South direction
rather than along the PA of the ALMA beam, but this choice has no appreciable
effects on the final results. Using a circular aperture would also make no
difference in our general results. We have 14, 8, and 6 apertures for NGC
5291N, NGC 5291S, and NGC 7252NW, respectively, for a total of 28 independent
measurements.

We extract the optical and CO($1-0$) spectra of each region using Casa. For
each H$\alpha$ and H$\beta$ spectra, we subtract continuum emission by
calculating the mean continuum flux from two narrow spectral regions on either
side of the emission profile. In all selected regions, the CO($1-0$),
H$\alpha$, and H$\beta$ lines are detected with a peak S/N ratio higher than
3.5, apart from region 8 of NGC 5291S in which H$\beta$ emission is
undetected. Figure 4 shows an example of the spectra from one region in NGC
5291N.

The H$\alpha$ and H$\beta$ lines have a nearly Gaussian shape, but the CO
lines do not (see Fig. 4). Most likely, the Gaussian shape of the MUSE
profiles is driven by the instrumental spectral resolution (FWHM of $\sim$80
km s-1 around the H$\alpha$ line), while the non-Gaussian shape of CO profiles
is intrinsic, given the ALMA spectral resolution of $\sim$10 km s-1 (after
Hanning smoothing). Rather than fitting a Gaussian function, therefore, we use
direct integration to estimate the integrated flux from the emission lines of
interest. The starting and ending frequency of integration were defined by
visual inspection; they are typically about 100 km s-1 wide. In regions with
low S/N, it is not trivial to identify the proper integration range. This is
especially the case for the H$\alpha$ line because the [N II] doublet blends
with the noise. In such cases, to avoid contamination from [N II] lines, we
estimate the redshifted wavelength of the [N II] doublet and consider
appropriate frequency ranges excluding the [N II] emission.

To extract CO spectra and measure CO fluxes, we used cubes that are _not_
corrected for the ALMA primary-beam attenuation because these cubes have
uniform and well-defined noise structure. In primary-beam-corrected cubes,
indeed, the noise varies from pixel to pixel, so it is challenging to define
the S/N ratio of the line. To recover the correct CO fluxes, instead, we use
the primary-beam map and compute the average primary-beam correction within
each region, then multiply the uncorrected CO flux by this value. Table 3
summarizes all our measurements.

Table 3: Line fluxes, molecular masses, and SFRs within the independent regions identified in Fig. 3. TDG | Region | SCO $\Delta\nu$ | Mmol | FHα | FHβ | Extinction | SFR
---|---|---|---|---|---|---|---
|  | (Jy km s${}^{-1})$ | (106 M⊙) | (10-16 ergs s-1 cm-2) | (10-16 ergs s-1 cm-2) | (mag) | ($M_{\odot}$ $yr^{-1}$)
| 1 | 0.03 | 1.0 | 1.2 | 0.4 | 0.4 | 0.0006
| 2 | 0.04 | 1.4 | 6.0 | 1.9 | 0.3 | 0.003
| 3 | 0.07 | 2.8 | 4.0 | 1.2 | 0.5 | 0.002
| 4 | 0.04 | 1.4 | 1.1 | 0.3 | 0.6 | 0.0007
| 5 | 0.04 | 1.5 | 0.9 | 0.2 | 0.8 | 0.0007
| 6 | 0.04 | 1.5 | 2.1 | 0.6 | 0.5 | 0.001
NGC 5291N | 7 | 0.07 | 2.7 | 65.4 | 18.4 | 0.5 | 0.04
| 8 | 0.03 | 1.1 | 15.7 | 4.7 | 0.4 | 0.008
| 9 | 0.2 | 7.4 | 111.6 | 29.8 | 0.7 | 0.08
| 10 | 0.2 | 6.5 | 161.2 | 41.1 | 0.8 | 0.1
| 11 | 0.04 | 1.5 | 7.1 | 2.1 | 0.4 | 0.004
| 12 | 0.08 | 3.3 | 66.5 | 17.2 | 0.8 | 0.05
| 13 | 0.04 | 1.5 | 11.8 | 3.6 | 0.4 | 0.006
| 14 | 0.03 | 1.3 | 14.1 | 4.2 | 0.4 | 0.007
| 1 | 0.03 | 1.2 | 55.0 | 14.5 | 0.7 | 0.04
| 2 | 0.05 | 1.9 | 21.6 | 6.0 | 0.6 | 0.01
| 3 | 0.04 | 1.5 | 7.6 | 2.3 | 0.3 | 0.004
NGC 5291S | 4 | 0.05 | 2.1 | 24.8 | 7.1 | 0.5 | 0.01
| 5 | 0.05 | 2.0 | 4.7 | 1.0 | 1.3 | 0.006
| 6 | 0.04 | 1.4 | 28.2 | 8.3 | 0.5 | 0.02
| 7 | 0.04 | 1.6 | 22.7 | 6.5 | 0.5 | 0.01
| 8 | 0.03 | 1.3 | 0.3 | - | - | 0.0001
| 1 | 0.03 | 1.5 | 6.9 | 1.9 | 0.6 | 0.005
| 2 | 0.03 | 1.6 | 1.8 | 0.5 | 0.6 | 0.001
NGC 7252NW | 3 | 0.05 | 2.1 | 0.4 | 0.08 | 1.3 | 0.0005
| 4 | 0.1 | 5.2 | 2.0 | 0.5 | 1.0 | 0.002
| 5 | 0.1 | 5.3 | 2.0 | 0.4 | 1.3 | 0.003
| 6 | 0.05 | 2.1 | 0.6 | 0.2 | 0.8 | 0.0006

###  3.3 Molecular gas masses & star-formation rates

We convert the CO line flux of each region (S${}_{\rm CO}\Delta\nu$) to a
total molecular gas mass (Mmol, including Helium and heavier elements)
assuming the Milky-Way CO-to-$M_{\rm mol}$ conversion factor $\alpha_{\rm CO}$
= 4.3 M⊙ (K km s-1 pc2)-1 or equivalently $X_{\rm CO}=2\times 10^{20}$ cm-2 (K
km s-1)-1. This corresponds to the following equation (Bolatto et al., 2017):

$M_{\rm mol}=1.05\times 10^{4}\,\frac{S_{\rm CO}\Delta\nu\,D_{\rm
L}^{2}}{(1+z)},$ (3)

where $M_{\rm mol}$ is in units of M⊙, $S_{\rm CO}\Delta\nu$ is in units of Jy
km s-1, $D_{\rm L}$ is the luminosity distance in Mpc and $z$ is the redshift.
The choice of conversion factor in Eq. 3 holds for disk galaxies similar to
the Milky Way. As discussed in Sect. 1, the same conversion factor is expected
to hold in TDGs because they retain the metallicity of the parent spiral
galaxies. Clearly, this is a simplifying assumption because the value of
$X_{\rm CO}$ varies even within “normal” disk galaxies and is known to
increase with decreasing metallicity (Bolatto et al., 2013). The metallicities
of our TDGs range from half solar to solar (Duc & Mirabel, 1998; Lelli et al.,
2015), so the variation in $X_{\rm CO}$ is expected to be null or small,
depending on the adopted model (cf. Bolatto et al., 2013). In the worst case
scenario, in some star-forming regions of our TDG sample, Mmol may be
underestimated by a factor of $\sim$2$-$3\. The molecular mass surface density
($\Sigma_{\rm mol}$) is derived by dividing Mmol by the ellipse area.

The H$\alpha$ flux provides an instantaneous measure of the SFR as nebular
emission is produced around young massive stars with masses greater than 10 M⊙
and lifetimes shorter than $10-20$ Myr (Kennicutt Jr, 1998a). SFR tracers
probing longer timescales have been studied and discussed in Boquien et al.
(2007, 2009, 2010). The primary contributor to systematic errors in
H$\alpha$-based SFRs is dust extinction, which can be accounted for by using
the Balmer decrement H$\alpha$/H$\beta$. We follow Bolatto et al. (2017) to
estimate the nebular extinction AHα:

$A_{\rm H\alpha}=5.86\,\log\,\frac{F_{\rm H\alpha}}{2.86\,F_{\rm H\beta}},$
(4)

where $F_{\rm H\alpha}$ and $F_{\rm H\beta}$ are the integrated fluxes. Here,
the case B recombination value of intrinsic Balmer decrement is considered to
be 2.86 as suggested by Storey & Hummer (1995) for H II regions at typical
electron temperatures and densities. The extinction corrected SFR was computed
as

${\rm SFR}=7.9\times 10^{-42}\,L_{\rm H{\alpha}}\,10^{({A_{\rm
H\alpha}/2.5})},$ (5)

where SFR is in units of M⊙ yr-1 and $L_{\rm H{\alpha}}$ is the luminosity of
H${\alpha}$ in units of ergs s-1. This equation assumes solar abundance and a
Salpeter initial mass function (IMF) with a mass range of 0.1 to 100 M⊙
(Kennicutt Jr, 1998a). Finally, we compute $\Sigma_{\rm SFR}$ (in units of M⊙
yr-1 kpc-2) dividing the SFR by the area of the region (equivalent to the beam
size, see Table 2).

###  3.4 The Kennicutt-Schmidt relation

As a first step, we locate our three TDGs on the spatially integrated KS
relation, which compares the total gas surface density (atomic plus molecular)
with the SFR. We use the data from Kennicutt & Evans (2012) because they are
internally self-consistent with our data: the SFRs are measured from
extinction-corrected H$\alpha$ fluxes and the molecular gas masses assume the
MW $X_{\rm CO}$ factor (cf. with Sect. 3.3). Figure 5 shows that TDGs follow
the same KS relation as “normal” galaxies, in agreement with earlier results
from Braine et al. (2001) and Boquien et al. (2011) using single-dish CO
observations. Notably, if we consider only molecular gas masses, TDGs would
strongly shift to the left of the relation because the total gas mass is
heavily dominated by atomic gas, unlike typical spiral galaxies.

Next, we study the spatially resolved SF relation (e.g., Bolatto et al., 2017;
Pessa et al., 2021; Lin et al., 2020). We compare our TDGs to 14 spiral
galaxies from the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey
(Lin et al., 2019, 2020). ALMaQUEST data represents the ideal comparison
sample because (i) molecular gas masses are estimated using CO($1-0$) data
from ALMA as in our work, (ii) SFRs are estimated using extinction-corrected
H$\alpha$ fluxes from IFS similar to our work, (iii) the same calibrations
have been adopted (Eqs. 3 and 5), and (iv) the angular resolution of ALMaQUEST
data ($\sim$2.5′′) is similar to that of our data ($\sim$2′′) albeit the
ALMaQUEST physical resolution ranges from 0.5 - 6.5 kpc depending on the
galaxy distance while our physical resolution is fixed at $\sim$0.6 kpc. In
addition, unlike other samples (Bolatto et al., 2017; Pessa et al., 2021), the
ALMaQUEST data covers low gas surface densities similar to those in TDGs.

Figure 5: The location of TDGs (blue diamond, yellow square, and pink
pentagon) on the spatially integrated Kennicutt-Schmidt relation (grey
symbols, from Kennicutt & Evans, 2012). The open symbols show the location of
TDGs if one considers only molecular gas, neglecting atomic gas. The red line
shows the best-fit line to the data; the dashed magenta lines correspond to
$\pm$3$\sigma_{\rm obs}$ where $\sigma_{\rm obs}$ is the observed vertical
scatter. Figure 6: The location of TDGs (blue diamonds, yellow squares, and
pink pentagons) on the spatially resolved Kennicutt-Schmidt relation from the
ALMaQUEST survey (cyan circles, from Lin et al. 2019). The solid red line
shows the best-fit line to the ALMaQUEST data; the dashed magenta lines
correspond to $\pm$3$\sigma_{\rm obs}$ where $\sigma_{\rm obs}=0.23$ dex is
the observed vertical scatter. Symbols with a thick border correspond to
regions in which young massive star clusters have been identified (see Fig.
3).

Unfortunately, we cannot study the spatially resolved SF relation considering
the total gas surface densities (molecular plus atomic gas) because the
angular resolution of the existing H I data of TDGs is too coarse. In fact,
the entire CO emitting area is within one H I beam. The ALMaQUEST data,
however, do not consider the H I surface densities as well, so our comparison
in Fig. 6 is self-consistent.

Figure 6 shows the location of TDGs on the spatially resolved SF relation from
ALMaQUEST. We fit the ALMaQUEST data with a linear relation using the Markov-
Chain Monte-Carlo (MCMC) software BayesLineFit (Lelli et al., 2019). The MCMC
fit returns a slope of 1.024 $\pm$ 0.008, an intercept of -2.964 $\pm$ 0.009,
and a vertical observed scatter $\sigma_{\rm obs}=0.23$ dex. The majority of
TDG points (16/27) lie on the same SF relation as spiral galaxies within
$\pm$3$\sigma_{\rm obs}$. The correlation between $\Sigma_{\rm SFR}$ and
$\Sigma_{M_{\rm mol}}$, however, is not evident when considering only TDG
data. On the one hand, this occurs because most TDG points cover a small
dynamic range in gas surface density (less than 1 dex) and the SF relation has
substantial scatter at fixed $\Sigma_{\rm mol}$. On the other hand, TDG data
display a larger scatter from the fitted line than the ALMaQUEST data: 0.7 dex
considering all TDG points and 0.4 dex considering only those within
$\pm$3$\sigma_{\rm obs}$. Indeed, only $\sim$30%-35% of TDG points lie within
$\pm$1$\sigma_{\rm obs}$ of the best-fit relation rather than the expected
68$\%$ for a Gaussian distribution.

The high scatter of TDG points may be due to small-number statistics or
systematic differences between our work and the ALMaQUEST analysis. If real,
instead, it could point to (i) the need of considering the total gas surface
density (atomic plus molecular gas) in H I-dominated galaxies, as in the case
of the spatially integrated SF relation in Fig. 5, (ii) high stochasticity in
the SF history of TDGs on small spatial scales (e.g., Boquien et al., 2010),
(iii) spatial variations in the $X_{\rm CO}$ factor due to additional effects
(beyond gas metallicity) such as gas temperature, gas pressure, and UV
background (e.g., Bolatto et al., 2013), (iv) differences in 3D volume
densities due to line-of-sight integration and variable disk thickness (e.g.,
Bacchini et al., 2019, 2020). Given the complex evolutionary status of TDGs,
which are possibly out of dynamical equilibrium (Lelli et al., 2015), it is
difficult to distinguish between these possibilities.

Interestingly, a substantial fraction of TDG regions (10/28) strongly deviate
from the observed SF relation and lie in the starburst zone above
$+$3$\sigma_{\rm obs}$. These starburst regions belong to NGC 5291N and NGC
5291S. Studies with the Hubble Space Telescope (HST) shows that these regions
are currently forming young star clusters with masses ranging from a few
$10^{3}$ M⊙ to a few $10^{5}$ M⊙ and ages from $\sim$1 Myr to $\sim$100 Myr
(Fensch et al., 2019). Thus, it is sensible that these areas have
exceptionally high SFEs.

Unlike NGC 5291N and NGC 5291S, NGC 7252NW does not show starburst regions
with high SFE, but most of its SF regions (4/6) fall below the average SF
relation. Consistently, visual inspection of the available HST images of NGC
7252NW does not reveal any clear young star cluster. Another two TDGs with
spatially resolved CO data (VCC 2062 from Lisenfeld et al. 2016 and J1023+1952
from Querejeta et al. 2021b) were also found to lie systematically below the
average SF relation, albeit Querejeta et al. (2021b) warn that the inclusion
or exclusion of diffuse CO emission (not contained in giant molecular clouds)
could result into a large difference. The different behaviours shown by
different TDGs may be related to the evolutionary status of the parent system
and the “age” of the specific TDG. According to numerical simulations, the gas
ring around NGC 5291 was formed by a head-on galaxy collision about $\sim$360
Myr ago (Bournaud et al., 2007). On the other hand, NGC 7252 is a late-stage
merger resulting from the interaction of two spiral galaxies about $\sim$700
Myr ago (Hibbard & Mihos, 1995; Chien & Barnes, 2010). One may speculate,
therefore, that the TDGs around NGC 5291 are young and experiencing a period
of peak SF activity due to efficient H I-to-H2 conversion, whereas those
around NGC 7252 are slightly older and more quiescent. A larger sample of TDGs
around more diverse interacting systems, together with detailed numerical
simulations of the system, is needed to study the relation between the
interaction stage and SF activity in tidal debris.

Broadly speaking, star-forming galaxies can be classified into “normal” and
“starbursts” using a 3$\sigma$ threshold from the mean KS relation. With such
a definition, starbursts represent a SF process that occurs with less than
99.7% chance for a Gaussian distribution of SFEs. The SF regions in our TDG
sample display a continuous range of SFEs, but a very large fraction of them
($\sim$40$\%$) proceed in starburst mode, resulting in molecular gas depletion
times as short as $10-100$ Myr. In the remaining $\sim$60$\%$ of TDG regions,
the SF activity proceeds in a similar way as in normal spiral galaxies,
regardless of the different environmental conditions. In this sense, TDGs are
“hybrid” systems because they contain some regions behaving as normal galaxies
and others as starbursts.

###  3.5 Timescales and evolution of TDGs

A spatially resolved KS relation with a slope of one (as observed) implies
that the molecular gas depletion time ($t_{\rm mol}=M_{\rm mol}$/SFR) is
nearly constant across spiral galaxies. Then, the intercept and observed
scatter imply that $t_{\rm mol}\simeq 1\pm 0.5$ Gyr. For our three TDGs, the
molecular gas depletion times are substantially smaller: 100 Myr for NGC
5291N, 70 Myr for NGC 5291S, and 300 Myr for NGC 7252NW. The molecular gas of
these TDGs, therefore, will soon be consumed by the SF activity unless it is
replenished by efficiently converting the substantial H I reservoir into H2
gas.

Considering the total H I mass associated with the TDG potential well (Lelli
et al., 2015, their Table 8), the atomic gas depletion time ($t_{\rm
atom}=M_{\rm atom}$/SFR) is about 2 Gyr for NGC 5291N, 4 Gyr for NGC 5291S,
and 7 Gyr for NGC 7252NW. These values of $t_{\rm atom}$ are substantially
smaller than those of low-surface-brightness (LSB) star-forming galaxies,
ranging between $10-100$ Gyr (e.g., McGaugh et al., 2017), but are comparable
to those of starburst dwarf galaxies (Lelli et al., 2014a), such as blue
compact dwarfs (BCDs). It is conceivable that both TDGs and BCDs are only able
to sustain the intense SF activity for a short period of time ($\sim$0.5-1
Gyr, e.g., McQuinn et al. 2010), so the starburst will not have enough time to
consume their entire H I reservoir. For example, if their SFR is going to
decrease in the next 500 Myr by a factor of $\sim$10, their $t_{\rm atom}$
will increase by a similar factor, reaching the high values observed in LSB
galaxies. Furthermore, there is a diffuse H I reservoir in the tidal debris
around the TDGs, which might replenish their gas content.

Another interesting timescale is the stellar mass growth time
($t_{\star}=M_{\star}$/SFR), which we compute using the stellar masses from
Lelli et al. (2015, their Table 8). For NGC 7252, we find $t_{\star}\simeq
940$ Myr. This is larger than the dynamical timescale of the galaxy merger
($\sim$700 Myr) inferred from numerical simulations (Chien & Barnes, 2010).
Assuming that the age of the TDG is equal to that of the merger, the current
SFR cannot explain the present-day stellar mass: the SFR was probably higher
in the past, indicating that the SF activity has been declining over time. A
possible caveat is that NGC 7252NW may contain old stars from the disk of the
parent galaxies, which would contribute to $M_{\star}$ beyond the mass formed
over the past 700 Myr. In any case, the situation of NGC 5291 appears
different: we find $t_{\star}\simeq 180$ Myr for NGC 5291N Myr and
$t_{\star}\simeq 270$ Myr for NGC 5291S, which are smaller than the dynamical
timescale of the galaxy collision ($\sim$360 Myr, Bournaud et al. 2007). Thus,
the current SFR can amply explain the present-day $M_{\star}$ of these two
TDGs. These facts are in line with the speculation in Sect. 3.4 that the TDGs
around NGC 5291 may be representative of early galaxy formation with efficient
H I-to-H2 conversion, high SFE, and short gas depletion times, while those
around NGC 7252 may represent a subsequent stage with more typical SF
activity. In addition, the initial conditions in the two systems may have been
different: while the TDGs around NGC5291 were born out of pure gaseous
condensation, those around NGC7252 may have been born in a less H I dominated
environment with both gas and stars from their progenitors.

##  4 Conclusions

We studied the molecular and ionized gas content of three bona-fide TDGs using
CO($1-0$) observations from ALMA and IFS data from MUSE. For the first time,
we locate TDGs on the spatially resolved KS relation. Our results can be
summarized as follows:

  1. 1.

CO($1-0$) and H$\alpha$ emissions in TDGs are very compact and cover a much
smaller area than the H I emission. Both CO and H$\alpha$ lines are not
suitable to study the internal kinematics of these TDGs. Most likely,
molecular gas is forming out of the more extended H I disk of TDGs, having
similar line-of-sight velocities.

  2. 2.

TDGs lie on the same spatially-integrated $\Sigma_{\rm SFR}-\Sigma_{\rm gas}$
relation of spiral galaxies but display a substantial scatter on the spatially
resolved $\Sigma_{\rm SFR}-\Sigma_{\rm mol}$ relation (which neglects atomic
gas due to the lack of high-resolution H I data).

  3. 3.

The majority (60$\%$) of SF regions in TDGs lie on the same spatially resolved
SF relation as spiral galaxies within $\pm$3 times the observed scatter but
display a larger dispersion from the mean relation. A substantial fraction
($\sim$40$\%$) of SF regions have exceptionally high SF efficiencies, lying in
the starburst regime of the KS relation. These regions belong to NGC 5291N and
NGC 5291S, and are associated with the formation of massive super star
clusters, which were previously identified by HST imaging (Fensch et al.,
2019).

The growing evidence about the existence of a fundamental SF relation compels
us to put the relation to test in a variety of star-forming environments. The
three TDGs analyzed here confirm the fundamental nature of the KS relation on
sub-kpc scales, albeit regions with exceptionally high SF efficiencies do
exist. Future studies may investigate the spatially resolved KS relation in
other bona-fide TDGs, probing even more diverse star-forming environments,
such as younger and/or older tidal debris.

## Acknowledgements

We are grateful to the anonymous referee for the useful comments that helped
to improve the paper. We thank Rob Kennicutt and Lin Li-Hwai Lin for providing
the KS-relation data in tabular form. N.K. and F.L. thank the School of
Physics and Astronomy of Cardiff University, where this work started as part
of a Master thesis project. N. K. acknowledges support from the Programa de
doctorado en Astrofísica y Astroinformática of Universidad de Antofagasta.
M.B. acknowledges support from FONDECYT regular grant 1211000 and by the ANID
BASAL project FB210003. U.L. acknowledges support by the research projects
AYA2017-84897-P and PID2020-114414GB-I00 from the Spanish Ministerio de
Economía y Competitividad, from the European Regional Development Funds
(FEDER) and the Junta de Andalucía (Spain) grants FQM108. This paper makes use
of the following ALMA data: ADS/JAO.ALMA#2015.1.00645.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. Based on observations collected at the European
Organisation for Astronomical Research in the Southern Hemisphere under ESO
MUSE program 60.A-9320(A) and 097.B-0152(A). Based on public data released
from the MUSE WFM-AO commissioning observations at the VLT under Programme IDs
60.A-9100 runs G & H.

## Data Availiability

The raw data used in this work are available in the ALMA and ESO archives.
Reduced data are available on request.

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