# The Lyman Alpha Reference Sample XIII:
High-Angular Resolution 21cm HI observations of Ly${\alpha}$ emitting galaxies

Alexandra Le Reste The Oskar Klein Centre, Department of Astronomy, Stockholm
University, AlbaNova, SE-10691 Stockholm, Sweden.  Matthew Hayes The Oskar
Klein Centre, Department of Astronomy, Stockholm University, AlbaNova,
SE-10691 Stockholm, Sweden.  John M. Cannon Department of Physics and
Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA.
Edmund Christian Herenz European Southern Observatory, Av. Alonso de Córdova
3107, 763 0355 Vitacura, Santiago, Chile  Jens Melinder The Oskar Klein
Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691
Stockholm, Sweden.  Veronica Menacho The Oskar Klein Centre, Department of
Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden.  Göran
Östlin The Oskar Klein Centre, Department of Astronomy, Stockholm University,
AlbaNova, SE-10691 Stockholm, Sweden.  Johannes Puschnig The Oskar Klein
Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691
Stockholm, Sweden.  Argelander-Institut für Astronomie, Auf dem Hügel 71,
53121, Bonn, Germany.  T. Emil Rivera-Thorsen The Oskar Klein Centre,
Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm,
Sweden.  Daniel Kunth  Institut d’Astrophysique de Paris, 98 bis Boulevard
Arago, F-75014 Paris, France  Nick Velikonja  Department of Physics and
Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA.

(Received May 19, 2021; Accepted June 9, 2022)

###### Abstract

The Ly${\alpha}$ emission line is one of the main observables of galaxies at
high redshift, but its output depends strongly on the neutral gas distribution
and kinematics around the star-forming regions where UV photons are produced.
We present observations of Ly${\alpha}$ and 21-cm HI emission at comparable
scales with the goal to qualitatively investigate how the neutral interstellar
medium (ISM) properties impact Ly${\alpha}$ transfer in galaxies. We have
observed 21-cm HI at the highest angular resolution possible ($\approx 3$″
beam) with the VLA in two local galaxies from the Lyman Alpha Reference
Sample. We contrast this data with HST Ly${\alpha}$ imaging and spectroscopy,
and MUSE and PMAS ionized gas observations. In LARS08, high intensity
Ly${\alpha}$ emission is co-spatial with high column density HI where dust
content is the lowest. The Ly${\alpha}$ line is strongly redshifted,
consistent with velocity redistribution which allows Ly${\alpha}$ escape from
high column density neutral medium with low dust content. In eLARS01, high
intensity Ly${\alpha}$ emission is located in regions of low column density
HI, below the HI data sensitivity limit ($<2\times 10^{20}\,$cm-2). The
perturbed ISM distribution with low column density gas in front of the
Ly${\alpha}$ emission region plays an important role in the escape. In both
galaxies, the faint Ly${\alpha}$ emission ($\sim 1\times
10^{-16}$erg.s-1cm-2arcsec-2) traces intermediate H$\alpha$ emission regions
where HI is found, regardless of the dust content. Dust seems to modulate, but
not prevent, the formation of a faint Ly${\alpha}$ halo. This study suggests
the existence of scaling relations between dust, H$\alpha$, HI, and
Ly${\alpha}$ emission in galaxies.

Interstellar atomic gas – Lyman-alpha galaxies – Starburst galaxies –
Interacting galaxies

††journal: ApJ††facilities: VLA, HST(ACS,COS), VLT:Yepun(MUSE), PMAS,
IRAM:NOEMA, SMA, ALMA ††software: Astropy (Astropy Collaboration et al., 2013,
2018), casa (McMullin et al., 2007)

##  1 Introduction

The Ly${\alpha}$ spectral line is one of the main probes of galaxies in the
high-redshift Universe, playing a central role in their detection and redshift
determination through either narrow-band filter or spectroscopic observations.
The possibility of using this line as a tracer of the first galaxies was
initially suggested by Partridge & Peebles (1967): it is indeed one of the
strongest intrinsic features in galaxy spectra, with 68% of ionizing radiation
from star-forming regions being reprocessed into the line, assuming case B
recombination (Dijkstra, 2019). Since the rest wavelength of the line at
$\lambda=1215.67$Å is located in the ultraviolet (UV) range, it is redshifted
out of atmospheric absorption and becomes observable from ground-based
facilities for $z\gtrsim 2$ objects, making the line easier to observe for
high-redshift objects.

Corresponding to the spontaneous transition between the first excited state of
the Hydrogen atom and its ground state, the Ly${\alpha}$ line is a resonant
transition and is absorbed and then shortly re-emitted by any neutral Hydrogen
atom it encounters, resulting in a scattering of the line both spatially and
spectrally. Since the rest-frame wavelength of the Ly${\alpha}$ line is
located in the UV range it is prone to dust absorption, which effectively
removes the absorbed photons from the Ly${\alpha}$ output of the galaxy. These
interactions with the interstellar medium (ISM) of galaxies result in a
complex radiative transfer of the line, that must be understood for the
Ly${\alpha}$ spectra of high-z galaxies to be interpreted correctly and to be
used to their maximum potential. However, studying the details of the ISM
conditions is not possible in the high-redshift Universe where only global
properties can be obtained, due to the loss of resolution and depth suffered
by observations. Moreover, no facilities will be built on the current horizon
that will detect 21cm emission directly from galaxies at the highest
redshifts.

The Lyman Alpha Reference Samples (LARS and eLARS) are two samples containing
42 local (z = 0.028 - 0.18) star forming galaxies that have been assembled to
study the parameters impacting the Ly${\alpha}$ radiative transfer in galaxies
(Östlin et al., 2014; Runnholm et al., 2020, Melinder et al., in prep.).
Originally based on Hubble Space Telescope (HST) imaging of the Ly${\alpha}$,
H$\alpha$ and FUV continuum emission as well as HST Cosmic Origin Spectrograph
(COS) spectroscopy, the number of additional observations has grown to include
a tremendous amount of supporting data allowing an in-depth investigation of
the galaxy properties that influence Ly${\alpha}$ emission. These observations
comprise deep optical imaging (Micheva et al., 2018), ionized gas imaging and
kinematics using the PMAS (Herenz et al., 2016) and MUSE Integral Field Units
(IFUs), Infrared observations using SOFIA and Herschel, molecular gas
observations with the IRAM 30m telescope and the Atacama Pathfinder EXperiment
(APEX) (Puschnig et al., 2020) and neutral gas imaging and kinematics with the
Green Bank Telescope (GBT) and the Karl G. Jansky Very Large Array (VLA); see
Pardy et al. (2014) and Le Reste et al. (in preparation).

The galaxies of the sample have high star formation rates (see Section 2) and
are close in properties to Lyman Break Galaxies (LBGs), making them analogs to
some high redshift galaxies (Hayes et al., 2014; Östlin et al., 2014). Several
studies have indeed pointed to compact star forming galaxies as local Universe
proxies for high-redshift star-forming galaxies (e.g., Stanway & Davies, 2014;
Henry et al., 2015; Izotov et al., 2015, 2021). The LARS and eLARS samples
therefore present an unprecedented and comprehensive multi-wavelength view on
the key properties driving Ly${\alpha}$ escape that can be reasonably compared
to galaxies at higher redshift.

Due to the resonant nature of the line, the Ly${\alpha}$ emission output is
expected to depend strongly on a combination of the content, geometry
(Giavalisco et al., 1996) and kinematics of the HI gas in the galaxy (Kunth et
al., 1998; Mas-Hesse et al., 2003; Wofford et al., 2013). High HI column
densities are thought to increase Ly${\alpha}$ scattering and thus the path
length of Ly${\alpha}$ photons. In the case where dust is present, this
severely increases the probability of Ly${\alpha}$ destruction. Therefore
processes disrupting the neutral ISM have been proposed to explain the escape
of Ly${\alpha}$ photons out of galaxies. Such processes include turbulence
(shifting the Ly${\alpha}$ photons out of resonance) or feedback (carving
channels in the ISM that lower the column density seen by the affected
Ly${\alpha}$ photons); (Keenan et al., 2017; Bik et al., 2018; Puschnig et
al., 2020). For a review on the parameters impacting Ly${\alpha}$ escape in
local galaxies, see Hayes (2019).

Neutral gas has previously been imaged at low resolution for a subset of the
galaxies of the LARS sample ($\sim$ 46″), thus allowing the comparison of the
global HI and Ly${\alpha}$ properties of galaxies (Pardy et al., 2014).
However, low-angular resolution observations of 21-cm HI trace the gas on
scales that are hardly comparable with those characterising the Ly${\alpha}$
emission at these distances. The neutral medium of the LARS galaxies was also
traced with metal absorption lines (Rivera-Thorsen et al., 2015), allowing a
comparison of Ly${\alpha}$ emission and the surrounding gas in one aperture
for each galaxy. UV absorption lines can be directly compared to Ly${\alpha}$
but most lines trace both the HI and HII gas, and are not unambiguous. For
example, CII and SiII are commonly used lines which have ionization potentials
of 24.4 eV and 16.3 eV respectively, while HI is ionized at 13.6 eV. Moreover,
absorption spectroscopy only provides information on a single line-of-sight,
and thus this technique does not assess the full distribution of neutral
medium throughout the galaxy, so that the exact impact of HI distribution on
Ly${\alpha}$ emission has never been observed directly. The preferred tracer
of the neutral gas medium, the 21cm HI emission line, is very faint compared
to e.g H$\alpha$ or Ly${\alpha}$ and so observing it at high angular
resolution requires significant surface brightness and long exposure times.
Additionally, resolution in the radio domain where the 21cm line is located is
inherently lower than in the optical or UV bands, making multi-wavelength
comparisons difficult. Therefore even at the comparatively low redshifts of
the galaxies in the LARS and eLARS samples, observing the HI 21cm emission
remains a challenge.

This study presents the first direct comparison of the resolved HI 21-cm and
Ly${\alpha}$ emission lines in galaxies. We have obtained VLA data of the HI
line at the highest angular resolution possible ($\sim$3″ beam) for two
galaxies, enabling the study of the two emission lines on scales that are
comparable. We also use a wealth of additional data including IFU ionized gas
observations and molecular gas data that allows the tracing of star formation
regions where Ly${\alpha}$ is produced and dust absorbing Ly${\alpha}$. By
examining the interstellar medium phases responsible for the creation,
destruction and scattering of Ly${\alpha}$ photons, this study depicts a
holistic view of the Ly${\alpha}$ radiative transfer in star-forming galaxies,
and tests current theories for Ly${\alpha}$ escape mechanisms.

We present the sample used for this study in section 2. The observations and
data reduction procedures are described in section 3. In section 4 we present
the HI data products and the main results obtained through the comparison of
various ISM phases and Ly${\alpha}$ emission. In section 5 the results are
discussed and we provide a summary and conclusion in section 6.

##  2 Sample selection

Table 1: Properties of the galaxies in the high angular resolution HI sample.

ID | RA | DEC | z | D | M${}_{*}^{a}$ | SFR a | 12 + log(O/H)a | L${}_{\textrm{Ly${\alpha}$}}\,^{a}$ | EW${}_{\textrm{Ly${\alpha}$}}\,^{a}$ | f${}_{esc}(\textrm{Ly${\alpha}$})^{a}$
---|---|---|---|---|---|---|---|---|---|---
|  |  |  | [Mpc] | [1010 M⊙] | [M⊙/yr] |  | [1041erg s-1] | [Å] | [%]
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11)
LARS08 | 12h50m13.5s | +07d34m42s | 0.038 | 168.3 | 11.4$\pm$ 0.12 | 60.53 $\pm$ 0.28 | 8.565$\pm^{0.011}_{0.013}$ | 3.94 $\pm$ 0.24 | 17.02 $\pm$ 0.53 | 0.40 $\pm$ 0.02
eLARS01 | 16h11m40.8s | +52d27m24s | 0.029 | 129.0 | 7.98$\pm$0.05 | 24.70 $\pm$ 0.03 | 8.758$\pm^{0.009}_{0.015}$ | 5.50 $\pm$ 0.08 | 22.70 $\pm$ 0.35 | 1.37 $\pm$ 0.03

Note. — Notes on columns: (1) - Galaxy ID. (2) - Right Ascension (J2000). (3)
- Declination (J2000). (4) - Redshift obtained from nebular line fitting of
the SDSS spectra. (5) - Luminosity distance calculated from the redshift
assuming $\Lambda$CDM cosmology with parameters $\Omega_{m}=0.3$,
$\Omega_{d}=0.7$, $H_{0}=70$ km s-1 Mpc-1 . (6) - Stellar Mass. (7) - Star
Formation Rate. (8) - Metallicity. (9) - Ly${\alpha}$ Luminosity. (10) -
Ly${\alpha}$ equivalent width. (11) - Ly${\alpha}$ escape fraction.

a Melinder et al. in preparation.

The two galaxies in this study were drawn from the LARS and eLARS galaxy
samples, aiming to investigate the physical properties that impact
Ly${\alpha}$ emission in local galaxies. The LARS sample contains 14 nearby
starburst galaxies ($0.028\leq z\leq 0.18$) that were selected for their
strong star formation by their H$\alpha$ equivalent width
(EW${}_{\textrm{H}\alpha}\geq 100$ Å) and FUV luminosity
($9.5\leq\log(\textrm{L}_{FUV}/\textrm{L}_{\odot})\leq 10.7$) (see Östlin et
al., 2014; Hayes et al., 2014, for more details). It was later extended with
the eLARS sample, containing 28 additional galaxies that evenly sample the
H$\alpha$ equivalent width and FUV luminosity parameter spaces (Melinder et
al., in prep.). Preliminary fluxes for the eLARS sample have been presented in
Runnholm et al. (2020). With lower H$\alpha$ equivalent width constraints
(EW${}_{\textrm{H}\alpha}\geq 40$ Å), the eLARS galaxies are less extreme in
their properties and are representative of the galaxy population dominating
the local FUV luminosity function. Together the LARS and eLARS samples are
suited to comprehensively study the parameters that impact the radiative
transfer of Ly$\alpha$ photons in local galaxies.

We have obtained VLA D and C configuration observations for nearly all the
galaxies in the samples and B configuration for 15 of them; these observations
will be presented in future work (Le Reste et al., in prep.). While the LARS
and eLARS samples were selected to contain galaxies comparable to those
identified in UV-based surveys at high-redshift (e.g. Cowie et al., 2011), the
targets selected for the highest angular resolution 21cm emission followup
(VLA A configuration) make up a small subset of only two galaxies: LARS08 and
eLARS01. Basic properties of the galaxies in the subsample are presented in
Table 1. In order to obtain the highest quality HI data with long baselines,
we began our campaign targeting the galaxies with the largest 21cm flux
integrals based upon VLA D and C imaging. This selection also leads to
targeting more massive and gas-rich galaxies, which may also be more metal and
dust-rich and thus have lower Ly${\alpha}$ output. This can be seen from the
data presented in Table 1, which shows that while the luminosities and EWs are
sufficient for them to be identified in high-$z$ Ly${\alpha}$ surveys, the
global Ly${\alpha}$ escape fractions are rather low at $\sim 1$ %.
Nevertheless, we stress that the emission physics, which depends significantly
upon HI and dust will be identical in all galaxies.

##  3 Observations and data reduction

###  3.1 VLA Observations

The galaxies in the sample were observed with the D, C, B and A configurations
of the Very Large Array111The VLA is operated by the National Radio Astronomy
Observatory. (project numbers 13A-181, 14A-077, 17A-240 and 18A-095).
Observations made use of the L-band centered on the galaxies respective
redshifted 21-cm HI frequency. Information about the observations for each
array configuration can be found in Table 2. Lower angular resolution
observations of LARS08 were previously presented in Pardy et al. (2014). We
designed our observations to reach the highest possible angular resolution by
following the guidelines spelled out by the
NRAO222https://science.nrao.edu/facilities/vla/docs/manuals/oss/performance/comb-
conf-mosaicking. These guidelines specify integration times should increase by
a factor of $\sim$3 when stepping through the longer array configurations to
reach homogeneous sensitivity limits; since we are concerned with HI emission,
such common limits would correspond to a fixed gas column density. Since
designing the observations we have become aware of the fact that these
guidelines may be most appropriate for relatively compact emission, while in
our case some of the HI gas – especially the warm neutral phase, tidal debris,
and circumgalatic gas – may be extended. Nevertheless, we believe this
strategy is still appropriate for several reasons. Firstly, we do not attempt
to use the A and B configuration data to study the large-scale emission for
which we rely solely upon imaging of the D+C data which has higher
sensitivity. The observed galaxies are at distances of 129 and 168 Mpc, which
are large compared to the distances of most galaxies with interferometric HI
observations. Thus our physical resolution elements are comparatively coarse
(2-3 kpc at half power) in comparison to the expected size of HI clouds. Thus
we expect these structures to appear compact and point-like, which would make
the choice of integration times appropriate. We stress that the
A-configuration data is used only when we make the comparisons on the smallest
scale (i.e. with UV/optical data) and that in these cases we are likely
sensitive only to the highest column density, clumpiest gas.

Table 2: Observation parameters for the high angular resolution LARS VLA
sample.

ID | Conf. | Date | $T_{int}[h]$ | Calibrator
---|---|---|---|---
L08 | D | Apr 2013 | 2.18 | J1254+1141
| C | Jun 2017 | 5.87 | $|$
| B | Nov 2017 - Jan 2018 | 12.95 | $|$
| A | May - Jun 2018 | 37.44 | $|$
eL01 | D | Aug 2014 | 1.63 | J1035+5038
| C | Jun 2017 | 5.81 | $|$
| B | Sep - Dec 2017 | 12.38 | $|$
| A | Mar - Jun 2018 | 36.40 | $|$

Note. — Notes on columns, from left to right: (1) - Galaxy name. (2) - VLA
array configuration. (3) - Observations date. (4) - Integration time. (5) -
Source used as phase calibrator.

###  3.2 HI Data reduction

####  3.2.1 RFI masking and Calibration

The VLA HI data was reduced with the casa software using standard
prescriptions. Radio-Frequency Interference (RFI) were removed from the data
to avoid the introduction of imaging artifacts in the final data cubes. A two
step procedure was applied to identify and exclude RFI. First, the automatic
RFI flagging task tfcrop was applied with parameters maxnpieces=3
timecutoff=3.0 and freqcutoff=3.0. This led to the removal of $\sim$1% of the
interferences in the time-frequency plane, consisting mainly of strong,
frequency-narrow RFI. This was followed by eye inspection (using the plotms
and viewer tasks) and hand-masking of the measurement sets using the flagdata
task. The resulting RFI-free measurement sets were then calibrated using
1331+305=3C286 as the bandpass calibrator and either J1254+1141 or J1035+5038
as the phase calibrator. The measurement sets containing all sources were
split using the task split to keep only the visibilities corresponding to the
target galaxies. The calibrated target-only measurement sets were continuum
subtracted by fitting a polynomial of order 1 on line-free channels of each
individual dataset using the task uvcontsub. The visibilities of each dataset
were reweighted according to their scatter using the task statwt on the same
channels as those where uvcontsub was applied, a necessary step when combining
data from different array configurations. Typically, we used 200 line-free
channels on each side of the HI line, making sure not to include channels at
the ends of the bandpass.

####  3.2.2 Cleaning

The resulting datasets were cleaned to 0.5$\sigma$ using the CASA task tclean
in interactive mode with the auto-multithresh algorithm (Kepley et al., 2020)
to identify regions containing emission. This was complemented by manual
inspection of the clean region selected. The use of auto-multithresh allowed
for the detection of a small HI object at the edge of the LARS08 field that
was missed by eye identification due to the size of the object and remote
location from the center of the field. It also helped speeding the clean
region selection process considerably, especially in the high-angular
resolution cubes. The clean images were set to have a common beam in the
tclean task and a Briggs weighting robust parameter of 0.5 was used for each
individual configuration, combining low and high angular resolution
configurations together (D+C, D+C+B and D+C+B+A). The spectral channel width
of the cubes was set to 10 km s-1. Additionally, a uv-taper of 50k$\lambda$
was used for the D+C+B+A cubes to increase the flux sensitivity while keeping
a reasonably high angular resolution. Table 3 summarizes the properties of
each data cube.

####  3.2.3 Emission detection

After data reduction, the emission regions were isolated from the noise. For
that purpose, the cubes were masked with dilated masking, using an algorithm
developed for the identification of CO emission in local galaxies (Rosolowsky
& Leroy, 2006; Sun et al., 2018). The algorithm first identifies regions above
a certain signal-to-noise ratio using an intensity threshold criterion snrhi
and a criterion on the number of consecutive channels nhi for which the
intensity threshold must be seen. This first mask is then extended spatially
and spectrally to include regions with a lower intensity threshold snrlo found
across a certain number of consecutive channels nlo. This type of emission
detection method allows for fainter emission regions to be detected, assuming
that they are connected to the higher intensity emission regions, and also
limits the noise included in the final data products. After conducting tests
on several datasets, we found that the parameter combination (snrhi=3.5-3,
nhi=3, snrlo=1.5-2, nlo=1) and extending the mask spatially by 2 pixels
allowed for the best emission recovery in our datasets while limiting noise
inclusion. The masked cubes were primary beam corrected to obtain correct flux
densities for the subsequent data products derivation.

###  3.3 HI data products

####  3.3.1 HI flux, mass, spectra

To obtain the flux and mass associated with the HI emission, the masked,
primary beam corrected cubes for the VLA D array configuration only were used.
This choice is motivated by the D configuration having the highest surface
brightness sensitivity, thus enabling the detection of HI flux on extended
scales. Furthermore the nominal D configuration beam size at 21cm ($\sim 40$”)
is well-matched to the angular sizes of the galaxies as shown in Figure 1.
Although it also contains information from the D array, the D+C+B+A
configuration data is more sensitive to higher surface brightness HI due to
the lower sensitivity of our longer baseline observations. Thus large-scale
emission is resolved out, leading to a loss of flux.

The flux SHI in a spectral cube containing a certain number N of pixels $i$
with flux density $I_{i}$ can be expressed as:

$\left(\frac{\textrm{S}_{HI}}{\textrm{Jy.km
s${}^{-1}$}}\right)=\frac{4\log(2){\ell_{pix}}^{2}}{\pi\theta_{min}\theta_{maj}}\sum_{i}^{N}\left(\frac{I_{i}}{\textrm{Jy/beam}}\right)\left(\frac{\Delta\textrm{v}}{\textrm{km
s${}^{-1}$}}\right)$ (1)

with $\theta_{min}$ and $\theta_{maj}$ the beam minor and major axis in
degrees, $\ell_{pix}$ the angular length of a pixel in degrees and
$\Delta\textrm{v}$ the channel width. The error on the flux was derived
accounting for the 3% typical flux error on the primary calibrator (Perley &
Butler, 2017) and the noise in the cube. Assuming that the gas is optically
thin, the HI gas mass can be derived from the flux using the following
expression, where D is the luminosity distance:

$\left(\frac{\textrm{M}_{HI}}{\textrm{M}_{\odot}}\right)=2.36\times
10^{5}\left(\frac{\textrm{D}}{\textrm{Mpc}}\right)^{2}\left(\frac{\textrm{S}_{HI}}{\textrm{Jy.km
s${}^{-1}$}}\right)$ (2)

Spectra were obtained by summing the flux in each spectral channel of the
masked, primary beam corrected D configuration data. We estimate the typical
r.m.s. in adjacent frequency channels devoid of 21cm flux by collapsing the
mask along the spectral direction, and computing the total noise level.

####  3.3.2 Moment maps

Moment maps were derived using the masked, primary beam corrected D+C+B+A
cubes with the spectral_cube Python package. Both cubes have an average
synthesized beam size of 3$\farcs$6\. Moment-0 maps (integrated emission maps)
are derived using:

$\textrm{M}_{0}=\sum_{v}I_{v}\Delta\textrm{v}$ (3)

Moment-1 maps are intensity-weighted velocity maps, corresponding to the
velocity centroid across the spectral axis:

$\textrm{M}_{1}=\frac{\sum_{v}v\,I_{v}\Delta\textrm{v}}{\textrm{M}_{0}}$ (4)

Moment-2 maps are linked to the velocity dispersion across the spectral axis,
and can be expressed as:

$\textrm{M}_{2}=\frac{\sum_{v}(v-\textrm{M}_{1})^{2}\,I_{v}\Delta\textrm{v}}{\textrm{M}_{0}}$
(5)

Velocity dispersion maps can be derived from the moment-2 maps using
$\sigma=\sqrt{\textrm{M}_{2}}$, although this method can sometimes lead to
overestimating the true velocity dispersion (Mogotsi et al., 2016).

Table 3: HI cube properties

ID | Conf. | Beam | rms | min NHI | scale | SHI | MHI | zHI
---|---|---|---|---|---|---|---|---
|  | (bmin,bmaj,pa) | [mJy/beam] | [cm-2] | [kpc/$\overline{beam}$] | [Jy km s-1] | [1010 M⊙] |
LARS08 | D | 45.2”,56.6”,4.5° | 0.74 | 6.30$\times$1018 | 41.5 | 1.75$\pm$0.09 | 1.17$\pm$0.06 | 0.03822
| D+C | 16.9”,18.6”,31.6° | 0.40 | 2.77$\times$1019 | 14.5 | 2.34$\pm$0.08 | 1.56$\pm$0.06 |
| D+C+B | 6.45”,7.09”,53.6° | 0.24 | 8.57$\times$1019 | 5.5 | 2.07$\pm$0.08 | 1.38$\pm$0.05 |
| D+C+B+A | 3.29”,3.95”,-85.3° | 0.16 | 2.0$\times$1020 | 2.9 | 1.35$\pm$0.06 | 0.9$\pm$0.04 |
eLARS01 | D | 42.2”,53.4”,67.5° | 0.85 | 8.22$\times$1018 | 29.9 | 4.3$\pm$0.21 | 1.69$\pm$0.08 | 0.02948
| D+C | 15.6”,16.2”,-43.1° | 0.42 | 3.62$\times$1019 | 9.9 | 2.93$\pm$0.1 | 1.15$\pm$0.04 |
| D+C+B | 5.3”,7.1”,-89.0° | 0.28 | 1.21$\times$1020 | 3.8 | 2.78$\pm$0.1 | 1.09$\pm$0.04 |
| D+C+B+A | 3.1”,4.1”,-80.7° | 0.17 | 2.18$\times$1020 | 2.2 | 1.69$\pm$ 0.07 | 0.67$\pm$0.03 |

Note. — Notes on columns, from left to right: (1) - Galaxy ID. (2) - Array
configuration data used to create the cube. (3) - Synthesized beam. (4) - Cube
rms. (5) - Limiting column density. These correspond either to the 2$\sigma$
or 1.5$\sigma$ limit, depending on the snrlo level specified during masking.
(6) - average physical diameter of the beam. (7) - HI flux. (8) - HI mass. (9)
- Redshift obtained from 21cm line centroid.

###  3.4 Additional data

####  3.4.1 HST data

HST Ly${\alpha}$ images were obtained using the Solar Blind Channel (SBC) of
the Advanced Camera for Surveys (ACS), using the F125LP and F140LP filters to
isolate Ly${\alpha}$ and the adjacent continuum, respectively. For details of
the observations and data-processing, see Hayes et al. (2014) and Östlin et
al. (2014); the full procedure will soon be described in Melinder (2022, in
prep). In short, we use two adjacent long-pass filters in the SBC to isolate
Ly${\alpha}$ and subtract the stellar continuum, using a spatially resolved
spectral modeling method relying upon 3 additional optical HST images and
spectral synthesis models.

We also use HST spectroscopic data from the Cosmic Origins Spectrograph (COS):
the LARS 08 spectrum was obtained as part of our own program and is presented
in Rivera-Thorsen et al. (2015), while the eLARS 01 spectrum was presented in
Leitherer et al. (2013). In both cases Ly${\alpha}$ was observed through the
G130M grating, which provides a nominal spectral resolution of $R\approx
17000$ at the wavelength of Ly${\alpha}$. The eLARS 01 data were re-processed
with the COS pipeline following the methods described in Rivera-Thorsen et al.
(2015).

####  3.4.2 MUSE data

Optical integral field spectroscopy of LARS08 was obtained from the Multi-Unit
Spectroscopic Explorer (MUSE; Bacon et al., 2010) at the Very Large Telescope
Unit Telescope 4 under ESO programme 0101.B-0703. Observations were obtained
on 18 May 2018, under a DIMM seeing of $\approx 0.9$ arcsec at full-width-
half-maximum. We obtained four individual exposures, each of 650 seconds and
rotated by 90 degrees between integrations. An independent sky frame (120
seconds) was also taken in order to improve the background subtraction. Data
were reduced by the standard MUSE pipeline Version 2.8.3 (Weilbacher et al.,
2020).

The reduced cube was continuum subtracted using a 151 pixel running median
filter in each spaxel. We then fit the fluxes and velocity parameters for the
nebular gas using the approximation of single-component Gaussian functions:
for every spatial pixel, we simultaneously fit a large number of optical lines
($\simeq$ 40). We adopt a method that recovers a single common recession
velocity and velocity dispersion for all spectral features (accounting for the
variation of the MUSE spectral resolution as a function of observed
wavelength), and only the normalization of each line (the flux) is allowed to
vary for the lines individually. In practice this allows all lines to
contribute to the velocity measurements at low SNR; at high SNR the strongest
lines of hydrogen and [O III] dominate in the recovery of the kinematic
properties, which in turn constrains the profile shape of the weaker lines.
This effectively removes degeneracies in the fitting of weaker lines, and
allows their flux to be better recovered.

####  3.4.3 PMAS data

PMAS Lens-Array (Roth et al., 2005) observations for eLARS01 were obtained
under overcast conditions on March 31st, 2016 (DIMM seeing $\sim$2.0”). The
observation is part of an integral field spectroscopic campaign to study the
ionised gas kinematics of the combined LARS+eLARS sample (Herenz et al., in
prep.). We used the 16$\arcsec\times 16\arcsec$ field sampled at
1$\arcsec\times 1\arcsec$ for each spectral pixel. Moreover, we used the
backwards-blazed R1200 grating and read out the CCD unbinned along the
dispersion axis. The exposure time was 1800s. No separate sky exposure was
taken, as the targeted H$\alpha$ emission line was not impacted by telluric
emission. We reduced the data with the p3d pipeline (Sandin et al., 2010)
following the same steps as in Herenz et al. (2016), except that we did not
sky-subtract and flux calibrate the data, as neither of those steps is
required for the extraction of kinematic information. Measuring the FWHM of
the calibration arc lamp lines we obtain $R\approx 5300$ as the final
resolving power (or $v_{\mathrm{FWHM}}=57$ km s-1) of the PMAS data at the
observed wavelength of H$\alpha$. Other H$\alpha$ kinematics observations of
eLARS01 can be found in Sardaneta et al. (2020).

##  4 Results

###  4.1 HI Properties

Figure 1: Moment maps for the 21cm HI emission line of LARS08 (left) and
eLARS01 (right), obtained with the VLA D+C+B+A configurations. White or black
contours characterizing the extent of the D+C data are overlaid
($N_{HI}=5\times 10^{19}$cm-2). From Top to bottom: integrated intensity
(moment-0) map in column density, rest-frame intensity-weighted velocity
(moment-1) map, velocity dispersion map. The size of the synthesized beams for
the D+C and D+C+B+A data are indicated on the bottom left of the maps by a
white or black ellipse. North and West direction are indicated on the moment-0
maps.

Moment maps of the HI emission in the D+C+B+A cubes for LARS08 (left) and
eLARS01 (right) are presented in Figure 1, along with black or white contours
showing the extent of the emission in the D+C cubes. Moment maps for the D,
D+C, D+C+B and D+C+B+A are shown in Appendix Figures 8 and 9. We present
channel maps for the D+C+B+A data in Figures 10 and 11. The extent of the 21cm
emission differs when probed by the different array configurations. This is
due to the most compact configuration data (D,C; low-resolution data) having
better surface brightness sensitivity, and thus being better suited at
detecting faint emission. Additionally, large-scale emission is resolved out
by the most extended configuration arrays (B,A) due to the lack of short
spacing between antennas. The combination of different array configuration
data thus yields a compromise between brightness sensitivity and angular
resolution that is sufficient to image the high surface brightness 21cm
emission at high angular resolution in the regions where Ly${\alpha}$ is
produced.

The HI emission of both galaxies shows signs of galaxy interaction. For
LARS08, the D+C contours indicate the presence of an HI component South West
of the center of the galaxy, with an extended HI feature resembling a tidal
tail likely due to a recent infall. The moment maps in Figure 8 also show they
are connected in velocity-space. The D+C+B+A data, which probes gas with
higher column density, does not show the companion object. Inspecting optical
data from SDSS (Ahumada et al., 2020) and the Pan-STARRS1 survey (Chambers et
al., 2016), two extremely faint optical objects were found on the Western edge
of the companion HI object, at projected angular distances of 14 arcsec and 7
arcsec from the center of the HI cloud. Due to their offset position and their
faintness, we could not unambiguously identify either of these two objects as
the counterpart of the HI companion. Another small, faint HI object was found
in the VLA field at a distance of 13.2 arcminutes to the center of LARS08,
with no detected optical counterpart. Due to the large distance of this object
to the galaxy and the lack of direct evidence for its connection, we did not
extend the area shown in the figure to display it, but note this suggests that
LARS08 is in a group environment.

The HI emission of LARS08 traces a prominent spiral arm in the galaxy and
resembles a rotating disk extending beyond the optical diameter of the galaxy.
An HI hole is present in the center of the galaxy. This feature linked to the
presence of star forming regions is commonly observed in galaxies, with atomic
hydrogen transitioning to a molecular phase in the center of the HI disk (see
e.g. Wong & Blitz, 2002; Blitz & Rosolowsky, 2004; Murugeshan et al., 2019).
The highest HI column density region of the galaxy ($N_{HI}>4\times
10^{21}\,$cm-2) is found immediately to the West of the HI hole.

eLARS01 shows a merger-like HI morphology, with tails of HI extending to the
North and South of the galaxy that extend on scales of $\sim 100$ kpc each.
The emission in the tails can be better appreciated in the D+C contours and
moment maps of the lower angular resolution data presented in Figure 9. Most
of the emission in the tails is not recovered with the D+C+B+A data despite
the 36 hours of integration with the VLA A configuration. This is probably due
to the large angular scale over which the HI emission is distributed and hence
a large fraction of the HI gas is found in an extended component that is
resolved out in the D+C+B+A configuration data. The decrease in flux recovered
between the D configuration-only and the D+C+B+A configuration data also
supports this hypothesis (see Table 3). The central part of the galaxy shows,
like in the case of LARS08, a central HI hole neighboring the highest
intensity region. However, careful examination reveals the presence of an
absorption feature centered on the core of one of the merging galaxies (see
Appendix Figure 12.

Comparable HI morpho-kinematics to those of LARS08 and eLARS01 have been seen
in low angular-resolution observations of some of the LARS sample galaxies
(see e.g. LARS03 in Pardy et al., 2014), other nearby Ly${\alpha}$-emitting
starburst galaxies (i.e. the extended HI tails of Tol 1924-416 in Cannon et
al. (2004), see also Hayes et al. (2005); Östlin et al. (2009)), and are
common in local dwarf star-forming galaxies (Jaiswal & Omar, 2020). This is
consistent with galaxy mergers playing a role in triggering starburst episodes
in galaxies.

Figure 2: 21cm HI emission line spectra of LARS08 and eLARS01, obtained from
the masked VLA D array configuration data. The gray shaded areas show the
typical error due to the flux calibration uncertainty and the cube rms. The
flux in masked velocity channels was obtained from a fixed 2D aperture
corresponding to the lines of sight containing emission in the cube. The
vertical dotted line shows the location of the central HI velocity, the
horizontal line shows the zero level. The gray area surrounding the spectrum
shows the amplitude of the error due to calibrator flux uncertainty.

Figure 2 shows the HI spectra of the galaxies obtained from the D
configuration data. The spectrum of LARS08 shows a plateau. The associated HI
flux is S${}_{HI}=1.75\pm 0.09$ Jy.km s-1 (with corresponding mass
M${}_{HI}=1.17\pm 0.06\times 10^{10}M_{\odot}$), in agreement within error
bars with the value of S${}_{HI}=1.8\pm 0.18$ Jy.km s-1 found with the VLA D
configuration presented in Pardy et al. (2014). As already noted in that
study, there is a strong discrepancy between the flux values found using the
VLA D configuration and the GBT which yields S${}_{HI,GBT}=3.4\pm 0.3$ Jy.km
s-1. We did not find any other HI source within the area covered by the 9
arcminutes beam of the GBT, thus the difference in flux value is most likely
due to the presence of low surface brightness emission that is either
spatially extended and resolved out by the array or below the sensitivity
limit of the VLA.

The spectral profile of eLARS01 is centrally peaked and we find an associated
flux S${}_{HI}=4.30\pm 0.21$ Jy.km s-1 (with corresponding HI mass
M${}_{HI}=1.69\pm 0.08\times 10^{10}M_{\odot}$). A previous HI flux
measurement with the single dish Nançay radiotelescope yielded S${}_{HI}=3.19$
Jy.km s-1 (Martin et al., 1991). This telescope is much less sensitive than
the VLA, with a typical rms of $\sim$1-2mJy. Error measurements on the flux
were not provided in that study, however we can reasonably conclude that the
lower flux value compared to the VLA is due to the large uncertainty on the
flux measurement. Table 3 shows the properties of the HI cubes corresponding
to the different VLA configuration combinations.

Figure 3: a: Pan-STARRS g,r and i stacked image (shown with asinh stretch) of
LARS08 (top) and eLARS01 (bottom) with VLA D+C+B+A (cyan) and D+C (black)
21-cm HI emission contours overlaid. HI contours increase from darker to
brighter. For LARS08 the contours correspond to $N_{HI}$=$5\times 10^{19}$,
$1\times 10^{21}$ and $2.5\times 10^{21}$ cm-2. For eLARS01 the contours
correspond to $N_{HI}$=$1\times 10^{20}$, $1\times 10^{21}$ and $3\times
10^{21}$ cm-2. The white box shows the extent of the images in b and c. b: HST
RGB color composite image with filters F336W (blue), F438W (green), and F775W
(red) for LARS08 and filters F336W (blue), F435W (green) and F814W (red) for
eLARS01 c: HST color composite showing Ly${\alpha}$ (blue), H$\alpha$ (red)
and FUV (green) emission (Melinder et al., in prep.)

Stacked Pan-STARRS and HST images with 21-cm HI contours of LARS08 and eLARS01
are presented in Figure 3. These figures are shown for purpose of comparison
between the HI and the optical emission. For both galaxies, low surface
brightness features can be seen in the optical, extending beyond the HI
contours. In LARS08, low surface brightness shell features in the optical
indicate that some minor companion has been tidally disrupted and annexed by
the main galaxy, which supports our hypothesis that the galaxy is in a group
environment.

###  4.2 Ly${\alpha}$ radiative transport - LARS08

In this section and the following, we analyze the multi-wavelength
observations to construct a picture of the Ly${\alpha}$ radiative transfer in
LARS08 and eLARS01. Here, we examine ISM phases presented in Figure 4 in turn:
the H$\alpha$ emitting regions, indicating where Ly${\alpha}$ is created, the
HI emitting regions, showing the transport and scattering medium of
Ly${\alpha}$ photons, and the dust, showing where Ly${\alpha}$ is absorbed.

Figure 4: Left panel: LARS08 D+C+B+A HI moment-0 map, with box indicating the
region presented on the right-hand panels overlaid in black. a: LARS08 D+C+B+A
HI moment-0 map with MUSE H$\alpha$ contours in units of 10-18 erg s-1 cm-2
arcsec-2 overlaid. Numbered arrows indicate the position of the strongest
H$\alpha$ regions. b: LARS08 D+C+B+A HI moment-0 map with HST Ly${\alpha}$
contours in units of 10-16 erg s-1 cm-2 arcsec-2 overlaid in orange. Numbered
arrows indicate the position of the strongest H$\alpha$ regions as shown on a.
c: Extinction map of LARS08 with Ly${\alpha}$ contours overlaid in black, with
levels corresponding to those in b.

####  4.2.1 Star formation and Ly${\alpha}$ creation - H$\alpha$ emission

Figure 4 a presents contours of H$\alpha$ emission on the HI column density
map. H$\alpha$ emission traces star-forming regions, and most importantly for
Ly${\alpha}$ transfer, the intrinsic production locations of Ly${\alpha}$
radiation within the galaxy. If we were to look at a galaxy without radiative
transfer effects due to HI gas or dust, the Ly${\alpha}$ emission morphology
would thus be the same as that of the H$\alpha$. We show four H$\alpha$
contours, corresponding to high H$\alpha$ intensity ($>6.0\times 10^{-15}$
erg/s/cm2/arcsec2, red contours), intermediate H$\alpha$ intensity ($2.5\times
10^{-17}$ erg/s/cm2/arcsec2 and $200\times 10^{-17}$ erg/s/cm2/arcsec2, orange
contours) and low H$\alpha$ intensity ($5\times 10^{-18}$ erg/s/cm2/arcsec2,
light orange contours). Regions of high H$\alpha$ intensity corresponding to
the sites of strongest star formation are indicated by numbered arrows on this
figure. The H$\alpha$ emission down to 5$\times 10^{-18}$ erg/s/cm2/arcsec2
outlines the spiral arm seen in HI. We have not represented all contours for
the H$\alpha$ emission in LARS08 to keep the H$\alpha$ levels comparable
between LARS08 and eLARS01, since the MUSE H$\alpha$ data used for LARS08 is
more sensitive than the HST data used for eLARS01. We note that the faintest
MUSE H$\alpha$ contours (down to 2.5$\times$10-18 erg/s/cm2/arcsec2) reach the
D+C HI contours for LARS08 and thus trace the full extent of the HI disk.

Figure 13 in the Appendix shows, in addition to the H$\alpha$ contours
presented here, CO(2-1) contours tracing molecular gas and thus reservoirs of
star-forming material. The peak of the CO emission is located in the HI hole,
indicating that the hydrogen in this region is found in molecular form. The CO
emission likely traces a gas reservoir for star formation, which is supported
by the central high intensity H$\alpha$ emission contours overlapping with the
CO emission. Star forming region 1 coincides with the HI hole and overlaps
with the CO emission present at this location. The other two strong star
forming regions that correspond to high intensity H$\alpha$ emission are found
to the West (region 2) and South-West (region 3) of the central star forming
region. These high intensity H$\alpha$ emission regions are all seen to border
the highest HI column density gas (N${}_{HI}>4.0\times 10^{21}\,$cm-2). The
strongest star formation regions are linked by an intermediate intensity
contour ($>$2.0$\times 10^{-15}$ erg/s/cm2/arcsec2), that overlaps with the
highest HI column density gas.

####  4.2.2 Ly${\alpha}$ transport - HI emission

We now investigate how the Ly${\alpha}$ emission compares in distribution and
geometry to the H$\alpha$ and HI emissions. Figure 4 b presents Ly${\alpha}$
emission contours overlaid on the HI column density map, with arrows
indicating regions of strongest star formation presented on Fig. 4 a for
reference. They indicate where most of the Ly${\alpha}$ emission is produced,
and where it would be observed in the absence of Ly${\alpha}$ radiative
transfer effects. Three contours are overlaid on the HI map, presenting the
high intensity Ly${\alpha}$ emission ($>5\times 10^{-15}$ erg/s/cm2/arcsec2,
dark red contour), intermediate intensity Ly${\alpha}$ emission ($>1\times
10^{-15}$ erg/s/cm2/arcsec2, orange contour) and faint Ly${\alpha}$ emission
($>1\times 10^{-16}$ erg/s/cm2/arcsec2, light orange contour). These contours
were selected to show the emission peaks, the intermediate Ly${\alpha}$
emission and the faintest contours enclosed within the HST field of view.

The strongest Ly${\alpha}$ emission ($>5\times 10^{-15}$ erg/s/cm2/arcsec2,
dark red contour) has a clumpy morphology and is found in two locations, North
of star-forming region 2 and between star-forming regions 2 and 3. The
strongest Ly${\alpha}$ emission borders, but does not coincide with, the
brightest HI emission regions (N${}_{HI}>4.0\times 10^{21}\,$cm-2). Instead,
it is found in regions with HI column density N${}_{HI}\sim 3.0\times
10^{21}\,$cm-2 at the edges of HI clouds. The fact that the highest intensity
Ly${\alpha}$ emission does not overlap with the highest HI density regions
could indicate that lower spatial scattering leads to stronger Ly${\alpha}$
emission on the line of sight. High intensity emission would thus be seen in
locations where Ly${\alpha}$ photons follow a path of least resistance through
the neutral gas medium. However, the HI column density values N${}_{HI}\sim
3\times 10^{21}\,$cm-2 where the emission is seen is still very high for
Ly${\alpha}$ radiation. We discuss this surprising result and potential
projection effects in section 4.2.4.

The intermediate intensity Ly${\alpha}$ emission ($>10\times 10^{-16}$
erg/s/cm2/arcsec2, orange contour) has a more extended geometry and surrounds
the strongest Ly${\alpha}$ emission contours. It partly overlaps with the high
HI column density emission regions N${}_{HI}>4\times 10^{21}\,$cm-2. This
supports the hypothesis that increased spatial scattering due to higher gas
column density leads to fainter Ly${\alpha}$ emission. The faintest
Ly${\alpha}$ contours ($>1\times 10^{-16}$ erg/s/cm2/arcsec2) broadly trace
the intermediate H$\alpha$ emission ($>2.5\times 10^{-17}$ erg/s/cm2/arcsec2)
where HI is detected in the D+C+B+A data (N${}_{HI}=2.0\times 10^{20}\,$cm-2).
Faint Ly${\alpha}$ thus reproduces the spiral structure apparent in both the
H$\alpha$ and HI data.

####  4.2.3 Ly${\alpha}$ destruction - dust

Figure 4 c shows the extinction E(B-V) map of LARS08, tracing the dust content
of the galaxy, with Ly${\alpha}$ contours from Fig. 4 b overlaid. Since dust
absorbs Ly${\alpha}$ photons this effectively shows the locations where
Ly${\alpha}$ photons are likely to be removed compared to where they are seen
in emission. The E(B-V) is calculated using the H$\alpha$ and H$\beta$
emission lines, assuming the extinction law from Cardelli et al. (1989) and an
intrinsic line ratio of 2.86, corresponding to ionized gas with an electron
density of 100 cm-3 and temperature of 10,000 K. Although the extinction map
does not cover the full extent of the HI disk due to low signal-to-noise of
the weaker and more obscured H$\beta$ line in the outer part of the galaxy,
some notable features can be seen. First, the high and intermediate intensity
Ly${\alpha}$ emission overlaps with regions of lower extinction and dust
content (E(B-V)$\lesssim$0.6), consistent with the picture where the presence
of dust-rich gas would lead to Ly${\alpha}$ absorption. However surprisingly,
the faint Ly${\alpha}$ emission is seen throughout the galaxy, regardless of
the dust content. This suggests that regardless of extinction, sufficient
spatial scattering will allow Ly${\alpha}$ emission to be eventually observed
on the line of sight. Dust thus seems to modulate, but not prevent, extended
Ly${\alpha}$ emission.

####  4.2.4 3D analysis - the view from the COS spectra

Figure 5: Left: Comparison of the normalised HI, Ly${\alpha}$, H$\alpha$
emission and SiIIλ1260 absorption spectra as a function of the rest-frame
velocity shift in LARS08. The spectra are extracted using the COS aperture,
and the rest-velocity is defined by the H$\alpha$ centroid at that position.
The HI spectrum is obtained from the unmasked D+C+B+A VLA data cubes. The
dashed vertical line indicates the position of the rest velocity as defined by
the H$\alpha$ centroid in the aperture, the black horizontal line indicates
the zero flux level in the case of the emission spectra, and the continuum
level in the case of the absorption spectrum. Right: Zoomed-in version of the
Ly${\alpha}$ HI, and H$\alpha$ intensity maps with COS aperture position
represented by a black circle.

Although the morphological comparison of Ly${\alpha}$ to the other phases of
the ISM has brought insight into what impacts the spatial scattering of
Ly${\alpha}$ photons in the galaxy, it does not solve the question of how the
photons can escape from an HI medium that is extremely opaque to Ly${\alpha}$
(N${}_{HI}\sim 10^{21}\,$cm-2 in LARS08). In this section, we make use of the
HST COS spectra of the galaxies and of the 3-dimensional nature of the ionized
gas IFU data and HI cubes to recover the spectral information of the various
phases of the ISM presented here.

Figure 5 shows the normalised HI and Ly${\alpha}$ spectra as well as the
H$\alpha$ emission and the SiIIλ1260 absorption as a function of the radio
velocity shift in the rest-frame (using the velocity of the ionized gas,
determined by the optical line fitting of the MUSE spectrum at the location of
the aperture) extracted using the COS aperture333We note that spectra
presented in this figure are shifted compared to those presented for LARS08 in
Rivera-Thorsen et al. (2015) due to the offset position between the SDSS
aperture used to recover the redshift in that previous study and that of the
COS aperture. This makes the outflow seen in LARS08 less extreme compared to
what had been presented previously. . We note that the COS aperture is just
2.5 arcsec in diameter, which is of the order of the synthesized beam in the
D+C+B+A data. Given that Ly${\alpha}$ is produced in the HII regions probed by
H$\alpha$, this figure shows the relative velocity of the material that
produces and scatters Ly${\alpha}$, together with the spectral profile of
Ly${\alpha}$ after the scattering process. The SiIIλ1260 absorption allows one
to probe the neutral medium on the same scales as Ly${\alpha}$ and at lower
column densities (NHI $\geq$ 1017cm-2).

We observe several important features in these spectra. First, the SiIIλ1260
absorption spectrum is blueshifted with respect to center at $\sim-140$km s-1
(determined by moment-1), indicating an outflow of the neutral gas at that
velocity. The absorption profile reaches 0, indicating high column density HI
gas on the line of sight, and essentially complete covering. This is evidence
that some of the neutral gas as seen in the 21cm HI map is indeed located in
front of the Ly${\alpha}$ emission source, and is not seen to be overlapping
due to projection effects.

The HI emission line is overlapping with the H$\alpha$ line, but the HI line
center is slightly redshifted. At the location of the COS aperture, the
Ly${\alpha}$ intensity increases as the intensity of the HI emission
decreases. The whole Ly${\alpha}$ profile is redshifted compared to the HI and
H$\alpha$ lines and there is negligible emission at line-centre, indicating
that the line must undergo a significant velocity redistribution on the red
side to escape the scattering medium. The spectral shift between the HI and
Ly${\alpha}$ lines explain how Ly${\alpha}$ emission can be seen to overlap
spatially with high column density gas in LARS08: since Ly${\alpha}$ is
redshifted it is effectively “seeing” lower column density gas through the
ISM. The Ly${\alpha}$ is seen to peak at $\sim 200$ km s-1, with a potential
secondary red peak at $\sim 330$ km s-1. While the main Ly${\alpha}$ peak is
likely produced by scattering in the red wing of the Ly${\alpha}$ line, the
potential presence of the secondary peak coincides with a value of $\sim$twice
the expansion velocity of an ionized shell produced by the outflow, and would
be expected from back-scattering on the expanding shell (Verhamme et al.,
2006). There is no apparent Ly${\alpha}$ emission on the blue side of the
Ly${\alpha}$ line, coherent with the suppression of the blue peak by neutral
outflowing gas with covering fraction of 1.

###  4.3  Ly${\alpha}$ radiative transport - eLARS01

Similarly to LARS08, we examine the ISM phases of eLARS01 presented in Figure
6 in turn.

Figure 6: Top panel: eLARS01 D+C+B+A HI moment-0 map, with box indicating the
region presented on the bottom panels overlaid in black. a: eLARS01 D+C+B+A HI
moment-0 map with HST H$\alpha$ contours in units of 10-18 erg s-1 cm-2
arcsec-2 overlaid in orange. Numbered arrows indicate the position of the
strongest H$\alpha$ regions. b: eLARS01 D+C+B+A HI moment-0 map with HST
Ly${\alpha}$ contours in units of 10-16 erg s-1 cm-2 arcsec-2 overlaid in
orange. Numbered arrows indicate the position of the strongest H$\alpha$
regions as shown on a.

####  4.3.1 Star formation and Ly${\alpha}$ creation - H$\alpha$ emission

Figure 6 a shows H$\alpha$ emission contours on the HI column density map.
Numbered arrows indicate strongest H$\alpha$ emission regions ($>$2.0$\times
10^{-15}$ erg/s/cm2/arcsec2). Additionally, the right panel of Figure 13 in
the appendix shows CO contours in addition to the H$\alpha$ contours presented
on Fig. 6 a. There are two main star forming regions in this galaxy. The high
H$\alpha$ intensity emission region to the North of the galaxy (region 1) is
co-located with the 21cm absorption feature (see Figure 12). The second region
overlaps with an HI hole and with the peak of the CO emission. This suggests
that part of the HI hole corresponds to a region where the hydrogen is found
in a molecular form and fuels star formation. The optical image of eLARS01
presented in Figure 3 indicates that the H$\alpha$ emission regions correspond
to the cores of the two galaxies that are merging. Additionally, the faintest
H$\alpha$ emission contour ($\sim$2.5$\times 10^{-18}$ erg/s/cm2/arcsec2)
extends into the gas traced by the HI maps. Although we used a similar
limiting surface brightness for our H$\alpha$ contours, the size difference of
the fields of view of MUSE used to get the H$\alpha$ information for LARS08
and HST used for eLARS01 make it harder to quantitatively compare the
H$\alpha$ emission of eLARS01 to its extremely extended HI emission and to the
H$\alpha$ emission seen in LARS08.

####  4.3.2 Ly${\alpha}$ transport - HI emission

Figure 6 b shows Ly${\alpha}$ emission contours overlaid on the HI column
density map, with regions of strongest star-formation indicated by the same
arrows as presented on Fig. 6 a. In eLARS01 the strong and intermediate
Ly${\alpha}$ emission overlaps spatially in a more direct manner with the
H$\alpha$ emission. This is possibly due to the lower column density HI gas in
front of the Ly${\alpha}$ photon production sites (N${}_{HI}<2.2\times
10^{20}\,$cm-2), leading to lower spatial scattering of the line before
escape. However similarly to LARS08, the faint Ly${\alpha}$ contour is seen to
trace the H$\alpha$ contour at 2.5$\times 10^{-17}$ erg/s/cm2/arcsec2, partly
following the HI emission in the D+C+B+A data. Overall, there is a very high
degree of correlation between the H$\alpha$ and Ly${\alpha}$ emission.

####  4.3.3 Ly${\alpha}$ destruction - dust

We cannot obtain the dust distribution in eLARS01 in the same manner as for
LARS08, due to the PMAS observational settings. Furthermore, E(B-V) maps
obtained using HST imaging have lower fidelity due to the low SNR of H$\beta$
emission and contamination of the H$\alpha$ by the NII emission. The HST
H$\alpha$/H$\beta$ map suggests that the extinction is E(B-V)$\sim$1 magnitude
on average in the Ly${\alpha}$ emitting region, but varies between 0.2 and 2,
indicating variations in the dust geometry. In this configuration, the high
intensity Ly${\alpha}$ emission can possibly escape through the regions with
lower dust content within the gas. Additionally, since the HI column density
is lower, Ly${\alpha}$ does not scatter as much, and is thus less likely to be
absorbed by dust.

####  4.3.4 3D analysis - the view from the COS spectra

Figure 7: Left: Comparison of the normalised HI, Ly${\alpha}$, H$\alpha$
emission and SiIIλ1260 absorption spectra as a function of the rest-frame
velocity shift in eLARS01. The spectra are extracted using the COS aperture,
and the rest-velocity is defined by the H$\alpha$ centroid at that position.
The HI spectrum is obtained from the unmasked D+C+B+A VLA data cubes. The
dashed vertical line indicates the position of the rest velocity as defined by
the H$\alpha$ centroid in the aperture, the black horizontal line indicates
the zero flux level in the case of the emission spectra, and the continuum
level in the case of the absorption spectrum. Right: Zoomed-in version of the
Ly${\alpha}$ HI, and H$\alpha$ intensity maps with COS aperture position
represented by a black circle.

Figure 7 shows the normalised HI, Ly${\alpha}$ and PMAS H$\alpha$ emission
spectra, as well as the SiIIλ1260 absorption spectrum as a function of the
radio velocity shift in the rest-frame extracted using the COS aperture. The
21cm spectrum extraction at the COS aperture in eLARS01 yields no measurable
HI. The Ly${\alpha}$ line peaking at 160 km s-1 is redshifted compared to the
H$\alpha$ emission, but this time some emission is seen at line center and a
blue bump is seen at -170 km s-1. The SiII absorption profile indicates an
outflowing gas shell with expansion velocity at $\sim-290$ km s-1, with the
absorption extending up to -750 km s-1. The kinematics of the Ly${\alpha}$ in
eLARS01, in particular the smaller red peak velocity compared to LARS08
indicates that Ly${\alpha}$ photons need less scattering to escape the neutral
gas medium, certainly due to the lower gas column density. The presence of the
blue bump could be due to low covering fraction of the outflowing neutral gas
in eLARS01.

##  5 Discussion

For the first time we have obtained Ly${\alpha}$ imaging and direct
observations of the atomic gas at angular resolutions where spatial comparison
with Ly${\alpha}$ becomes meaningful. We support these observations with
optical integral field spectroscopy of the ionized gas obtained with the MUSE
and PMAS instruments. Here we discuss the results of the comparison between
different phases of the ISM and Ly${\alpha}$ emission in the observed
galaxies. We recall that the H$\alpha$ emission – probed both spatially and
kinematically by the MUSE and PMAS data – represents the intrinsic
Ly${\alpha}$ distribution. After production by recombination events in the HII
regions, the Ly${\alpha}$ photons then scatter in the HI material mapped by
the VLA 21cm imaging. This scattering process will continue until the photons
either escape, in which case we might observe them in the HST imaging if they
do so in the line of sight, or are absorbed by dust, which we probe for one of
the galaxies using the H$\alpha$/H$\beta$ line ratio assuming a dust screen
geometry.

###  5.1 The role of mergers on Ly${\alpha}$ emission from galaxies

The HI morphologies of the two galaxies presented in this study support that
both are undergoing some environmental interactions. The optical image of
eLARS01 clearly shows that the two galaxies of the system are currently in a
major merging event that likely triggered the starburst episode. For LARS08,
shell structures in the optical image indicative of such interaction were
already known (Micheva et al., 2018), but the optical evidence for the infall
of a galaxy is opposite to the newly detected HI tail. The connection between
LARS08 and the gas cloud without optical counterpart on the Western side of
the galaxy, where the gas is metal-poor (Figure 14) could point to the
accretion of metal-poor HI gas via tidal stripping, fueling the ISM of the
galaxy with neutral gas. The presence of another faint HI object detected with
the VLA in the vicinity of LARS08 seem to further indicate that the galaxy is
located in a group environment. HI gas is very sensitive to environmental
interactions, additionally more than half of the galaxies in the LARS and
eLARS sample display signs of galaxy interactions or merger in the optical.
This indicates that mergers might play a decisive role in fragmenting the
neutral ISM and in creating the conditions that facilitate Ly${\alpha}$
escape.

###  5.2 The escape of high intensity Ly${\alpha}$ radiation from galaxies

We have found ISM conditions helping the escape of Ly${\alpha}$ photons in
both galaxies. First, we note that high star formation rates and low HI column
density seem to enable the escape of high intensity Ly${\alpha}$ with little
spatial scattering. In eLARS01, we indeed observe that the Ly${\alpha}$
emission morphology is very similar to the H$\alpha$ emission, and is
coincident with a location where the column density of the HI gas overlapping
with Ly${\alpha}$ and H$\alpha$ sites is below the detection limit
($<2.2\times 10^{20}\,$cm-2). Our high-angular resolution observations are
particularly sensitive to high-surface brightness clouds. Thus, even if not
detected with the 21cm data, it is likely that some neutral gas is present in
front of the Ly${\alpha}$ regions in a more diffuse state, or in smaller,
fainter clumps. The comparison of the HI spectra also shows that the
Ly${\alpha}$ in eLARS01 needs a smaller velocity offset compared to the
H$\alpha$ to be able to escape the galaxy. This suggests that spatial
redistribution by scattering in the surrounding gas is negligible, likely due
to the presence of pathways for the Ly${\alpha}$ emission to escape directly.
Furthermore, the COS Ly${\alpha}$ spectrum shows a blue bump peaking around
v$\sim$-170 km s-1, which should be suppressed by the outflow of neutral gas.
In order for the Ly${\alpha}$ emission to escape and have such morphology, a
relatively low dust content mixed with the HII gas in the regions where we see
Ly${\alpha}$ is also needed, although we could not evaluate this precisely
with the available data. However, the H$\alpha$/H$\beta$ line ratio obtained
from HST imaging suggests a very heterogeneous dust medium, with escape
through dust-free channels possible thanks to the lower HI column density
which limits spatial scattering along the line of sight.
High star formation rates, high HI column density, and low dust contents on
the line of sight also enable the escape of high intensity Ly${\alpha}$
emission under certain gas kinematics. In LARS08, the highest intensity
Ly${\alpha}$ emission is clumpy and is found around regions of intense star
formation that border the highest column density HI clouds in regions with low
dust content (Figure 4). The fact that the high intensity Ly${\alpha}$
emission does not overlap with the highest intensity gas clouds despite the
intense star formation taking place on the line of sight suggests that
Ly${\alpha}$ photons follow paths of least resistance when scattering through
the gas by avoiding the densest clouds. However, it is surprising that
Ly${\alpha}$ can escape such dense medium at all. Absorption in the
Ly${\alpha}$ spectrum (Figure 5) is visible along the line-of-sight out to
velocities of almost -1000 km s-1, which implies significant absorption of the
UV continuum along the line-of-sight, with column densities in excess of 1020
cm-2, which is optically thick to Ly${\alpha}$($\tau\sim 10^{5}$).
Furthermore, on the line of sight where the COS spectrum is obtained, the SiII
absorption line reaches zero, implying a covering fraction of 1. Figure 5
shows the Ly${\alpha}$ is strongly redshifted with respect to rest velocity
defined by nebular lines in the MUSE spectrum. The Ly${\alpha}$ emission peaks
at a velocity of $\sim$200 km s-1, where co-spatial H$\alpha$ decreases
significantly, but where the HI density is significantly lower. Furthermore
the Ly${\alpha}$ emission as seen in the COS spectrum extends to significantly
redder velocities, well beyond where detectable amounts of HI are found.
Therefore, Ly${\alpha}$ photons seem to scatter in velocity space in order to
“bypass” the high column density HI gas on the line of sight and escape on the
red side. We conjecture that the mechanism responsible for the velocity-space
scatter of the Ly${\alpha}$ line at the COS aperture position is a combination
of scattering in the red wing of the line and back-scattering of the
Ly${\alpha}$ photons on the HI material of an expanding medium. Indeed
examining the neutral gas in the COS aperture through SiII line absorption,
one finds gas moving out to high velocities ($-$140 km s-1 on average and
$-$365 km s-1 at maximum), guaranteeing the presence of a strong outflow. This
is not seen in the HI data, however SiII probes column densities $\geq
10^{17}$ cm -2 and can be found both in HII and HI gas, and hence is not
tracing exactly the same medium as the 21-cm line is. The potential presence
of a secondary peak would be expected from back-scattering of the Ly${\alpha}$
photons on the expanding shell. The velocity redistribution of Ly${\alpha}$
photons is probably facilitated by the low dust obscuration on this line of
sight, meaning that Ly${\alpha}$ can undergo many scattering events with
limited absorption, leading to high intensity emission. On the other hand, no
strong or intermediate intensity Ly${\alpha}$ emission is seen overlapping
with star forming region 1 despite the high star formation intensity, probably
due to the high dust obscuration (E(B-V)$>0.6$), which seems to significantly
dampen the intensity of the emission at this location. Thus dust content seems
to impact the escape of high intensity Ly${\alpha}$ emission out of galaxies.

###  5.3 Faint Ly${\alpha}$ emission and HI structure

We now discuss our results regarding the faint Ly${\alpha}$ emission. We find
that in both galaxies, the faint Ly${\alpha}$ emission (1-1.5 10-16
erg/s/cm2/arcsec2) broadly follows the H$\alpha$ emission at $5-25\times
10^{-18}$ erg/s/cm2/arcsec2 where HI is detected with the D+C+B+A data
(N${}_{HI}\geq 2\times 10^{20}$cm-2). In LARS08, the faint Ly${\alpha}$ traces
the spiral arm, regardless of dust content on the line of sight. Thus while
dust seems to impede high intensity Ly${\alpha}$ emission, it does not prevent
the formation of low intensity Ly${\alpha}$ halo. Hayes et al. (2013) found
that the size of Ly${\alpha}$ halos in LARS galaxies was strongly correlated
with quantities that also scale with dust content. This suggests that dust
modulates Ly${\alpha}$ emission by either dampening the Ly${\alpha}$ output or
allowing only the escape of Ly${\alpha}$ emission from unobscured star forming
regions, resulting in a faint Ly${\alpha}$ halo. We note that the Ly${\alpha}$
emission contours we can access are limited by the HST field of view. Thus our
data cannot answer the question of how fainter Ly${\alpha}$ is distributed
relative to the neutral gas and whether it traces the extent of the HI in the
galaxies we study here.

##  6 Conclusions and Outlook

We have presented the first comparative study of Ly${\alpha}$ and 21-cm HI
emission at high angular resolution in galaxies, supported by molecular and
ionized gas observations. This study of two low-z starburst galaxies shows the
observational link between Ly${\alpha}$ creation, scattering and destruction
and the local structure of the interstellar medium. The main results of our
study are summarized hereafter:

  * -

Both galaxies are undergoing interactions which has possibly played a role in
triggering the starburst episodes they are experiencing. In eLARS01 a major
merger is ongoing, while LARS08 is interacting with a companion object with no
optical counterpart. The companion is either a low-surface brightness dwarf
galaxy or a tidal remnant which has been imaged here for the first time.
Additional deep optical imaging reveals shell features indicative of a prior
merger.

  * -

We find that high-intensity Ly${\alpha}$ emission ($>5\times 10^{-15}$
erg/s/cm2/arcsec2) is found overlapping with regions of intense star formation
traced by the H$\alpha$ emission. We find two ISM states facilitating the
emergence of high-intensity Ly${\alpha}$ emission. In eLARS01, with low column
density HI on the line of sight, high intensity Ly${\alpha}$ emission can
escape with little velocity redistribution and the Ly${\alpha}$ morphology is
very similar to that of the H$\alpha$ emission. In LARS08, we observe that
high intensity Ly${\alpha}$ emission can escape in the presence of high column
density HI with low dust extinction, but the line needs significant velocity
redistribution on the red side.

  * -

Fainter Ly${\alpha}$ emission ($1-1.5\times 10^{-16}$ erg/s/cm2/arcsec2)
broadly traces regions of intermediate H$\alpha$ intensity where we observe HI
in the D+C+B+A data. For the galaxy where we can measure dust obscuration, it
is present regardless of dust content on the line of sight, implying that
increased scattering will eventually lead to the formation of a faint
Ly${\alpha}$ halo. In this context dust obscuration seems to modulate, but
does not impede the formation of large-scale Ly${\alpha}$ emission. Lower
intensity Ly${\alpha}$ emission might trace HI structures on larger scales but
the limited field of view of HST compared to the 21cm data does not allow us
to assess the full scale of the Ly${\alpha}$ emission.

The results we have obtained highlight the importance of the intermediate-
scale ($\sim$kpc) ISM structure to Ly${\alpha}$ escape, in particular high
intensity Ly${\alpha}$ emission. Although the analysis has remained very
exploratory, we have been able to explain Ly${\alpha}$ escape in both galaxies
using simple scenarios that suggest that scaling relations between HI,
H$\alpha$, dust and Ly${\alpha}$ could be found.

The two galaxies presented here are interesting case studies, but they
represent a very small sub-sample of the LARS+eLARS galaxies, so a
quantitative analysis will require a larger sample size. Obtaining further VLA
A-configuration observations for the 15 galaxies already having intermediate
angular scale observations (D+C+B) would be a way to extend the results
presented here to a larger range of galaxies and ISM environments. However,
the use of the highest angular resolution data presents challenges in terms of
exposure time and signal-to-noise ratio, that make this approach impractical
for a large sample of galaxies. Additionally, we identified that the
combination of D, C, and B data allows to identify several of the HI
substructures and represents a compromise between angular resolution and
sensitivity (see Figures 8 and 9). Thus in future work, we will exploit the
VLA B-configuration data already obtained for 15 LARS galaxies to explore the
link identified here between Ly${\alpha}$ escape and resolved ISM properties.
Some future work will also include detailed analysis of the HI emission
properties and kinematics of the full LARS + eLARS galaxies at lower angular
resolution (VLA D+C data), to quantify the impact of global HI properties on
Ly${\alpha}$ emission. These will be important steps to quantify the impact of
neutral gas on Ly${\alpha}$ radiative transfer at a variety of physical
scales.

A.L.R. would like to thank the referee for their comments and members of the
Stockholm University Astronomy Department galaxy group for their feedback
throughout the project.
M.H. is fellow of the Knut and Alice Wallenberg Foundation.
E.C.H thanks the staff at Calar Alto Observatory for help with the visitor
mode observations.
G.O. was supported by the Swedish Research Council (Vetenskapsrådet).
J.M.C. and N.V. acknowledge support from Macalester College. DK is funded by
the CNES/CNRS agreement N. 180027. The National Radio Astronomy Observatory is
a facility of the National Science Foundation operated under cooperative
agreement by Associated Universities, Inc.
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained
from the data archive at the Space Telescope Science Institute. STScI is
operated by the Association of Universities for Research in Astronomy, Inc.
under NASA contract NAS 5-26555.
Based on observations collected at the European Southern Observatory under ESO
programme 0101.B-0703(A). Data here reported were acquired at Centro
Astronómico Hispano Alemán (CAHA) at Calar Alto operated jointly by Instituto
de Astrofísica de Andalucía (CSIC) and Max Planck Institut für Astronomie
(MPG). Centro Astronómico Hispano en Andalucía is now operated by Instituto de
Astrofísica de Andalucía and Junta de Andalucía.

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##  Appendix A Lower-angular resolution HI maps of LARS08 and eLARS01

Figure 8: Moment maps for the 21cm HI emission line of LARS08, obtained with
the VLA D, D+C, D+C+B and D+C+B+A configuration data. From left to right:
column density map, rest-frame intensity-weighted velocity (moment-1) map,
velocity dispersion map. Figure 9: Moment maps for the 21cm HI emission line
of eLARS01, obtained with the VLA D, D+C, D+C+B and D+C+B+A configuration
data. From left to right: column density map, rest-frame intensity-weighted
velocity (moment-1) map, velocity dispersion map.

##  Appendix B HI channel maps

Figure 10: Full HI channel map for the D+C+B+A data of LARS08 with contours of
0.24 mJy/beam in grey and 0.4 mJy/beam in black corresponding to the
1.5$\sigma$ and 3$\sigma$ noise levels. Figure 11: Full HI channel map for the
D+C+B+A data of eLARS01, with contours of 0.26 mJy/beam in grey and 0.51
mJy/beam in black corresponding to the 1.5$\sigma$ and 3$\sigma$ noise levels.

##  Appendix C 21cm absorption in eLARS01

Figure 12: Left panel: 21cm HI moment-0 map with absorption contours overlaid
in white in units of [0.5, 1.0, 1.5] $\times 10^{18}\,Ts/f$ cm-2 . The
aperture used to extract the spectrum is shown by a black circle. Top right
panel: 21cm absorption profile extracted from the aperture shown on the left
panel. Bottom right panel: Total 21cm absorbing column density as a function
of spin temperature Ts and covering fraction $f$ for temperatures
characteristic of the cold neutral medium. The solid line corresponds to the
observed self-shielding limit of HI (Kanekar et al., 2011), the dotted line
corresponds to the threshold for the formation of molecular hydrogen (Savage
et al., 1977) and the dashed line to the threshold where most of the neutral
gas is molecular (Schaye, 2001).

Figure 12 shows the absorption profile extracted from an aperture the average
size of the beam in eLARS01. The velocity offset has been calculated using the
H$\alpha$ centroid at the aperture location. The HI gas seen in absorption is
redshifted compared to the nebular gas, indicating an inflow of neutral gas
towards the star-forming region. Although a Ly${\alpha}$ spectrum is not
available at this location, the presence of the neutral gas inflow suggests
that we would observe a Ly${\alpha}$ profile with a strong blue peak at the
location of 21cm absorption. It is interesting to find evidence of a neutral
gas inflow overlapping with the Northern star forming region in eLARS01, just
5 kpc away from the second star forming region where the COS spectrum was
obtained, and where the SiII absorption profile indicates a neutral gas
outflow. This suggests that the feedback processes occurring in the two star-
forming regions are different.

The absorbing column density in cm-2 can be calculated using
$N_{HI}=1.822\times 10^{18}\ T_{s}/f\,\frac{F_{HI,abs}}{F_{HI,cont}}\,\Delta
v$, where $T_{s}$ is the spin temperature in K, $f$ is the covering fraction,
$F_{HI,abs}$ is the absorbing flux in mJy and $F_{HI,cont}$ the continuum flux
in mJy. We find that the continuum flux at the 21cm frequency is $F_{HI,cont}$
= 28.9$\pm$0.9mJy. The significant uncertainty on the spin temperature Ts and
the covering fraction $f$ limits our ability to calculate the column density
of the absorbing gas. On the bottom right panel of Figure 12 we show the
column density as a function of the spin temperature and covering fraction for
Ts values characteristic of the cold neutral medium (40-500K, Kanekar et al.
(2011)). We find that the absorbing gas is above the effective self-shielding
limit for HI $N_{HI}>2\times 10^{20}$cm-2 (Kanekar et al., 2011), consistent
with the gas being observed in front of a star-bursting location emitting UV
emission. Without self-shielding the HI at this location would likely be
ionized. Since the peak of the absorption feature is found just next to the
location where molecular gas is observed (see Figure 13), we conclude that the
column density is likely above the threshold $N_{HI}=5\times 10^{20}$cm-2
where HI starts to become molecular (Savage et al., 1977), but below
$N_{HI}<1\times 10^{22}$cm-2, the threshold for which hydrogen is found in a
predominantly molecular form (Schaye, 2001). These limits poorly constrain the
range of values for the spin temperature and the covering fraction, however
the similarity between the H$\alpha$ and Ly${\alpha}$ morphologies at this
location indicate that Ly${\alpha}$ photons escape with relatively little
scatter through the neutral gas of the galaxy. This could be explained by the
neutral gas having a low covering fraction, which is not excluded by the
limits, with covering fractions as small as 3% being possible for a spin
temperature T${}_{s}<55\,$K.

##  Appendix D CO in LARS008 and eLARS01

In Figure 13, we show H$\alpha$ and CO(2-1) contours on the HI D+C+B+A
moment-0 maps of LARS08 and eLARS01 (The CO data will be presented in Puschnig
et al, in prep.). Since CO(2-1) traces molecular gas, the CO contours show
star-forming regions. CO contours overlap with regions of strong star
formation and low HI column density in both galaxies, suggesting the drop in
HI column density is linked to the presence of Hydrogen in a molecular, rather
than atomic form.

Figure 13: Zoomed-in portion of the HI column density map, with MUSE H$\alpha$
contours overlaid in black and CO(2-1) contours in yellow. For LARS08 (left),
the H$\alpha$ contour levels increase from the outer part of the galaxy, with
the same levels as in Fig. 4 a displayed. For eLARS01 (right), the H$\alpha$
contour levels increase from the outer part of the galaxy, with the same
levels as in Fig. 6 a displayed. CO(2-1) contours representing the extent and
peak of the emission are shown in arbitrary units. In each panel, arrows
labelled with numbers indicate the position of the regions with strongest
H$\alpha$ emission. The size of the VLA D+C+B+A synthesized beam is indicated
on the bottom left of the mom-0 map by a white ellipse.

##  Appendix E Metallicity map of LARS08

Figure 14: N2 metallicity map of LARS08.

Figure 14 presents the N2 metallicity map of LARS08, derived from the
[NII]λ6584/H$\alpha$ ratio in the MUSE data cube using the Marino et al.
(2013) calibration. The metallicity exhibits a radial gradient, and is lower
along the main spiral arm, especially in the outer part to the South and West
of the galaxy. The strongest metallicities are found in the inner part of the
galaxy, to the East in particular.