# The connection between galactic outflows and the escape of ionizing photons

Ramesh Mainali Observational Cosmology Lab, Code 665, NASA Goddard Space
Flight Center, Greenbelt, MD 20771, USA  Department of Physics, The Catholic
University of America, Washington, DC 20064, USA  Jane R. Rigby Observational
Cosmology Lab, Code 665, NASA Goddard Space Flight Center, Greenbelt, MD
20771, USA  John Chisholm Astronomy Department, University of Texas at Austin,
2515 Speedway, Stop C1400, Austin, TX 78712-1205, USA  Matthew Bayliss
Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA
Rongmon Bordoloi Department of Physics, North Carolina State University,
Raleigh, North Carolina, 27695  Michael D. Gladders Department of Astronomy &
Astrophysics, The University of Chicago, 5640 S. Ellis Avenue, Chicago, IL
60637, USA  T. Emil Rivera-Thorsen The Oskar Klein Centre, Department of
Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden  Håkon
Dahle Institute of Theoretical Astrophysics, University of Oslo, P.O. Box
1029, Blindern, NO-0315 Oslo, Norway  Keren Sharon University of Michigan,
Department of Astronomy, 1085 South University Avenue, Ann Arbor, MI 48109,
USA  Michael Florian Steward Observatory, University of Arizona, 933 North
Cherry Ave., Tucson, AZ 85721, USA  Danielle A. Berg Astronomy Department,
University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX
78712-1205, USA  Soniya Sharma  Observational Cosmology Lab, Code 665, NASA
Goddard Space Flight Center, Greenbelt, MD 20771, USA  M. Riley Owens
Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA
Karin Kjellgren  The Oskar Klein Centre, Department of Astronomy, Stockholm
University, AlbaNova, SE-10691 Stockholm, Sweden  Keunho J. Kim Department of
Physics, University of Cincinnati, Cincinnati, OH 45221, USA  Julia Wayne
Department of Physics, University of Cincinnati, Cincinnati, OH 45221, USA
<EMAIL_ADDRESS><EMAIL_ADDRESS>
###### Abstract

We analyze spectra of a gravitationally lensed galaxy, known as the Sunburst
Arc, that is leaking ionizing photons, also known as the Lyman continuum
(LyC). Magnification from gravitational lensing permits the galaxy to be
spatially resolved into one region that leaks ionizing photons, and several
that do not. Rest-frame ultraviolet and optical spectra from Magellan target
ten different regions along the lensed Arc, including six multiple images of
the LyC leaking region, as well as four regions that do not show LyC emission.
The rest-frame optical spectra of the ionizing photon emitting regions reveal
a blue-shifted ($\Delta V$=27 km s-1) broad emission component (FWHM=327 km
s-1) comprising 55% of the total [OIII] line flux, in addition to a narrow
component (FWHM = 112 km s-1), suggesting the presence of strong highly
ionized gas outflows. This is consistent with the high-velocity ionized
outflow inferred from the rest-frame UV spectra. In contrast, the broad
emission component is less prominent in the non-leaking regions, comprising
$\sim$26% of total [OIII] line flux. The high ionization absorption lines are
prominent in both leaker and non-leaker but low ionization absorption lines
are very weak in the leaker, suggesting that the line of sight gas is highly
ionized in the leaker. Analyses of stellar wind features reveal that the
stellar population of the LyC leaking regions is considerably younger ($\sim$3
Myr) than the non-leaking regions ($\sim$12 Myr), highlighting that stellar
feedback from young stars may play an important role in ionizing photon
escape.

cosmology: observations - galaxies: evolution - galaxies: formation -
galaxies: high-redshift

##  1 Introduction

Understanding the epoch of reionization, when the last phase transition of the
Universe occurred, is one of the longstanding goals of extragalactic
astronomy. The massive stars residing in these high-redshift star-forming
galaxies are suspected to be the dominant ionizing agents that drive
reionization (e.g. Robertson et al., 2015; Finkelstein et al., 2019), although
some studies suggest that low luminosity active galactic nuclei (AGN) may play
a significant role in reionization (e.g. Madau & Haardt, 2015; Kulkarni et
al., 2019). The study of reionization requires understanding the rate at which
ionizing photons are injected into the intergalactic medium (IGM) by the
galaxy population, known as ionizing emissivity (e.g. Robertson et al., 2015;
Finkelstein et al., 2019). In practice, this quantity can be measured by
combining three key factors: the rest-frame ultraviolet (UV) luminosity
density ($\rm\rho_{UV}$), the ionizing photon production efficiency
($\rm\xi_{ion}$), and the fraction of ionizing photons that escapes from
galaxies ($\rm f_{esc}$). Soon The James Webb Space Telescope (JWST) will
measure the first two quantities at the epoch of reionization, however, the
measurement of $\rm f_{esc}$ will heavily rely on indirect probes at high
redshift or work on analogs at lower redshift since ionizing fluxes cannot be
measured at $z>6$ due to the large opacity of the IGM. Further progress hinges
on understanding the detailed astrophysics of ionizing photons escape from
Lyman continuum (LyC) emitting systems at lower redshift.

One of the main suspects in aiding escape of ionizing photons from galaxy is
galaxy scale outflows (e.g. Weiner et al., 2009; Heckman et al., 2011; Newman
et al., 2012; Jones et al., 2013; Bordoloi et al., 2014; Rubin et al., 2014;
Alexandroff et al., 2015; Chisholm et al., 2017; Kim et al., 2020). These
outflows are expected to remove surrounding neutral gas, thus clearing a
pathway for ionizing photons to efficiently escape. However, it is often
challenging to reconcile the timescales of such strong galactic outflows with
the timescale of production and escape of ionizing photons in a galaxy. This
has been demonstrated in cosmological simulations where a detailed treatment
of the multiphase interstellar medium (ISM) and feedback were possible, yet
very low $\rm f_{esc}$ of only a few percent level were predicted (e.g. Gnedin
et al., 2008; Kim et al., 2013; Wise et al., 2014; Ma et al., 2015; Rosdahl et
al., 2018). In these simulations, most of the ionizing photons are consumed by
surrounding neutral gas clouds (e.g. Ma et al., 2015; Kimm et al., 2017;
Kakiichi & Gronke, 2021). The feedback from supernovae activity (SNe) may help
in the ionizing photon escape but SNe first occur after about 3 Myr of massive
star formation and peaks around 10 Myr. On the other hand, the ionizing photon
production from massive stars begins to decline after 3 Myr, and thus allows
for a very narrow timing window for the escape of ionizing photons (Ma et al.,
2015).

Figure 1: HST F814W image of the Sunburst Arc showing the positions of FIRE
and MagE slits. The red rectangles show the position and orientation of 10
FIRE slits, and the black rectangles show the position and orientation of 9
MagE slits. The solid and dashed rectangles indicate LyC photons leaking and
non-leaking knots, respectively. The numbers indicate multiple images of a LyC
emitting region.

Recent progress on the above picture has come from high-resolution
cosmological simulations of LyC emitting sources in the EoR (Ma et al., 2020),
studying $z>5$ simulated galaxies from the Feedback in Realistic Environments
project (FIRE: Hopkins et al. 2018). The latest version of the simulations
incorporates the multi-phase ISM, star formation, and stellar feedback. The
study finds that a majority of LyC escape comes from the very young ($<$10
Myr), vigorously star-forming regions of a galaxy, with negligible
contribution from an older ($>10$ Myr) stellar population. The LyC escaping
sites are often characterized by feedback-driven kpc-scale superbubbles which
clear out the neutral gas column to allow the escape of ionizing photons.

In order to test the theoretical picture discussed above, we must study LyC
emitting galaxies in detail to determine the astrophysics that regulate the
escape of LyC photons into the IGM. Galaxies at redshifts above $\sim 0.1$ are
in general too distant to study in any detail. Rest-UV spectra of nearby
galaxies require observations with the Cosmic Origins Spectrograph on HST,
whose 2$\farcs$5 aperture makes it impossible to pinpoint LyC sources on
scales smaller than a 100s of pc for the closest galaxies. However, the lower
redshift LyC emitters are very rare; only 3 have been discovered at $z<0.1$
(Leitet et al., 2011, 2013; Leitherer et al., 2016). The average measured LyC
escape fractions are found to be only a few percent suggesting very little
ionizing radiation of stellar origin escapes from galaxies. At slightly higher
redshifts, $z\sim$ 0.1–0.4, “Green Pea” galaxies are found to be LyC emitters,
with escape fractions up to $\sim 75\%$ (Borthakur et al., 2014; Izotov et
al., 2016, 2018, 2021; Flury et al., 2022a). Recently, several programs have
discovered LyC emitting galaxies at $1.5\lesssim z\lesssim 4$; only a handful
of these galaxies show large LyC escape fractions (Vanzella et al., 2016;
Shapley et al., 2016; Bian et al., 2017; Vanzella et al., 2018; Fletcher et
al., 2019; Steidel et al., 2018; Rivera-Thorsen et al., 2019). Overall, the
results from lower redshift LyC studies suggest that the LyC escape from an
average galaxy population is significantly lower than that required to drive
cosmic reionization. It is thus crucial to understand the key differences
between lower redshift and reionization era galaxies, and how they impact the
overall escape of LyC photons.

Highly magnified lensed galaxies at $z>1$ with significant LyC leakage may
present the most tractable sites to study the escape of ionizing photons at
any redshift. This point is powerfully demonstrated by the _Sunburst Arc_ at z
= 2.37 (Dahle et al., 2016; Rivera-Thorsen et al., 2017, 2019; Chisholm et
al., 2019), also known as PSZ1-ARC G311.6602–18.4624, one of the brightest
gravitationally lensed galaxies known. HST observations in the rest-frame LyC
(the observed F275W band) clearly show that this galaxy appears 12 times in
the image plane with a significant amount of ionizing photons leakage (Rivera-
Thorsen et al., 2019). The LyC emitting region has a highly unusual, triple-
peaked Lyman-$\alpha$ line profile, which Rivera-Thorsen et al. (2017)
interpret as evidence that the radiation escapes through a narrow, empty
channel in an otherwise optically thick medium.

This paper leverages rest-frame UV and optical spectroscopy of multiple
distinct regions within the Sunburst arc, to compare regions that are leaking
LyC photons to those that do not. This allows us to connect the properties of
the outflows to their local driving sources. Rest-frame ultraviolet spectra
from the Magellan Echellette (MagE: Marshall et al., 2008) instrument on the
Magellan/Baade telescope constrain the massive stellar population as well as
the interstellar medium. Rest-frame optical spectra from the Folded-port
InfraRed Echelle (FIRE: Simcoe et al., 2008) instrument on Magellan/Baade
constrain the nebular gas.

The paper is organized as follows. We discuss our observational strategy and
data reduction in §2. In §3, we discuss the methodology, and present analysis
in §4. We discuss implications for the physical picture of ionizing photon
escape and inferred size scale of outflowing gas in §5, and summarize our
findings in §6. Throughout this paper, we adopt a $\Lambda$-dominated, flat
universe with $\Omega_{\Lambda}=0.7$, $\Omega_{M}=0.3$ and
$\rm{H_{0}}=70\,\rm{h_{70}}~{}{\rm km\,s}^{-1}\,{\rm Mpc}^{-1}$. All
magnitudes are quoted in the AB system. Equivalent widths are quoted in rest-
frame, unless stated otherwise.

FIRE slit | MagE slit | RA | DEC | UT Date | time (ks) | LyC leaker | zspec | PA | Magnification($\mu$)
---|---|---|---|---|---|---|---|---|---
F-0 | M-0 | 15:50:04.317 | -78:10:59.87 | 2016 Mar 30 | 3.6 | Yes | 2.37014 | 140.2 | 7.8${}^{+15}_{-5.9}$
F-1 | … | 15:50:06.008 | -78:10:58.07 | 2017 Mar 28 | 3.6 | Yes | 2.37009 | 30.2 | 40.3${}^{+1.0}_{-4.7}$
F-2 | M-2 | 15:49:59.662 | -78:11:13.48 | 2017 Mar 28 | 3.6 | Yes | 2.37017 | 100.2 | 30.7${}^{+6.1}_{-2.9}$
F-3 | M-3 | 15:50:00.488 | -78:11:10.00 | 2017 Mar 28 | 4.8 | No | 2.37025 | 125.2 | 36.7${}^{+4.5}_{-4.9}$
F-4 | M-4 | 15:50:04.824 | -78:10:59.20 | 2017 Mar 29 | 3.6 | No | 2.37073 | 140.2 | 14.7${}^{+0.7}_{-2.6}$
F-5 | M-5 | 15:50:01.005 | -78:11:08.07 | 2017 Mar 29 | 2.7 | No | 2.37086 | 165.2 | 49.4${}^{+5.0}_{-11.3}$
F-6 | M-6 | 15:50:06.557 | -78:10:57.52 | 2017 Mar 29 | 2.7 | No | 2.37021 | 32.2 | 141.0${}^{+35.0}_{-27.0}$
F-7 | M-7 | 15:50:07.238 | -78:10:57.22 | 2017 Mar 29 | 2.7 | Yes | 2.37044 | 20.2 | 34.4${}^{+2.4}_{-5.6}$
F-8 | M-8 | 15:49:59.834 | -78:11:12.48 | 2017 Aug 28 | 3.6 | Yes | 2.37024 | 100.2 | 28.9${}^{+6.1}_{-2.9}$
F-9 | M-9 | 15:50:00.251 | -78:11:10.71 | 2017 Aug 27 | 5.4 | Yes | 2.37030 | 100.2 | 30.9${}^{+3.8}_{-3.2}$
Table 1: Observation log for Magellan/FIRE. The first and second columns
represent FIRE slit names and the closest corresponding MagE slit names. The
remaining columns are Right ascension (J2000), Declination (J2000), Universal
date, Universal time, integration time in kiloseconds, whether that source is
a LyC leaker according to the HST F275W imaging, spectroscopic redshift
measured from optical lines, the position angle (degrees E of N) of the FIRE
slit and the average magnification factor ($\mu$) within the slit.

##  2 Observations

###  2.1 Magellan/FIRE near-IR spectroscopy

We observed the Sunburst Arc over the course of three observing runs from
2016–2018. We used the FIRE (Simcoe et al., 2008) instrument on Magellan in
echelle mode, providing continuous spectral coverage for wavelengths between
0.82 and 2.51 $\rm\mu$m. We adopted a slit width of 0$\farcs$6, resulting in a
resolving power of $R=6000$. Ten different regions within the arc are
observed, including both LyC-emitting and non-emitting regions. The
orientations of the 10 different slits are shown in Figure 1. The exposures
were carried out using two dither positions separated by 3$\farcs$0\. We
obtained total exposure of 9.08 hr with an average exposure of 54.5 min per
slit position. Slits were positioned on the sky by directly acquiring one of
two nearby reference stars (see Figure 1) and then applying small
($\lesssim$11″) coordinated offsets to move the slit onto the desired location
along the arc.

The FIRE spectra were reduced using standard routines in the FIREHOSE data
reduction
pipeline111wikis.mit.edu/confluence/display/FIRE/FIRE+Data+Reduction. The
pipeline calculates two-dimensional sky models iteratively, following Kelson
(2003). The wavelength solutions are provided by fitting OH skylines in the
science spectra. Flux calibration and telluric corrections are applied using
A0V star observations that were obtained immediately before or after science
observations. The one-dimensional spectra were extracted by using optimal
extraction, obtained by calculating spatial profiles of the strongest, well-
isolated emission lines in the FIRE spectra (i.e. [OIII]$\lambda$5007).

###  2.2 Near-infrared HST grism spectroscopy

Near-infrared spectra of the Sunburst Arc were obtained using the HST WFC3/IR
G141 grism from program GO-15101 (PI: H. Dahle). The WFC3/IR G141 grism covers
wavelengths from 1.075 $\mu m$ to 1.7 $\mu m$ and has a resolving power of
$R\sim 130$. At the redshift of Sunburst Arc ($z=2.37$), the G141 grism covers
rest-frame wavelength of $\sim$3200 to 5040 Å, allowing measurements of all
emission lines from [O II] 3727 to [O III] 5007. The grism observations were
performed at two different orientation angles (27.37∘ and 355.37∘), as is
standard practice to mitigate contamination from cluster galaxies. The total
grism integration time was 8.42 Ks.

The HST grism spectra were reduced following the procedures described in
Florian et al. (2021). To summarize, we used the software package Grizli13
(Brammer, 2018), following the standard steps plus an extra step of GALFIT
(Peng et al., 2010) modeling of the contamination from more than 80 foreground
stars and cluster galaxies. Since the Galactic coordinates of the Sunburst Arc
are close to the Galactic plane, there are more foreground stars in the field
of view than is typical for lensed galaxies. Most of the grism spectra are
thus contaminated by foreground star lights at several locations. In this
paper, we focus on grism spectrum of LyC emitting knot 3 which is devoid of
any contaminant, allowing reliable measurement of both continuum and emission
lines. The continuum and emission lines in the spectra were fit as described
in Rigby et al. (2021).

###  2.3 Magellan/MagE optical spectroscopy

Observations of the Sunburst Arc with the MagE spectrograph (Marshall et al.,
2008) were made as a part of the extension to The Magellan Evolution of
Galaxies Spectroscopic and Ultraviolet Reference Atlas (MegaSaura). The main
MegaSaura survey is described in Rigby et al. (2018); the MegaSaura extension
will be described in Rigby et al. (in prep.) Here we summarize key
observational details. Observations were conducted with MagE mounted on the
Baade Magellan telescope. Target acquisition was performed by offsetting from
a nearby bright star, with acquisition verified from the slit-viewing guide
camera. Nine slit positions were obtained; all used the 0.85″ slit width
except slit position M-0, which used the 1″ slit width — motivated by the
atmospheric seeing during the observing runs. These observations overlap with
9 of the 10 FIRE slit positions. This overlap enables joint analysis of the
rest-frame ultraviolet and optical spectra. The MagE slit positions
corresponding to FIRE positions are given in Table 1.

The MagE spectra cover observed wavelengths of 3200 to 8280 Å. For the
Sunburst Arc at z=2.37, this results in rest-frame wavelength coverage of 950
Å to 2457 Å. The spectral resolution, as measured from the widths of night
skylines, was $5300\pm 300$ for the observations with the 0.85″ width slit,
and $4700\pm 200$ for the observations with the 1″ width slit.

The data were reduced as described in Rigby et al. (2018).

###  2.4 X-ray Observations

The field containing the Sunburst Arc was observed with the Chandra X-ray
Observatory under observation ID 20442 (PI: Bayliss). The purpose of these
observations was to test for the presence of an X-ray bright active galactic
nucleus (AGN) as a possible source of ionizing radiation in the lensed galaxy.
The observations consist of a single 39.53 ks exposure with the aimpoint
located near the center of the I3 chip in the ACIS-I array in VFAINT telemetry
mode. All Chandra data reductions and analysis were performed using the
Chandra Interactive Analysis of Observations (CIAO v4.13) with CALDB v4.9.6.
We reprocessed the data using the CIAO chandra_repro routine, and filtered the
data for flares using the lc_sigma_clip function in the lightcurves Python
package that is included in CIAO. The resulting “clean” integration time is
38.53 ks. We apply an energy filter of 0.5–7 keV, visually identify bright
sources in the data and perform an initial background estimation by masking
out those visually identified sources. We then run the wavdetect procedure
using the initial background estimate to identify point sources in the data.
We then mask the point sources and the bright emission from the galaxy cluster
responsible for lensing the Sunburst Arc, and use the resulting “source-free”
data to perform a final measurement of the background statistics.

Figure 2: Magellan/FIRE spectra (normalized by [OIII]$\lambda$5007) showing
key spectral features in the stacked spectrum of LyC leaking regions (blue)
and non-leaking regions (red). The corresponding error spectrum is shown in
dotted blue (leaker) and dotted red (non-leaker). Figure 3: Magellan/MagE
spectra showing the stacked spectrum of LyC leaking regions (blue) and non-
leaking regions (red). The leaker spectrum is normalized by a median continuum
level between 1400 Å to 1500 Å, while the non-leaker is further offset by 0.5
for clarity of plotting. The corresponding error spectrum is shown in dotted
blue (leaker) and dotted red (non-leaker). The figure shows several emission
(black label) and absorption (red label) features. The emission lines are more
prominent in the leaker spectrum while absorption lines, particularly low
ionization interstellar features (see §3 for more details), are prominent in
the non-leaker spectrum.

##  3 Methods

| Leaker | Non-Leaker
---|---|---
Line | Narrow | Broad |  | Narrow | Broad
$\rm{[OII]3727}$ | 4.9$\pm$0.2 | 4.5$\pm$0.3 |  | 0.6$\pm$0.1 | 0.6$\pm$0.2
$\rm{[OII]3729}$ | 5.3$\pm$0.2 | 4.8$\pm$0.3 |  | 0.8$\pm$0.1 | 0.8$\pm$0.2
$\rm[NeIII]3869$ | 3.0$\pm$0.2 | 7.5$\pm$0.4 |  | 0.5$\pm$0.3 | 0.7$\pm$0.9
H$\beta$ | 6.8$\pm$0.3 | 14.2$\pm$0.4 |  | 1.5$\pm$0.2 | 1.5$\pm$0.4
$\rm{[OIII]5007}$ | 64.8$\pm$0.3 | 77.8$\pm$0.4 |  | 12.2$\pm$0.1 | 4.3$\pm$0.3
H$\alpha$ | 27.1$\pm$0.5 | 47.1$\pm$0.6 |  | 5.6$\pm$0.2 | 3.9$\pm$0.3
$\rm{[NII]6584}$ |  $<$0.6 |  $<$1.1 |  |  $<$0.3 |  $<$0.6
Table 2: Two component gaussian fits to the stacked FIRE spectra of the LyC leaker and the non-leaker. The table shows line fluxes in both the narrow and broad emission components. Line fluxes are given in the units of 10-17 erg s-1 cm-2. MagE slit | E(B-V) | $\sigma$ | Metallicity (Z⋆/Z⊙) | Age (Myr)
---|---|---|---|---
M-0∗ | $0.146$ | $0.003$ | $0.55$ | $2.92$
M-2∗ | $0.059$ | $0.003$ | $0.50$ | $3.04$
M-3 | $0.018$ | $0.003$ | $0.23$ | $4.03$
M-4 | $0.078$ | $0.005$ | $0.36$ | $10.1$
M-5 | $0.054$ | $0.004$ | $0.24$ | $12.4$
M-6 | $0.080$ | $0.004$ | $0.28$ | $11.3$
M-7∗ | $0.098$ | $0.007$ | $0.40$ | $2.60$
M-8∗ | $0.040$ | $0.002$ | $0.40$ | $2.65$
M-9∗ | $0.049$ | $0.005$ | $0.44$ | $2.84$
Table 3: Derived E(B-V) reddening, stellar metallicity, and stellar age, from stellar population fits to the MagE data for the Sunburst arc. The asterisk denotes the MagE slits targeting LyC leaking regions. Line Ratios | Leaker | Non-Leaker
---|---|---
O32 | 7.5$\pm$0.9 | 6.1$\pm$1.9
$\rm{[OIII]5007}$/H$\beta$ | 6.6$\pm$0.8 | 5.3$\pm$1.5
R23 | 9.4$\pm$1.1 | 8.3$\pm$2.3
$\rm{[NII]6584}$/H$\alpha$ |  $<$0.002 |  $<$0.009
Table 4: Table showing optical line diagnostic ratios (combined broad and
narrow) for the leaker and non-leaker stacks. The limits are 3-$\sigma$

###  3.1 Stacking the MagE and FIRE spectra

As demonstrated in Rivera-Thorsen et al. (2019), all the LyC leaking regions
in the image plane that were targeted with Magellan/FIRE and Magellan/MagE are
in fact multiple images of a single physical region in the source plane. It is
therefore appropriate to combine the spectra from these “leaker” slits, since
they arise from the same physical source. By contrast, the 4 slits that target
non-leaking regions (see Figure 1) correspond, according to the lens model, to
at least seven different physical regions. While it is desirable to study
these regions separately, for some applications stacking is a practical
necessity, as the surface brightness of the non-leaking regions is low. As
such, we separately stacked the spectra of LyC leaking regions and the non-
leaking regions, to study characteristic differences.

The FIRE stacking was conducted as follows. As Table 1 lists, 6 FIRE slits
(F-0, F-1, F-2, F-7, F-8, F-9) cover the LyC leaker, and 4 (F-3, F-4, F-5,
F-6) cover non-leaking regions. Of the 4 slits covering non-leaking regions,
we exclude slit F-3 for further analysis since this region may contain an
atypical extragalactic object (Vanzella et al., 2020). The remaining spectra
are then normalized by their respective [OIII]$\lambda$5007 flux density. We
then perform average stacking of FIRE spectra of the 6 LyC leaker slit
positions and 3 non-leaking slit positions. Figure 2 shows the resulting
stacked FIRE spectra.

We also produced stacked rest-frame ultraviolet spectra of the LyC leaker and
the non-leaking regions. All but one FIRE slits (i.e. F-1) have corresponding
MagE spectra. We also exclude the MagE spectrum corresponding to the FIRE F-3
position (i.e M-3) for further stacking analysis, resulting in 5 leaker (M-0,
M-2, M-7, M-8, M-9) and 3 non-leaker (M-4, M-5, M-6) MagE spectra. Since all
LyC leaker images are physically the same star-forming region (Rivera-Thorsen
et al., 2019), leaving one leaker region (compared to FIRE stack) without
being included in the stacked spectrum other than slightly lowering the signal
to noise. We then normalized all the leaker and non-leaker spectra with their
continuum flux density from the rest frame 1400 Å to 1500 Å. After
normalization, we perform average stacking to produce the stack rest-frame
ultraviolet spectra for leakers and non-leakers. Figure 3 shows the stacked
rest-frame UV spectra of leaker (blue) and non-leaker (red).

Line | $\rm{[OII]}$ | $\rm[NeIII]+H8+HeI$ | [NeIII]+H$\epsilon$ | H$\delta$ | H$\gamma$+[OIII]4363 | H$\beta$ | $\rm{[OIII]4959}$ | $\rm{[OIII]5007}$
---|---|---|---|---|---|---|---|---
Flux | 19.2$\pm$1.3 | 19.3$\pm$4.0 | 7.5$\pm$2.2 | 7.4$\pm$1.9 | 19.5$\pm$4.8 | 36.6$\pm$2.0 | 80.5$\pm$0.7 | 242.5$\pm$2.1
EW (Å) | 46.7$\pm$3.1 | 52.8$\pm$10.9 | 22.0$\pm$6.5 | 42.1$\pm$6.2 | 74.7$\pm$18.4 | 169.9$\pm$9.3 | 373.4$\pm$3.2 | 1119.7$\pm$9.7
Table 5: HST grism measurements of the LyC emitting region (knot 3). Line
fluxes and uncertainties are given in the units of 10-17 erg s-1 cm-2.

###  3.2 Emission line fitting in the FIRE spectra

The complex emission line profiles shown in Figure 2 require two Gaussian
components for each emission line. We fit the observed emission line profiles
by assuming that all emission lines can be fit by the same number of
components at the same velocity, but with varying total fluxes. We fixed the
line flux ratio of [OIII]$\lambda\lambda$5007,4959 to be 3.01 (Storey &
Zeippen, 2000). We identify [OIII]$\lambda$5007 as the highest signal-to-noise
emission line which is not impacted by any sky residual. We fit
[OIII]$\lambda$5007 by a two-component Gaussian model, and identify the
systemic redshift traced by the narrow component.

For all other emission lines, we fixed the systemic redshift as well as line
widths of two Gaussian components traced by the [OIII]$\lambda$5007 (after
accounting for instrument resolution). We also fixed the relative offset
between the two components as given by the [OIII]$\lambda$5007 line profile.
However, we set the narrow to broad component integrated flux ratio to be a
free parameter. The measured emission line fluxes from the two-component
Gaussian model are given in Table 2, for both the leaker and non-leaker stack.

###  3.3 Systemic Redshift

The best published systemic redshift for the Sunburst Arc, $2.3709\pm 0.001$,
comes from a stellar population fitting to the MagE M-0 spectrum (Chisholm et
al., 2019). The high signal-to-noise of the emission lines in the FIRE data
potentially enables a more precise measurement. We measured the velocity
centroids of the narrow components of the brightest line, [OIII]$\lambda$5007,
in each FIRE pointing. The resulting redshifts are given in Table 1. The
average redshift from [OIII]$\lambda$5007 emission is 2.37034$\pm$0.00024,
consistent with the previous result.

###  3.4 Stellar Population fitting to the MagE spectra

Rest-UV spectra contain several key features that indicate the age and
metallicity of the massive stellar population. The stellar population fitting
of the stacked spectra followed the procedure described in Chisholm et al.
(2019). To summarize: we assume that the spectra can be fit by a linear
combination of bursts of stellar populations, each with a given age and
metallicity, with a single reddening E(B-V) value. For the theoretical stellar
models, we considered single-star Starburst99 models (Leitherer et al., 1999,
2010, 2014). We assume a Kroupa initial mass function (IMF; Kroupa, 2001) with
a broken power-law with a high-(low-) mass exponent of 2.3 (1.3) and a high-
mass cut-off of 100 M⊙. We then propagated the intrinsic spectra through a
uniform dust screen using the Reddy et al. (2016) attenuation law. We fit the
observed spectra between rest frame wavelengths of 1220 Å to 2000 Å. The
outputs of this process are the linear coefficients for each of the 50 single-
age, single-metallicity models, as well as an E(B-V) reddening value.

###  3.5 Lensing Magnification

We define pixel-level masks that identify the FIRE slit positions along the
giant arc from the available HST imaging by flagging which pixels fall inside
or outside of each observed FIRE slit position. Only pixels that have more
than half of their area falling within the slit aperture are counted as inside
the slit, and any effects from fractional slits should be extremely minor
considering the small pixel size ($0.03$″) in our drizzled HST images relative
to the FIRE slit size. The slit definition masks are then used to define an
aperture mask for each position that represents the intersection of the FIRE
slit aperture and the emission from the giant arc. These apertures represent
HST pixel-level masks indicating the region on the sky from which light
entered the FIRE slit at each position listed in Table 1.

For the purpose of defining these apertures we use the HST F125W (the band
closest to the J-band used in the FIRE acquisition camera). We compute the sky
background statistics in F125W and manually mask out any other sources that
fell into some slits. We then define the emission from the giant arc as
originating from all pixels within a given FIRE slit that are 8-$\sigma_{sky}$
above the sky background level. We experimented with several different integer
multiples of $\sigma_{sky}$ as the threshold, and found that 8-$\sigma_{sky}$
strikes a good balance between robustly identifying the giant arc emission
without including spurious sky pixels, while also capturing the large majority
($\gtrsim 90$%) of the total giant arc flux in each slit.

The average magnification within each aperture is determined as follows. We
first ray-trace each aperture from the image plane to the source plane using
the lens model deflection map outputs. We then calculate the area enclosed by
the aperture in the source plane and in the image plane. The average
magnification is defined as the ratio of the image plane to source plane
areas.

Some of the apertures cross the critical curves and include portions of the
arc that traverse it, causing the source-plane projection of the aperture to
fold over itself (see, e.g., Figure 6 of Sharon et al., 2012, for an
illustration of this behavior). Therefore, care must be taken in deriving
their average magnification. In such cases, the overlapping region in the
source plane should only be counted once and the average magnification
accounts for the fact that the slit aperture encloses two images of the same
source region.

Magnification uncertainties are estimated by running the same process on
deflection maps calculated from sets of parameters in the MCMC that sample
$1-\sigma$ in the parameter space. The resulting magnifications and their
uncertainties are tabulated in Table 1.

##  4 Results

###  4.1 Nebular Reddening

We applied a nebular reddening correction using the Balmer lines (H$\alpha$ &
H$\beta$) measured in the leaker stack spectrum. While H$\beta$ is slightly
impacted by an underlying skyline in the individual spectra, we successfully
recover it in the stack spectrum. We used the observed H$\alpha$/H$\beta$ line
ratio of 3.533, and first applied Milky Way extinction correction using
E(B-V)=0.094 (Green et al., 2015). In the next step, we assumed Case B
recombination to infer intrinsic value of $(H\alpha/H\beta)_{in}$=2.86
corresponding to a temperature of $T=10^{4}$K and an electron density of
$n_{e}=10^{2}$ cm-3. For an extinction law of Cardelli et al. (1989) and an RV
value of 3.1, this corresponds to a nebular reddening of
E(B-V)=0.195$\pm$0.025. Recovering reliable H$\beta$ measurement in the non-
leaker stack is challenging for two reasons. First, non-leaking regions have
lower surface brightness and hence lower single-to-noise in individual
emission lines. Second, only 3 spectra are stacked for non-leaker (compared to
6 in leaker), again resulting in lower single-to-noise emission lines. This
prevents us from estimating a reliable H$\alpha$/H$\beta$ ratio for the non-
leaker. We, therefore, assume that the nebular reddening value of the leaker
is representative of the whole Sunburst Arc.

###  4.2 Stellar Reddening

The stellar reddening is inferred by fitting the rest-frame UV MagE data (see
§2.4). These results are given in Table 3, and cover a larger range of
physically distinct regions than is currently available for the reddening
data. We, therefore, use this stellar reddening information to check whether
there are large spatial gradients in the reddening. The results presented in
Table 3 show no apparent trend of $E(B-V)_{*}$ with whether the slit covers
the leaking or non-leaking regions. This supports the validity of using a
single nebular reddening value for the whole of the Sunburst Arc.

Figure 4: Stellar population synthesis fit to the leaker and non-leaker
spectra. The left panel shows P-Cygni NV features in the leaker (top) and non-
leaker (bottom), the middle panel shows P-Cygni CIV features in the leaker
(top) and non-leaker (bottom) and the right panel shows He II 1640 feature in
the leaker (top) and non-leaker (bottom). The orange line represents the best-
fit light-weighted SB99 model: the inferred age is 3.3 Myr for the leaker and
11.8 Myr for the non-leaker.  Figure 5: The ionization diagram showing the
location of leaker (blue) and non-leaker (red) using combined broad and narrow
components. The grey squares represent SDSS data points and green circle are
z$\sim$2.3 galaxies from MOSDEF survey (Sanders et al., 2016). The dotted line
represents AGN/SF demarcation from Kauffmann et al. (2003).

###  4.3 Comparison of UV features in leaking and non-leaking regions

Figure 3 plots the MagE spectra for the leaker and the non-leaker. The two
spectra are quite different in several ways. First, as shown in the figure
inset, the Ly$\alpha$ velocity profiles are radically different. The leaker
shows a triple-peaked profile which is a characteristic feature of ionizing
photons leaking from the galaxy. This feature is clearly absent in the non-
leaker stack spectrum. More specifically, the non-leaker lacks the blue-
shifted Ly$\alpha$ emission, which would suggest that the line of sight ISM is
sufficiently neutral to absorb Ly$\alpha$ photons. This confirms, with deeper
data, the results seen by Rivera-Thorsen et al. (2017).

Second, the rest-frame UV nebular emission lines, namely CIII]
$\lambda\lambda$ 1907, 1909, OIII] $\lambda\lambda$1661, 1666, Si III]
$\lambda\lambda$1883, 1892, have much larger equivalent widths in the leaker
stack than the non-leaker stack. These features are characteristics of
galaxies dominated by young massive star populations (Stark et al., 2013; Berg
et al., 2016; Du et al., 2018; Rigby et al., 2021).

Third, the spectral wind features that are characteristics of massive stars
are much stronger in the leaker than in the non-leaker. These features are
described in detail in Chisholm et al. (2019). Figure 4 compares these
features for the leaker and the non-leaker, namely NV, C IV, and He II
emissions. In particular, the leaker shows strong P-cygni NV and C IV
emission, which is characteristic of an extremely young (2–5 Myr) stellar
population. In contrast, these features are very weak in the non-leaker
(Figure 4). The leaker shows CIV emission, both a narrow component and a broad
component which again is characteristic of massive star winds. The narrow
component is likely nebular emission, which is commonly detected in LyC
emitting sources (e.g. Naidu et al., 2022; Schaerer et al., 2022). CIV also
shows the blue-shifted absorption that is characteristic of stellar winds.
These features are much weaker in the non-leaker. Additionally, the leaker
shows broad He II emission that is entirely absent in the non-leaker. This is
only found in massive Wolf-Rayet stars and must indicate that there is a very
recent burst of star formation. Together, these spectral features indicate a
very different light-weighted age for the leaker stack compared to the non-
leaker stack, which we now quantify.

Figure 6: HST WFC3/IR G141 grism spectrum of a LyC leaking region (knot 3) in
Sunburst Arc. The black line represents grism spectrum, the red dotted line
shows noise level and the blue line shows the best fit to the data. Figure 7:
[OIII]$\lambda$5007 emission line profile in leaking (left) and non-leaking
(right) regions. The black curve represents FIRE spectra. We fit the data with
a two-component gaussian model where the red dashed line represents the narrow
component and the blue dashed line represents the broad line. The green dashed
line represents best-fit model.

###  4.4 Stellar age and metallicities

The stellar population synthesis fits use both the strong stellar wind lines
(such as N V and C IV) and the weak stellar photospheric absorption features
to match the observations as a best-fit stellar templates. Using this
information, the fitting estimates a UV light-weighted age of $3.3\pm 0.5$ Myr
and a metallicity of 0.46$\pm$0.06 Z⊙ for the leaking region, and $11.8\pm
0.9$ Myr and 0.30$\pm$0.05 Z⊙ for the non-leaking region.

As discussed in Section 4.3, Figure 4 compares the observations (in black) to
the data (in gold) for the leaker (upper panel) and non-leaking (lower panel)
spectra. The left and middle panels show the stellar population fits to the
strong N V and C IV stellar wind lines. These wind lines display the classic
P-Cygni wind feature with blue shifted absorption and redshift emission
arising from gas launched off the photospheres of massive stars. While both
the leaking and non-leaking fits match the absorption components, the C IV
feature for the leaker shows evidence of narrow C IV emission that is not
well-fit by the model. The He II region is well-fit in the non-leaker case,
but poorly fit in the leaker case. This is due to broad ($\sim 2000$ km s-1)
emission seen in the leaker spectrum. Broad He II emission is typically seen
in massive, moderately metal-rich Wolf-Rayet stars with very young ages. Thus,
the observed broad He II is consistent with the young estimated age of the
leaker region, and the absence of broad He II in the non-leaking region is
consistent with the older fitted stellar population. The fact that the
Starburst99 models do not match the He II feature is a common problem with the
current generation of stellar models because the stellar tracks do not
populate the Wolf-Rayet stars that produce this feature (Leitherer et al.,
2010; Chisholm et al., 2019). Thus, the dichotomy of stellar properties –
young and moderate metal-rich population in the leaking region and a older
population in the non-leaking region – is consistent with the observed stellar
features.

###  4.5 Rest-frame optical diagnostic line ratios

The rest-frame optical strong emission lines form the classic line ratio
diagnostics that are used to classify galaxies as dominated by star formation
or active nuclei, and are sensitive to the metallicity and ionization state of
the nebular gas. In Table 4 we report the measured values for the standard
emission line ratios (for combined broad and narrow components) after
accounting for the internal extinction discussed above. Both the leaker and
non-leaker show larger than average O32 and [OIII]5007/H$\beta$ of typical
$z\sim 2$ galaxies suggesting that the galaxy is in higher ionization state
compared to typical galaxies at similar redshift (Steidel et al., 2016;
Sanders et al., 2016). O32 is often thought of as a proxy of LyC escape (e.g.
Jaskot & Oey, 2013; Nakajima & Ouchi, 2014), but is often not seen to scale
with LyC escape at low-redshift (Izotov & Thuan, 2008; Flury et al., 2022a).
Interestingly, these line ratios are not very different for individual leaker
and non-leaker stacks. We note that the non-leaker stack doesn’t include all
non-leaking regions identified in the Sunburst Arc.

In Figure 5, we plot the classic BPT diagram (Baldwin et al., 1981) in [O
III]/H$\beta$ vs [N II]/H$\alpha$ plot. The grey data points indicate SDSS
galaxies (z$\sim$0) and the green circles indicate z$\sim$2.3 galaxies from
the MOSDEF survey (e.g. Sanders et al., 2016), while the dotted line indicates
AGN versus star-forming galaxies classification based on Kauffmann et al.
(2003) model. The line flux ratios of both leaker (blue) and non-leaker (red)
are shown in the diagram. Since [N II] is not detected in either stack, we
only have limits on the [N II]/H$\alpha$ ratio, but these limits tightly
constrain both the leaking and non-leaking portions of the Sunburst Arc to the
far upper left wing of the “seagull”–shaped cloud that SDSS galaxies inhabit.
Both leaker and non-leaker stacks show high [OIII]/H$\beta$, distinct from
typical star forming galaxies in SDSS sample. However, such a high
[OIII]/H$\beta$ is consistent with young metal-poor galaxy population
typically observed at the intermediate redshifts (e.g. Berg et al. 2018;
Vanzella et al. 2017; Mainali et al. 2020).

###  4.6 X-ray constraints on AGN

We search for X-ray emission from the lensed Sunburst Arc to look for direct
evidence of any possible active galactic nucleus (AGN) that could be
contributing substantially to the ionizing radiation in the galaxy. We define
an aperture for the Chandra data by convolving the Chandra PSF with contours
that trace the optical emission of the Sunburst Arc in HST imaging. The
bright, extended X-ray emission from the foreground lensing galaxy cluster
overlaps with the location of the lensed images of the Sunburst Arc,
effectively acting as another source of background noise for the purpose of
constraining the emission from the arc. To account for the X-ray photons from
the cluster we fit a radially symmetric $\beta$-profile to the cluster
emission. We use the model to subtract off the cluster emission underneath the
giant arc aperture, resulting in a measurement of the X-ray emission from the
Sunburst Arc; the result is a non-detection of the giant arc, with a 0.5–7 keV
2-$\sigma$ limiting flux of $6.6\times 10^{-15}$ erg s-1 cm-2. Assuming a
power-law spectra shape with $\Gamma=1.8$ (reasonable for AGN), this
corresponds to a rest-frame 2–10 keV flux limit of $3.6\times 10^{-15}$ erg
s-1 cm-2 at $z=2.37$, and an upper limit on the intrinsic 2–10 keV luminosity
of L${}_{\rm 2-10}<1.6\times 10^{44}$ erg s-1. These limits do not account for
the lensing magnificiation, which reduces the constraint on the upper limit by
the magnification, $\mu$. We estimate the median magnification factor based on
our strong lensing model of the Sunburst Arc system using the full aperture
within which the Chandra upper limit was measured; the resulting magnification
is $\mu\simeq 38$. Applying this magnification factor to the upper limit on
the X-ray luminosity constrains the intrinsic X-ray luminosity of the Sunburst
Arc to be L${}_{\rm 2--10}\lesssim 7.6\times 10^{42}$ erg s-1, ruling out the
presence of an X-ray bright AGN in the Sunburst Arc.

| Centroid (Narrow) | FWHM (Narrow) | Centroid (Broad) | FWHM (Broad) | Fractional light in broad component
---|---|---|---|---|---
| km s-1 | km s-1 | km s-1 | km s-1 | %
Leaker | 0 | 112$\pm$1 | -27$\pm$1 | 326$\pm$2 | 54.5$\pm$0.3
Non-leaker | 0 | 115$\pm$2 | -43$\pm$14 | 454$\pm$47 | 26.1$\pm$7.0
Table 6: Breakdown of narrow and broad components in leaker vs non-leaker measured from [OIII]$\lambda$5007 emission. FIRE slits | Physical regions | Distance (kpc)
---|---|---
F-0 | 1 | 0
F-4 | 3,4,7,9,10 | 1.53${}^{+0.50}_{-0.23}$
F-5 | 3,8 | 1.06${}^{+0.20}_{-0.11}$
F-6 | 2 | 0.47${}^{+0.03}_{-0.07}$
Table 7: Distinct physical regions of Sunburst Arc targeted by FIRE spectra (Sharon et al., 2022). From left to right, the columns are: FIRE slit name, distinct physical regions within the FIRE pointing and average distance (pc) of the physical regions from LyC emitting region in source plane Diagnostic | leaker (cm-3) | non-leaker (cm-3)
---|---|---
CIII]$\lambda\lambda$1907,1909 | 66000${}^{+5000}_{-3000}$ | 19000${}^{+7000}_{-4000}$
[OII]$\lambda\lambda$3727, 3729 | 335${}^{+20}_{-21}$ | 340${}^{+1040}_{-340}$
Table 8: Electron density measurements for the leaker and the non-leaker using
[OII] and CIII] doublets.

###  4.7 Equivalent widths of the rest-frame optical emission lines

The equivalent width of nebular emission lines (EW) tells us about the
relative strength of the nebular emission compared to the stellar emission.
Because it is a ratio, it is not affected by lensing magnification. It
therefore tells us about the intensity and age of the massive stellar
population. The non-detection of continuum in the FIRE data precludes us from
a reliable equivalent width measurement, but we can measure them using the
grism data.

We plot the leaker spectrum in Figure 6 and present the measurements in Table
5, whereas a similar measurement for non-leaker is currently not possible with
the grism data. This is because continuum is weakly detected in the non-leaker
owing to its lower surface brightness and is further contaminated by nearby
starlights. For the leaker, the equivalent widths are very high, which is
typical of LyC emitting galaxies (e.g. Izotov et al., 2018; Flury et al.,
2022b). Such large equivalent widths are also commonly observed among extreme
emission line galaxy populations (Mainali et al., 2020; Amorín et al., 2017;
Labbé et al., 2013; Berg et al., 2021).

###  4.8 Evidence for ionized outflows in the stacked spectra

The strongest optical emission line in the FIRE data is [OIII] $\lambda$5007,
which is also free of contamination from telluric skylines. Examining Figure
2, it is immediately obvious that the stacked leaker and stacked non-leaker
FIRE spectra show very different velocity profiles for [O III] 5007. The
leaker stack shows a strong broad wing of emission, which is considerably
weaker in the non-leaker stack. The leaker stack also shows a broad red wing.

Table 2 reports the relative fluxes of the broad and narrow components for
both the leaker stack and the non-leaker stack. Figure 7 shows in detail the
best-fit broad and narrow components of the [O III] $\lambda$5007 velocity
profile. The observed line widths (FWHM) of the narrow component and broad
components in the leaker are $\sim$112 km s-1 and $\sim$326 km s-1. The broad
emission is blue-shifted from the narrow component by $\Delta V$ = 27 km s-1.
For [O III] 5007, 54.5$\pm$0.3% of the line flux is in the broad component for
the leaker stack; and 26.1$\pm$7.0% for the non-leaker stack. The measurements
are given in Table 6.

Though it suffers from some skyline contamination (particularly in the non-
leaker stack), the H$\alpha$ velocity profiles tell the same story: the leaker
stack shows a broad blue wing that is much stronger than in the non-leaker
stack. H$\alpha$ does appear to have a higher fraction of the flux in the
broad component compared to [OIII] 5007 line (63.4$\pm$1.5% & 41.1$\pm$9.3%
for leaker and non-leaker, respectively).

###  4.9 Evidence for ionized outflows in the individual spectra

The above analysis was performed on the stacked FIRE spectra. Each spectrum
that makes up the stacked leaker spectrum is intrinsically the same source,
multiply imaged. There is therefore no value in analyzing the kinematics of
each individual leaker spectrum. By contrast, the non-leaker stack is
comprised of six different physical regions based on the lens model (Sharon et
al., 2022), even though only four FIRE slits target non-leaking regions. This
is because a FIRE slit may target several unresolved physical regions. The
leaker FIRE spectrum F-0 targets Clump 1 while non-leaker spectrum F-4 targets
Clumps 2,3,4,9, and 10, F-5 targets Clumps 3,8, and 10 and F-6 targets Clump 2
(Table 7). We therefore further examine the [O III] 5007 in the individual
(non-stacked) non-leaker FIRE spectra to study any spatial variation in the
line profile. We calculated angular separation in object planes for different
star-forming clumps in the leaker and non-leaker spectra. Then we converted
them to average distance (in parsec) from the leaking region to regions
targets by F-4, F-5 and F-6 (see Table 6).

In Figure 8 we show the intrinsic line flux in the broad component of [OIII]
5007 line as a function of the average distance from the leaker. The intrinsic
line flux is obtained after applying magnification correction presented in
Table 1 to the observed line fluxes. Clump 2 (at 400 pc from the leaker) lies
very close to the critical curve in the source plane resulting in extremely
large magnification ($\mu$=637). This implies intrinsically negligible flux
contribution from Clump 2. While we see a broad outflow component in every
physical region, the figure demonstrates that the ionized gas outflow is
strong in the leaker, and that it is weak at the 0.5 – 1 kpc distances probed
by the FIRE pointings that cover non-leaking regions.

###  4.10 The outflow as traced in absorption of low-ionization and high-
ionization gas

The rest-UV stacks shown in Figure 3 show several interstellar absorption
features which tell us about the line of sight absorbing gas kinematics. These
absorption features are blue-shifted with respect to the systemic velocity
which indicates the presence of outflowing gas, consistent with outflows
traced in the emission lines discussed above. While flux in the emission lines
depends on n${}_{e}^{2}$, the outflows traced by line of sight absorption
scales as ne. Absorption is more sensitive to gas at larger distances, and gas
with lower densities. Thus, the absorption lines probe different physical
regions of outflow than the emission lines. The absorption lines also have
sensitivity to a range of ionization states.

In Figure 9, we show the different interstellar absorption features identified
in the leaker and non-leaker stack, and compare them with the gas kinematics
observed with the [O III] 5007 line. The resolution of the MAGE spectra
provides resolved velocity profiles of the interstellar absorption lines. We
normalize the spectra by the continuum level obtained from the stellar
population fitting procedure as discussed in §3.4.

For the non-leaker, the stacked MagE spectrum shows prominent absorption from
low ionized gas (Si II 1260, O I 1302, C II 1334, Al III 1670). The absorption
peaks at $\sim$-150 km s-1 relative to systemic, and extends from $\sim$+50 km
s-1 to $\sim$-500 km s-1. The highly ionized gas, as traced by Si IV and C IV,
shows similar gas kinematics.

By contrast, for the leaker, the stacked MagE spectrum shows extremely weak
absorption from the same four tracers of low ionized gas. A small amount of
blue-shifted absorption appears at $\sim$-50 km s-1 and $\sim-375$ km s-1
relative to systemic. The high ionization transitions show strong absorption
at these same two velocities, and in addition, show a tail of high-velocity
absorption extending out to $\sim$-750 km s-1.

Comparing the absorption velocity profiles between the leaker and the non-
leaker, while both show significant absorption from highly ionized gas, for
the leaker the absorption extends to higher velocities (-750 km s-1) than the
non-leaker (-500 km s-1).

Figure 8: Intrinsic line flux (magnification corrected) of outflow component
measured from the broad component in [OIII] 5007 line for the leaker (square)
and three individual non-leakers (red square) probed by FIRE slits. In the
x-axis, we plot physical distances between the leaker and the three non-
leakers in the source plane as given by the lens model (Sharon et al., 2022).

Figure 9: Comparison of outflowing gas inferred from ultraviolet absorption
lines and optical emission lines for the leaker (top panel) and the non-leaker
(bottom panel). The velocity profiles from narrow (red curve) and broad (blue
curve) components of [OIII]$\lambda$5007 emission line are plotted against low
ionization absorption lines (left panel) and high ionization absorption lines
(right panel).

##  5 Discussion

###  5.1 Outflows traced by UV and optical spectra

Outflowing gas in star-forming galaxies at z$\sim$1-3 has been reported both
from optical emission lines (e.g. Genzel et al., 2011; Newman et al., 2012;
Davies et al., 2019) as well as ultraviolet absorption lines (e.g. Weiner et
al., 2009; Steidel et al., 2010; Martin et al., 2012; Bordoloi et al., 2014,
2016; Jones et al., 2018). These studies have found that the outflows traced
by optical emission lines typically imply a dense phase that extends out to
kpc scales, and is driven by recent star formation activity. On the other
hand, outflows traced by ultraviolet absorption can trace a diffuse phase that
extends over larger galactic scales, accumulated over longer timescales.
However, quite rare in the literature are studies of outflow, at any redshift
that are traced by both ultraviolet absorption lines and optical emission
lines, due to the need for high signal-to-noise data that covers both the
rest-frame UV and rest-frame optical bandpasses (e.g. James et al., 2014; Wood
et al., 2015).

In the Sunburst Arc, we confidently detect outflowing gas from emission lines
as well as ultraviolet absorption lines. As presented in §4, the blue-shifted
broad component in [OIII] 5007 implies outflowing gas with $v_{out}$ =
327$\pm$2 km s-1 for the leaker and $v_{out}$ = 454$\pm$47 km s-1 for the non-
leaker. While we detect outflowing gas from ultraviolet absorption lines in
both the leaker and non-leaker, the maximum velocity of the outflow in the
leaker is considerably larger ($\sim$750 km s-1 as measured from high
ionization absorption lines) than the maximum value measured from emission
lines. The non-leaker does not show this effect; the maximum velocities traced
by the ultraviolet absorption lines and broad emission components are mostly
consistent within uncertainties.

This difference in maximum velocities traced by emission and absorption lines
in the leaker may indicate different regimes of the outflow. As shown in
Figure 9, the absorption lines indicate outflowing material at higher
velocities than implied by optical emission lines. Some of this outflowing
material, traced by Si IV, is likely in a phase with similar ionization state
to that traced by [O III] emission. The lack of the outflow in emission at
higher velocity may directly result from a density gradient (Wood et al.,
2015). Indeed in the leaker, [OII] and CIII] trace very different gas
densities: the electron density from [OII] (335 cm-3) is similar to typical
z$\sim$2-3 galaxies (e.g Bian et al., 2010; Sanders et al., 2015; Steidel et
al., 2016), whereas the density measured from CIII] (66000 cm-3) is
significantly higher (Table 8).

Interestingly, the high-velocity tail of the broad emission component aligns
very well with the blue-shifted Ly$\alpha$ emission. This effect appears to be
true for both the leaker and non-leaker, although both broad-component and
blue shifted Ly$\alpha$ emission is weaker in the non-leaker. This might
indicate that the high-ionization, high-density, and high-velocity outflow
regions are the same regions of low neutral gas column density that enable the
Ly$\alpha$ (and LyC) photons to escape (Gazagnes et al., 2020).

###  5.2 Why is the leaker leaking?

LyC escape requires both production and leakage of UV photons. LyC production
is likely associated with the most vigorously star-forming region in a galaxy
(Ma et al., 2020). In the Sunburst Arc, the leaker is responsible for
18$\pm$4% of the rest-frame 1500 Å flux, which can serve as a proxy for star
formation rate (Kennicutt, 1998). As such, the leaker is the most vigorous
site of star formation within its parent galaxy and is also the dominant
producer of UV photons.

As shown in Figure 4, production of UV photons is particularly high in the
leaking region, which shows the spectral hallmarks of very massive stars
(P-Cygni features in NV and CIV), the kind that are most effective at
generating ionizing photons. A young stellar population with stellar ages of
only a few million years is the primary production of LyC photons in a galaxy.
Previous studies have shown that the majority of LyC photons are produced by
1–3 Myr stellar population with some contribution from 3–10 Myr stars (Ma et
al., 2015, 2020; Kimm et al., 2017; Kim et al., 2019; Kakiichi & Gronke,
2021). We have shown that the portion of the Sunburst Arc that is emitting LyC
is considerably younger than the rest of the galaxy, with light-weighted ages
of 3.3 Myr versus 11.8 Myr. Thus, we attribute the youth of the leaker’s
stellar population to its prodigious production of ionizing photons. By
contrast, the older stellar population of the rest of the galaxy is unable to
produce ionizing photons.

Thus, the generation of LyC photons has a clear physical cause: young stars.
By contrast, the physical processes responsible for the escape of those
photons are not well established. Two mechanisms have been proposed in the
literature. The first mechanism is mechanical feedback, in the form of SNe
activity or stellar winds (which occur at younger timescales than SNe), that
would clear out neutral gas surrounding the young stellar cluster, such that
LyC photons can eventually escape (e.g. Wise & Cen, 2009; Heckman et al.,
2011; Sharma et al., 2017; Hogarth et al., 2020). The second mechanism is the
ionization of surrounding gas from young stellar populations, thus creating a
density-bounded H II region in which LyC photons penetrate through the low
column density region (Jaskot & Oey, 2013; Nakajima & Ouchi, 2014). Both
processes may act together to facilitate LyC escape.

The rest-frame UV spectra (Figure 9) imply that the LyC photons are escaping
because there is a deficit of low-ionization gas, which is present in
abundance only $\sim$ 400 pc away. The presence of some neutral gas along the
line of sight is evident from the weak low-ionization absorption gas. This
supports some neutral gas along the line of sight and that models of the Lyman
alpha line favor a model with narrow ionized channels as per Rivera-Thorsen et
al. (2017).

###  5.3 Physical scale of ionized outflow

A key open question of modern astrophysics is how the “feedback” from young
star clusters is able to regulate star formation over the spatial scales of a
whole galaxy. Fortunately, gravitational magnification allows the spatially
resolved study of these processes in the Sunburst Arc, where a single star-
forming region dominates the ionizing photon production and escapes, at a
spatial resolution that is difficult to achieve in the local universe and
impossible without lensing in the distant universe.

The leaking region shows clear evidence of ionized outflowing gas (Figure 7),
which is considerably weaker in the non-leaking portions of the galaxy. Such a
strong gas outflow is likely driven by recent star formation activity in the
LyC leaking region. It is unclear whether the weak broad component seen in the
non-leaker arises in the non-leaking region, or whether it is just the
extension of the leaker’s outflow that reaches to the non-leaking regions.
Whichever the case, it is clear that the strongly ionized outflows are
spatially associated with the leaker. The high ionization absorption lines
further support this – they are much stronger in the leaker compared to the
non-leaking regions (Figure 9).

Further examination of the individual non-leaking regions allows us to
constrain the spatial extent of the outflow. Three different FIRE pointings
targeting non-leaking regions probe physical regions in the source plane that
are at different distances from the leaker (Sharon et al., 2022). It is clear
from Figure 8 that the outflows probed by [OIII]5007 emissions are strongest
in the LyC region. At the distances of 400–700 pc that the non-leaking FIRE
pointings probe, the intrinsic line flux in outflow reduces by $>$80 %. Thus,
we constrain the physical scale of the ionized outflow to be $<$400 pc.

###  5.4 Evidence of an ionized supper bubble in the Sunburst Arc?

A detailed picture of the physics of LyC escape requires probing the ISM
structure and feedback from young stars on sub-kiloparsec spatial resolution
scales. This has been challenging to achieve both theoretically and
observationally. Most recently, Ma et al. (2020) studied LyC escape from 34
z$>$5 galaxies in high-resolution cosmological simulations. They found that
majority of LyC escape come from very young ($<$10 Myr), vigorously star-
forming regions of the galaxy. These regions may reside in a kpc-scale ionized
bubble presumably created by SNe activity from 3–10 Myr stars. Low column
density sight lines can be fully ionized by young stars allowing the escape of
ionizing photons. In contrast, the galaxy may contain several other non-
leaking regions dominated by young stars but still embedded in dense neutral
clouds. Feedback from young stars in the non-leaking regions is not strong
enough to clear pathways for LyC escape.

The LyC emitting region in the Sunburst Arc is the most vigorous star-forming
region in the galaxy and includes the youngest stars ($\sim$ 3 Myr) in the
galaxy. The stellar feedback from such a young stellar population is likely
driving the ionized outflowing gas. Over the time (3–10 Myr), such ionized
outflowing gas may turn into a giant ionized super bubble surrounding the star
formation site. In such a scenario, the strongly localized outflows seen in
emission lines of the leaker may indicate that the LyC region lies within a
super bubble.

Recently, Menacho et al. (2019) reported evidence of a kpc-scale super bubble
in a local LyC emitter (Haro 11), which may have cleared channels for LyC
escape in the galaxy. This seems quite analogous to the $<$400 pc bubble of
ionized gas in the Sunburst Arc. Super bubbles may be important mechanisms for
enabling LyC photons to escape galaxies. An upcoming IFU study with JWST (GO:
2555, PI: T Rivera-Thorsen) will measure the physical size of the superbubble
in the Sunburst Arc, and obtain a detailed picture of the gas kinematics in
this remarkable ionizing lensed galaxy.

##  6 Summary and conclusions

In this paper, we present a spectroscopic study of multiple star-forming
regions within the LyC emitting galaxy, the Sunburst Arc, at $z$=2.37. We
obtained near-infrared spectra from Magellan/FIRE and optical spectra from
Magellan/MagE. One of the primary goals of the paper is to understand the
physical conditions that facilitate the production and escape of LyC photons
from a galaxy. In order to achieve our goal, we generated rest-frame
ultraviolet and rest-frame optical stacked spectra for two regions: those that
emit ionizing photons (leaker stack) and those that do not emit ionizing
photons (non-leaker stack).

The rest-frame optical spectra reveal highly ionized gas flowing out of the
Sunburst Arc. The leaker shows blue shifted broad component optical emission
(FWHM = 327 $\pm$ 2 km s-1) in addition to a narrow component (FWHM = 112
$\pm$ 1 km s-1) component, which indicates the presence of strongly ionized
outflowing gas. This outflow is strongest in the spectra of the leaker.
Indeed, most (54.5$\pm$0.3%) of the [O III] 5007 emissions from the leaker
emerges from a broad component. The stacked non-leaker also shows this broad
component, but it accounts for only 26.1$\pm$7.0% of the [O III] 5007
emissions. Examining the spectra of the individual non-leaking components (Fig
8), the broad component decreases with increasing distance from the leaker.

The high-ionization gas traced by rest-frame UV absorption lines echoes the
above picture: both the leaker and the non-leaker show blue-shifted gas. In
the non-leaker this gas is centered at $-$200 km s-1 and extends to 600 km
s-1; in the leaker the absorption has two peaks at $-$50 km s-1 and $-$350 km
s-1, and extends all the way to $-$750 km s-1. Thus, the spectra of both the
leaking and non-leaking regions show evidence for a high-velocity, high-
ionization outflow, which in the leaker extends to higher velocities. The
rest-frame UV absorption lines provide something the rest-frame optical
emission lines do not: a picture of the low ionization gas, as traced by Si II
1260, CII 1334, and Al III 1670. The non-leaker shows absorption centered at
$-$200 km s-1, extending from 0 to $-$500 km/s. In the leaker, this low-
ionization gas is much weaker, with two very weak absorption features at
$\sim-$50 km s-1 and $\sim-$375 km s-1.

The rest-frame UV stack spectra show features that are characteristics of the
massive stars (Figure 4). In particular, the leaker shows strong P-Cygni NV
and CIV features as well as broad He II emission; all are characteristics of
young massive stars. In contrast, these features are weaker in the non-leaker
stack. The stellar population fitting to the rest-frame UV spectra reveals
that the stellar population of leakers is considerably young ($\sim$3 Myr)
than non-leaker ($\sim$12 Myr).

Overall, the presence of relatively young massive stars in the leaker suggests
that ionizing photons are being produced within the leaking region. These
ionizing photons are being carried out along the channels that are mostly
cleared out by the stellar feedback as evident in the high ionization
absorption lines and optical emission lines. There is evidence of little
neutral gas along the line of sight as indicated by weak low ionization
absorption lines (Figure 9). This picture supports a picture in which LyC
photons stream out of channels cleared through the interstellar medium. On the
other hand, the absence of a younger stellar population in the non-leaking
regions indicates that these are not actively producing ionizing photons.
Additionally, stronger low ionization absorption features suggest the presence
of substantial neutral gas along the line of sight which would absorb any
ionizing photons being carried out. Together, these data support a picture in
which a young ( 3Myr) vigorous star cluster not only prodigiously generates
ionizing photons, but also drives a high-velocity outflows that clear out
neutral gas along certain sight-lines, enabling the escape of ionizing
photons.

The rest-frame UV stack spectra show features that are characteristic of
massive stars (Figure 4). In particular, the leaker shows strong P-Cygni NV
and CIV features as well as broad He II emission; all are characteristic of
young massive stars. By contrast, these features are weaker in the non-leaker
stack. Stellar population fitting to the rest-frame UV spectra reveals that
the leaker is considerably young ($\sim$3 Myr) than the non-leaker ($\sim$12
Myr).

Thus, we see a consistent picture, in which the only region of the Sunburst
Arc that is known to be leaking ionizing photons — the “leaker” — is also the
only region showing spectral signatures of the young massive stars that are
capable of producing large numbers of ionizing photons. The sightlines to the
leaker appear to have little neutral gas along the line of sight, as indicated
by weak low ionization absorption lines. What gas is present appears to be
highly ionized, as evident in the high ionization absorption lines. The broad
component of the [O III] emission suggests that stellar feedback is what has
cleared out these channels. This picture supports a model for LyC escape, in
which LyC photons stream out of channels cleared through the interstellar
medium.

By contrast, the non-leaking regions lack the signposts of a very young
stellar population, have stronger low ionization absorption features that
suggest substantial neutral gas along the line of sight, and have a much
weaker broad velocity component of [O III]. In other words, in the non-leaking
regions, we see no evidence that ionizing photons are being created, nor do we
see clear paths by which ionizing photons could escape, nor do we see evidence
of stellar feedback that could clear out these paths. The Sunburst Arc may be
one of the best places to catch these feedback processes in action.

##  7 Acknowledgments

.

We thank the referee for their useful comments. The author is grateful to
Daniel Stark for enlightening conversation. The material is based upon work
supported by NASA under award number 80GSFC21M0002. Based on observations made
with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities for
Research in Astronomy, Inc., under NASA contract NAS 5-26555. These
observations are associated with program GO-15101. Support for Program number
GO-15101 was provided through a grant from the STScI under NASA contract
NAS5-26555. Support for this work was provided by the National Aeronautics and
Space Administration through Chandra Award Number GO8-19084X issued by the
Chandra X-ray Center, which is operated by the Smithsonian Astrophysical
Observatory for and on behalf of the National Aeronautics Space Administration
under contract NAS8-03060. The scientific results reported in this article are
based on observations made by the Chandra X-ray Observatory, and this research
has made use of software provided by the Chandra X-ray Center (CXC) in the
application package, CIAO. This paper includes data gathered with the 6.5 m
Magellan Telescopes located at Las Campanas Observatory, Chile. We thank the
staff of Las Campanas for their dedicated service, which has made possible
these observations. We thank the telescope allocation committees of the
Carnegie Observatories, The University of Chicago, The University of Michigan,
and Harvard University, for supporting this observing program over several
years.

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