# The effects of LMC-mass environments on their dwarf satellite galaxies in
the FIRE simulations

Ethan D. Jahn,1 Laura V. Sales,1 Andrew Wetzel,2 Jenna Samuel,2 Kareem El-
Badry,3Michael Boylan-Kolchin,4 and James S. Bullock5
1Department of Physics and Astronomy, University of California, Riverside, CA
92507, USA
2Department of Physics and Astronomy, University of California, Davis, CA
95616, USA
3Department of Astronomy and Theoretical Astrophysics Center, University of
California Berkeley, Berkeley, CA 94720, USA
4Department of Astronomy, The University of Texas at Austin, 2515 Speedway
Blvd, Stop C1400, Austin, TX 78712, USA
5Department of Physics and Astronomy, University of California, Irvine, CA
92697, USA E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

Characterizing the predicted environments of dwarf galaxies like the Large
Magellanic Cloud (LMC) is becoming increasingly important as next generation
surveys push sensitivity limits into this low-mass regime at cosmological
distances. We study the environmental effects of LMC-mass halos ($M_{200m}\sim
10^{11}$ M⊙) on their populations of satellites ($M_{\star}\geq 10^{4}$ M⊙)
using a suite of zoom-in simulations from the Feedback In Realistic
Environments (FIRE) project. Our simulations predict significant hot coronas
with $T\sim 10^{5}$ K and $M_{\text{gas}}\sim 10^{9.5}$ M⊙. We identify
signatures of environmental quenching in dwarf satellite galaxies,
particularly for satellites with intermediate mass ($M_{\star}=10^{6-7}$ M⊙).
The gas content of such objects indicates ram-pressure as the likely quenching
mechanism, sometimes aided by star formation feedback. Satellites of LMC-mass
hosts replicate the stellar mass dependence of the quiescent fraction found in
satellites of MW mass hosts (i.e. that the quiescent fraction increases as
stellar mass decreases). Satellites of LMC-mass hosts have a wider variety of
quenching times when compared to the strongly bi-modal distribution of
quenching times of nearby centrals. Finally, we identify significant tidal
stellar structures around four of our six LMC-analogs, suggesting that stellar
streams may be common. These tidal features originated from satellites on
close orbits, extend to $\sim$80 kpc from the central galaxy, and contain
$\sim 10^{6-7}$ M⊙ of stars.

###### keywords:

galaxies: dwarf – galaxies: formation – galaxies: evolution – galaxies: star
formation – Local Group

††pubyear: 2015††pagerange: The effects of LMC-mass environments on their
dwarf satellite galaxies in the FIRE simulations–The effects of LMC-mass
environments on their dwarf satellite galaxies in the FIRE simulations

##  1 Introduction

Dwarf galaxies, due to their shallow potentials and low baryon fractions,
represent ideal laboratories within which to study both galaxy formation and
dark matter. Understanding the effects of the environments they evolved in is
important to producing accurate constraints on such physics. Present-day
sensitivity limits have restricted the observational study of dwarf galaxies,
especially the faintest ones, to the Local Group (LG; the Milky Way (MW),
Andromeda, and all galaxies within $\sim$2 Mpc of each). Therefore, much
effort has been put to studying MW-mass halos or LG-like pairs (e.g. Klypin et
al., 1999; Springel et al., 2008; Sawala et al., 2016; Applebaum et al., 2021,
etc.) and the formation of dwarf galaxies within such environments (Brooks &
Zolotov, 2014; Garrison-Kimmel et al., 2014; Wetzel et al., 2016; Simpson et
al., 2018; Garrison-Kimmel et al., 2017, 2019a; Buck et al., 2019; Akins et
al., 2021).

$\Lambda$CDM predicts the presence of dark matter (DM) structure and
substructure at all scales, with galaxies populating halos down to a certain
limit (White & Rees, 1978; Moster et al., 2010; Jethwa et al., 2018). Dwarf
galaxies often exist within substructure of larger halos as well as being
hosts to their own sub-substructure (Sales et al., 2013; Wheeler et al.,
2015), with the Large Magellanic Cloud (LMC; the MW’s largest satellite) as a
well-studied example of this intermediate scale. Recent works investigating
the observed and predicted satellite population of the LMC have revealed that
these environments, while less disruptive than MW-like centrals (Jahn et al.,
2019), can be host to significant populations of dwarf galaxies (D’Onghia &
Lake, 2008; Sales et al., 2011; Wetzel et al., 2015a; Deason et al., 2015;
Jethwa et al., 2016; Sales et al., 2017; Dooley et al., 2017; Kallivayalil et
al., 2018; Pardy et al., 2020; Erkal & Belokurov, 2020; Santos-Santos et al.,
2021).

Environment – the relative proximity to higher mass structures – has been
repeatedly shown to correlate with morphology, color, and star formation rate
(SFR) in dwarf galaxies (Balogh et al., 2004; Kauffmann et al., 2004; Baldry
et al., 2006; Cooper et al., 2006; Lisker et al., 2007; Bamford et al., 2009;
Wang et al., 2014), though the majority of such surveys are limited to
relatively massive environments and satellite galaxies with $M_{\star}\gtrsim
10^{8}$ M⊙. Studies of LG and near-field dwarf galaxies have further
corroborated this trend by demonstrating the dependence of HI abundance and
star formation history (SFH) on environment (Grebel et al., 2003; Grcevich &
Putman, 2009; Weisz et al., 2011). More detailed studies of the LG have shown
that a majority of satellite dwarf galaxies in the LG are quenched at stellar
masses below $10^{8}$ M⊙ (Weisz et al., 2015; Wetzel et al., 2015b), while
field dwarfs remain star-forming (Geha et al., 2012, though this work is
mostly limited to bright dwarf galaxies with $M_{\star}\gtrsim 10^{7}$M⊙). In
contrast to the universally high quenching fractions found around the LG, Mao
et al. (2021) showed via the Satellites of Galactic Analogs (SAGA) survey that
MW-mass systems outside the LG may have systematically lower quenched
fractions, even at lower masses. This suggests that the SFHs of LG satellites
might not be typical of MW-mass hosts.

Plausible physical mechanisms for the quenching of star formation have been
identified. Small-scale hydrodynamic effects are known to either remove gas
from a star-forming galaxy via ram-pressure stripping (Gunn & Gott, 1972;
Abadi et al., 1999), prevent the infall of cold gas that fuels star formation
leading to ‘starvation’ (Larson et al., 1980), and/or disrupt the structure of
the galaxy during close interactions with other galaxies (Moore et al., 1996;
Pearson et al., 2016).

Since dwarf galaxies have shallow potentials and massive hosts such as the MW
are know to host hot gaseous halos (Gupta et al., 2012), much attention has
been given to the effects of ram-pressure stripping in low-mass satellites.
For example, Fillingham et al. (2015); Wetzel et al. (2015b) demonstrated a
mass-dependent quenching model in which satellites with $M_{\star}=10^{6-8}$
M⊙ have short quenching time-scales consistent with ram-pressure stripping,
while the longer quenching time-scales of higher mass satellites
($M_{\star}\gtrsim 10^{8}$ M⊙) are consistent with starvation. Fillingham et
al. (2016) further showed that a clumpy gaseous halo with local densities
$\sim 2-20$ times the mean gas density increases the efficacy of ram-pressure
stripping and reproduces the high quenched fraction of MW satellites. This
model breaks down at the lowest galaxy masses ($M_{\star}\lesssim 10^{5}$ M⊙),
where ram-pressure and starvation have been shown to be unable to reproduce
the universally early quenching times (Emerick et al., 2016; Rodriguez
Wimberly et al., 2019), thus pointing to heating from the ionizing UV
background as the quenching mechanism at such scales.

While the quenching fraction of LG satellites is known to rise as satellite
mass falls (Weisz et al., 2011; McConnachie, 2012; Wetzel et al., 2015b), high
resolution studies of nearby satellites and simulated LG-like environments
have enabled the characterization of the full star formation histories (SFHs)
of dwarf galaxies, and the study of their dependence on both satellite mass
and environment. Weisz et al. (2015) characterized a bimodal mass-dependence
for quenched fraction, with the highest ($M_{\star}\sim 10^{11.5}$ M⊙) and
lowest ($M_{\star}<10^{5}$ M⊙) mass galaxies holding high quenched fractions,
and galaxies with $M_{\star}=10^{8-10}$ M⊙ having the lowest, suggesting this
mass range may be the most difficult to quench. Comparing to infall time
estimates from Rocha et al. (2012), they also find that higher mass satellites
tend to quench $1-4$ Gyr after infall, while lower mass satellites quench
prior to infall. Wetzel et al. (2016) used FIRE-2 simulations (Hopkins et al.,
2018b) to reproduce properties of satellites around MW-like hosts, in
particular, the wide scatter in SFHs and the stellar mass dependence thereof.
Garrison-Kimmel et al. (2019b) looked at a sample of $\sim$ 500 dwarf galaxies
from the FIRE suite to investigate the effect of various environments (LG vs
MW vs centrals thereof vs highly isolated centrals), finding that LG and MW-
like environments quench similarly, and form their stars earlier than dwarf
centrals, supporting the host-satellite interaction model for quenching. They
also find that higher mass dwarf galaxies are more likely to form a higher
fraction of their stars at later times, in agreement with observed LG SFHs.

Other simulations of MW and LG-like environments, such as APOSTLE (Digby et
al., 2019), Auriga (Simpson et al., 2018) and NIHAO (Buck et al., 2019) have
demonstrated consistent findings, particularly that satellites form earlier
than centrals. Recently, Akins et al. (2021) used the DC Justice League
simulation suite to show that ram-pressure is the source of short quenching
time-scales for satellites of intermediate mass ($M_{\star}=10^{6-8}$ M⊙), as
well as the diversity of satellite SFHs and the trend of increasing quenched
fraction with decreased $M_{\star}$.

While interactions with the host environment are known to affect satellite
star formation, it can also affect morphology through gravitational
interactions. Such interactions can be strong enough, depending on the
satellite’s physical size, proximity of the host, and the mass of the host, to
produce observable stellar features known as tidal streams. Tidal streams
around our Galaxy have been studied extensively, starting with the Sagittarius
(Sgr) dSph tidal stream (Ibata et al., 1994; Belokurov et al., 2006), and with
dynamical models of stream kinematics revealing multiple close encounters of
satellites with the MW (Johnston et al., 1995; Majewski et al., 1996; Helmi &
White, 2001; Peñarrubia et al., 2005). Such features have also been
observationally identified in galaxies beyond the LG (e.g. Malin & Hadley,
1997; Martínez-Delgado et al., 2008; Martínez-Delgado et al., 2010). Stellar
streams give insight and evidence to the hierarchical nature of galaxy
assembly. Given this hierarchical nature and the confirmation of a population
of satellites around the LMC, tidal streams should presumably be detectable
around galaxies of lower mass than the MW. A handful of tidal streams have
indeed been discovered around dwarf galaxies (Martínez-Delgado et al., 2012;
Carlin et al., 2019), but their cosmological frequency remains unknown.

Much of the literature on satellites and the interactions with their host
environments is confined to the scale of the MW/LG. This is because our
highest resolution observations of dwarf galaxies exist within this volume. As
next-generation surveys such as DELVE (Drlica-Wagner et al., 2021) MADCASH
(Carlin et al., 2016) and LBT-SONG (Davis et al., 2021) come online, it will
be important to characterize the environments of lower-mass systems. We aim to
extend the analysis of previous works listed above to the scale of the Large
Magellanic Cloud (LMC), about an order of magnitude smaller than the MW. For
example, Carlin et al. (2021) recently discovered two ultrafaint dwarf
satellites of LMC-mass hosts approximately 3 Mpc from the MW. Previous works
have characterized properties of LMC analogs (Chan et al., 2015; Chan et al.,
2018; El-Badry et al., 2018) or the predicted satellite population (Jethwa et
al., 2016; Sales et al., 2017; Kallivayalil et al., 2018; Jahn et al., 2019;
Pardy et al., 2020; Erkal & Belokurov, 2020; Santos-Santos et al., 2021), but
limited work has been done on characterizing the influence such environments
have on observable properties of the satellite population, in particular their
SFHs, quenched fractions, and tidal structures.

This paper is organized as follows: the simulations and sample are presented
in Section 2; overall trends in satellite quenching are investigated in
Section 3.1, while we investigate the environmental quenching of individual
satellites in Section 3.3; tidal features around our LMC-mass hosts are
presented in Section 4.

Simulation | $m_{\text{bary}}$ | $M_{\text{200m}}$ | $M_{\star}$ | $r_{\text{200m}}$ | r50⋆ | min. $M_{\star}$ | N${}_{\text{satellite}}$ | N${}_{\text{central}}$ | Reference
---|---|---|---|---|---|---|---|---|---
| (M⊙) | (M⊙) | (M⊙) | (kpc) | (kpc) | (M⊙) |  |  |
m11c | 2100 | 1.5e11 | 8.2e8 | 167.4 | 2.80 | 5.89e4 | 2 | 10 | 1
m11d | 7070 | 2.8e11 | 4.1e9 | 203.9 | 6.75 | 2.29e5 | 6 | 1 | 2
m11e | 7070 | 1.5e11 | 1.4e9 | 166.0 | 3.31 | 2.46e5 | 3 | 1 | 2
m11h | 880 | 1.8e11 | 1.1e8 | 174.0 | 1.44 | 1.25e4 | 10 | 5 | 2
m11q | 880 | 1.5e11 | 3.4e8 | 168.7 | 2.71 | 1.23e4 | 5 | 9 | 1
m11v | 7070 | 2.9e11 | 2.4e9 | 210.5 | 2.61 | 1.39e5 | 4 | 5 | 1
|  |  |  |  |  | total | 30 | 31 |
Table 1: Properties of the host halo of all FIRE simulations analyzed. Column
1 is the name of each run; column 2 is the minimum baryonic particle mass;
column 3 ($M_{\text{200m}}$) is the mass of DM contained within
$R_{\text{200m}}$; column 4 ($M_{\star}$) is the stellar mass of the primary
central of each zoom-in region; column 5 ($R_{\text{200m}}$) is the radius at
which the mean interior DM density is equal to 200 times the average matter
density of the universe; column 6 ($r_{50\star}$) is the half mass radius of
stars in the primary central galaxy; column 7 (min $M_{\star}$) is minimum
stellar mass of any object examined in each run (an effective resolution
limit); column 8 is the number of satellites around the primary central;
column 9 is the number of resolved galaxies outside $R_{\text{200m}}$ of the
primary central; and column 10 is the reference in which the simulation was
first presented. The total count for satellites and centrals analyzed herein
is shown in the bottom row. Satellites are luminous (sub)-halos located within
the host $R_{\text{200m}}$ at $z=0$, while centrals are located outside the
host halo. We make further resolution cuts on contamination by low-resolution
particles ($M_{\text{lowres}}/M_{\text{200m}}$ < 3 per cent) and a minimum of
20 star particles formed within the progenitor halo of each object. Naturally,
the runs with higher particle mass are less capable of resolving low-mass
galaxies, leading to incompleteness of the faint end. Note that m11d, m11e,
and m11v are unable to resolve ultrafaints ($M_{\star}<10^{5}$ M⊙) when
applying our resolution criteria. References: [1] Hopkins et al. (2018b); [2]
El-Badry et al. (2018)

##  2 Simulations

We analyze six cosmological zoom-in simulations of isolated LMC-mass halos
from the Feedback In Realistic Environments
project111http://fire.northwestern.edu (FIRE). These runs used the FIRE-2
model (Hopkins et al., 2018b) via the cosmological hydrodynamics code
GIZMO222http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html (Hopkins,
2015), a multi-method gravity plus hydrodynamics code, in its meshless finite-
mass (MFM) mode. GIZMO implements an improved version of the Tree-PM solver
from GADGET-2 (Springel, 2005) with fully-adaptive conservative gravitational
force softenings for gas (Price & Monaghan, 2007).

Simulations are
initialized333http://www.tapir.caltech.edu/~phopkins/publicICs/ with second-
order Lagrangian perturbation theory at $z=99$ using the MUSIC code (Hahn &
Abel, 2011) and evolved within a low-resolution cosmological box. The intended
“zoom-in” (Katz & White, 1993; Oñorbe et al., 2015) volume is selected as a
convex Lagrangian region containing all particles within
${\sim}5\,$$R_{\text{200m}}$ at $z=0$ with no similar or higher mass halos as
the primary, and is then reinitialized with higher resolution. This procedure
is iterated until convergence at the intended resolution, with a buffer of
low-resolution particles surrounding the main volume. The simulation is then
evolved until $z=0$. Since our hosts are isolated LMC-mass halos that are not
embedded within the environment of the LG, small deviations in the assembly
history and satellite population may be expected in comparison to the real
LMC. However, the physical mechanisms explored here, in particular the
environmental effects of LMC-mass hosts, are expected to apply in the case of
the real LMC in addition to influence from its evolution in proximity to the
LG environment.

The FIRE-2 code calculates heating and cooling rates from 10 – 1010 K, using
CLOUDY ionization states for free-free, photoionization & recombination,
Compton scattering, photoelectric, metal-line, molecular, fine structure, dust
collisional, uniform cosmic ray heating, against a spatially uniform,
redshift-dependent UV background (Faucher-Giguère et al., 2009).

Star particles are formed in gas that is required to be locally self-
gravitating, self-shielded, Jeans unstable, and with density
$n_{H}>n_{\text{crit}}=1000$ cm-3; inheriting mass and metallicity from their
progenitor gas particles. To calculate stellar feedback, a Kroupa (2001) IMF
is assumed in each star particle, with feedback quantities tabulated from the
STARBUST99 stellar population model (Leitherer et al., 1999), including
supernova Type Ia, II, and stellar winds, as detailed in Hopkins et al.
(2018b) and Hopkins et al. (2018a). Radiative feedback is modeled using the
Locally Extincted Background Radiation in Optically-thin Networks (LEBRON),
accounting for absorbed photon momentum, photo-ionization, and photo-electric
heating.

Dark matter (DM) particles are assigned to halos and subhalos through 6+1
dimensional phase space analysis via the ROCKSTAR halo finder (Behroozi et
al., 2013a), which determines gravitationally bound particles and assigns mass
through a spherical overdensity calculation relative to a threshold, such as
the critical density density of the universe. Stellar properties are computed
for each subhalo during an iterative post-processing procedure in with star
particles within 80 per cent of a halo’s radius and slower than
2$\times$$V_{\text{max}}$ with respect to the halo center are selected and
refined until the stellar mass converges to $<1$ per cent. More details on
this process can be found in Samuel et al. (2020). Progenitors of $z=0$ halos
are traced through time using consistent-trees (Behroozi et al., 2013b) to
construct merger trees. We use properties generated by the above methods to
initially determine the stellar mass and star formation histories of dwarf
galaxies, but we make further cuts on resolution as described below.

###  2.1 Selecting the Sample

Table 1 lists the simulations used in our sample, including the resolution,
the halo and stellar masses of the host galaxies, as well as the number of
satellites and centrals identified within each simulation volume. Satellites
are identified as being within $R_{\text{200m}}$ of the main host halo at
$z=0$, where $R_{\text{200m}}$ is defined as the radius within which the mean
DM density is equal to 200 times the average matter density of the Universe.
We classify centrals as any galaxy that falls outside $R_{\text{200m}}$ but
within the high-resolution region of each simulation. Well-resolved centrals
are further selected as having a maximum contamination of low resolution
particles at 3 per cent of their $z=0$ halo mass. This cut is unnecessary for
satellites, because they naturally fall within the high-resolution region.

Figure 1: Star formation histories (SFHs) of simulated LMC satellites (right
sub-panels) and centrals (left sub-panels), colored and separated according to
stellar mass. Satellites identified as being environmentally quenched (see
Section 3.3) have been highlighted with thicker line styles. All colored lines
are simulated data, while black/grey lines are observational data. Note that
all observed data is for Local Group (LG) satellites/centrals. Individual
observed SFHs of satellites of the MW/M31 (Weisz et al., 2014; Skillman et
al., 2017) are shown as faint dashed lines (style according to legend), with
medians in each bin shown as thicker dashed black lines. Blue lines represent
the median SFHs of satellites of LG-like hosts simulated using the FIRE-2
code, and as presented in Garrison-Kimmel et al. (2019b) (see Table 1 therein
for more information, including resolution). Histograms on the top of each
panel represent the formation time of the youngest star particle for simulated
LMC satellites or nearby centrals (i.e. they do not include observed
satellites of the MW/M31, nor simulated FIRE satellites of LG-like hosts). The
majority of centrals are quenched prior to $t=4$ Gyr if they are not presently
star-forming. Satellites, however, are more prone to influence by the
environment of their LMC-mass hosts, and exhibit a greater diversity of
quenching times. Overall, the SFHs of LMC satellites do not differ
substantially from the SFHs of LG satellites. This is perhaps a counter-
intuitive result given the reduced stellar mass to halo mass ratio of LMC-mass
halos, and their less disruptive nature (Jahn et al., 2019).

To study the SFHs and to remove spuriously assigned particles, we track the
location of star particles assigned to each (sub)halo and select only the ones
which were formed within half of the halo’s radius (of bound DM particles, as
determined by ROCKSTAR). We place a minimum cutoff of 20 such star particles
for each galaxy analyzed. In some cases, merger events led to large amounts of
stars formed outside the halo being assigned at later times, but as these are
physically meaningful associations, they were retained.

We have explicitly checked for splashback galaxies – those that entered
$R_{\text{200m}}$ of the primary central at some point, but exited at a later
time – and find that all but one such objects are satellites at present day.
Note that this is in contrast with the splashback population of MW-mass hosts,
which tend to be significant outside of $R_{\text{200m}}$ at $z=0$ (Sales et
al., 2007; Garrison-Kimmel et al., 2014; Fillingham et al., 2018), an effect
which is likely due to the difference in halo mass and infall rates of low-
mass galaxies. The splashback process can potentially affect the evolution of
the object and contaminate the sample of centrals, which are intended to
represent the evolution of similarly-massed galaxies in FIRE that are not
affected by the host (or at least affected to a substantially smaller degree).
The individual splashback central is a low-mass galaxy
($M_{\star}$$\approx$3$\times 10^{5}$ M⊙) that was quenched at $t\approx 1.2$
Gyr while it was $\sim$1000 kpc from the central, first entering the primary
halo at $t\approx 11$ Gyr, suggesting that its quenching was not
environmentally induced. Our entire population of centrals is therefore
unlikely to have been directly influenced by the environment of the primary
LMC-mass halo.

We note that the following predictions are limited by both resolution and
sample size. Only three of the runs are able to resolve galaxies with
$M_{\star}<10^{5}$ M⊙, leading to a limited variety of cosmological volumes
being sampled at this mass scale. Indeed, at all resolved scales, six zoom-in
hosts is not a sufficient sample size to make statistically robust
predictions, especially due to the relatively small number of predicted
satellites of LMC-mass halos compared to that found around MW/LG-mass hosts.
We consider this to be a case study in the formation of LMC satellites, rather
than a statistically complete cosmological prediction.

Figure 2: Histogram of quenching times for all centrals and $z=0$ satellites
of LMC-mass hosts. Here, we define $t_{\text{quench}}$ as the formation time
of the youngest star particle associated with the galaxy. Environmentally
quenched satellites are highlighted in yellow and selected as having 4 Gyr $<$
$t_{\text{quench}}$$<$ 13.2 Gyr, as well as quenched within twice the virial
radius of their host. To distinguish galaxies that were recently quenched from
those that are actively star-forming, we have placed galaxies that formed a
star particle within the last 500 Myr in the $14-15$ Gyr bin, labeled as ‘star
forming.’ While both centrals and satellites follow bimodal distributions,
centrals exhibit clearly defined peaks and low scatter, while satellites
exhibit a wider distribution. This indicates that the environments of LMC-mass
hosts are the source of additional quenching mechanism(s), which we
investigate in Section 3.3.

##  3 Satellite Quenching

###  3.1 Comparison to Centrals

Figure 1 shows the SFHs of dwarf galaxies separated by association (satellites
of an LMC mass host versus nearby centrals) as well as separated by stellar
mass bins. All simulated data is shown as solid lines, while observed data for
satellites of the MW/M31 is shown via dashed lines (these objects are shown
for comparison, and are not direct analogs of our simulated satellites of LMC-
mass hosts). We highlight LMC-satellites that have been identified as
environmentally quenched (see Section 3.3) in a thicker line. In this
subsection, we examine only the objects from our sample of LMC-mass hosts,
with individual SFHs shown as solid colored lines. Satellites are defined as
being located within its host virial radius at $z=0$, while centrals are
defined as being located outside this radius at $z=0$ but within the high-
resolution region of the simulated zoom-in volume. We combine the
$M_{\star}=10^{7-8}$ M⊙ and $M_{\star}=10^{8-9}$ M⊙ bins due to the low number
count, late quenching times, and general similarity of SFHs of galaxies in
these bins.

In every mass bin, satellites (right panels) exhibit a wider range in
quenching times than do centrals (left panels). We find that all ultrafaint
(UF; $M_{\star}\approx 10^{4-5}$ M⊙) galaxies quench early on, as expected
from the heating effects of the ionizing UV background, preventing the
accretion and subsequent cooling of gas in low-mass halos. The FIRE-2
simulations implement a spatially uniform UV background, so such effects would
not be due to patchy reionization (Hopkins et al., 2018b). The latest forming
UF satellite ($t_{\text{quench}}$$\sim$ 4.6 Gyr) has been identified as
environmentally quenched, and inhabiting a more massive halo than other
satellites (when comparing the highest mass ever achieved by each halo to
account for mass loss due to tidal stripping). This galaxy would have likely
continued forming stars and be included in a higher mass bin if star formation
were not shut off due to environmental effects. Excluding this object, we find
that UF satellites quench at $t=2.0\pm 0.8$ Gyr, and centrals quench at
$t=1.3\pm 0.6$ Gyr, using the age of the last star particle formed as
quenching time. When looking at the 90 per cent star formation time-scale, we
find that satellites (again excluding those that are environmentally quenched)
have $\tau_{90}=1.8\pm 0.7$, and centrals have $\tau_{90}=1.3\pm 0.5$. We
therefore find the distribution in quenching times between UF satellites and
centrals to be statistically indistinguishable. We also note that UF dwarf
galaxies are only present in the three highest resolution runs: m11c, m11h,
and m11q.

In the next mass bin, we find that satellites with $M_{\star}=10^{5-6}$ M⊙
also exhibit a wider range of quenching times than their predominantly early-
quenched central counterparts. Notably, there are two late-quenching centrals,
which formed stars until $t\approx 13$ Gyr. These galaxies inhabit somewhat
larger DM halos ($M_{\text{200m}}$$\approx$5.5$\times 10^{9}$ M⊙) than most
other galaxies in this stellar mass range, with the average halo mass excluded
these two objects being $M_{\text{200m}}$$\approx$2$\times 10^{9}$ M⊙. This
difference might seem small, but leads to a factor of 2 difference in their
virial temperatures (4.6$\times 10^{4}$K versus 2.3$\times 10^{4}$K),
increasing the temperature limit of gas that will remain bound to the halos
during heating due to reionization and star formation feedback. The latest
forming satellites in this mass bin, as in the previous, were identified as
environmentally quenched and as possessing more massive DM halos than the rest
of the satellites in this bin. Therefore, these objects are more consistent
with failed versions of centrals in the next stellar mass bin.

Moving up in stellar mass to $M_{\star}=10^{6-7}$ M⊙, we now find that the
majority of centrals are star-forming (the exception again being an outlier in
halo mass, this time much lower than average), while all satellites are
quenched at various intermediate times, $t_{\text{quench}}$$\approx 4.5-12.5$
Gyr. This is a strong indication that the environment of LMC-mass hosts is
able to quench star formation in its dwarf companions.

In contrast, the highest mass dwarf satellites ($M_{\star}=10^{7\sim 9}$ M⊙)
form universally late, with all centrals and four of six satellites remaining
star-forming at $z-0$. The two quenched satellites ceased forming stars at
$t\approx 11-12$ Gyr, and have stellar masses of $M_{\star}=10^{7-8}$ M⊙. All
galaxies with $M_{\star}>10^{8}$M⊙ are star-forming, regardless of
environment, further supporting the claim that such galaxies are the most
resilient to quenching (Wheeler et al., 2014; Weisz et al., 2015; Wetzel et
al., 2015b; Fillingham et al., 2015).

Figure 2 shows the histogram of quenching times for all satellites and
centrals. We define star-forming galaxies as those which have formed at least
one star particle in the last $\sim$ 500 Myr, while quenched galaxies did not
form any star particles in the same time interval. Quenching times for such
objects are defined as the formation time of their youngest associated star
particle. We include the count of star-forming galaxies in the $t=14-15$ Gyr
bin to differentiate this population from those that formed their last star
particle between $t=13-13.2$ Gyr. We find that centrals demonstrate strongly
bi-modal quenching behavior, either halting their star formation by $t=4$ Gyr,
or continuing until $z=0$. In contrast, satellites exhibit a wider variety of
quenching times, with 8 having clear signs of environmental quenching
(highlighted in yellow). These galaxies were selected with 4 Gyr $<$
$t_{\text{quench}}$$<$ 13.2 Gyr, and
$d_{\text{host}}(t=t_{\text{quench}})<2$$R_{\text{200m}}$$(t)$. That is, they
are selected as having quenched late enough that reionization heating is
unlikely to be the culprit, and close enough to their host halo to be
influenced by its circum-galactic medium (CGM). We investigate these objects
further in Section 3.3.

In summary, we predict that isolated LMC-mass galaxies should host a
population of mostly quenched low-mass dwarf galaxies. More specifically,
these galaxies should be host to $\gtrsim 3$ ultrafaint ($M_{\star}=10^{4-5}$
M⊙) satellites with ancient stellar populations, $1-4$ intermediate mass
($M_{\star}=10^{5-7.5}$ M⊙) dwarf satellites with a variety of quenching
times, some of which may have been environmentally quenched by the host, and
lastly, one bright star-forming companion of $M_{\star}\gtrsim 10^{7.5}$ M⊙,
though not all simulations contain such an object. The number and mass
distribution is in agreement with recent predictions of the LMC satellite
population (Jethwa et al., 2016; Sales et al., 2017; Kallivayalil et al.,
2018; Pardy et al., 2020; Jahn et al., 2019; Erkal & Belokurov, 2020).

Figure 3: Stellar mass versus quiescent fraction in our simulations of LMC-
mass hosts versus observed (points) LG galaxies and simulated (shaded regions)
LG or MW-like halos. MC12 (McConnachie, 2012) and W15 (Weisz et al., 2015)
report data for the LG itself, while M21 (Mao et al., 2021) report data for
observed MW analogs. We define the quiescent fraction as the number of
galaxies in each mass bin that have $t_{\text{quench}}$$<13.2$ Gyr (that is,
selecting all galaxies that have not formed a star particle within the last
500 Myr) to allow for variations that may arise from finite time-steps and
star particle mass limits. We find that LMC-mass hosts are able to quench
their satellite population to nearly the same degree as MW or LG-like hosts,
perhaps a surprising result given their significantly smaller halo masses and
baryonic content. References: FIRE MW+LG - Samuel et al. in prep; CHANGA -
Akins et al. (2021); MC12 - McConnachie (2012) (as compiled by Wetzel et al.
2015b); W15 - Weisz et al. (2015); M21 - Mao et al. (2021)

###  3.2 Comparison to Local Group Environments

We find that satellites of LMC-mass hosts quench their satellites similarly to
the LG, with simulated data from the FIRE simulations for such objects from
Figure 4 of Garrison-Kimmel et al. (2019b) shown as the blue line in the right
panel for each mass bin of Figure 1 (where $M_{\star}=10^{4-5}$ M⊙ is not
included due to resolution limits). These simulations of LG-like environments
include two MW-mass ($\sim 10^{12}$ M⊙) halos, and satellites are defined as
being with 300 kpc of one of those MW-like halos. For our $M_{\star}=10^{7-9}$
M⊙ bin, we show the mean reported SFH for the bins $M_{\star}=10^{7-8}$ M⊙ and
$M_{\star}=10^{8-9}$ M⊙. In each mass bin where simulated LG-satellites are
available, the overall shape of SFHs for such objects are consistent with LMC
satellites.

We also compare to observed SFHs for satellites of the MW or M31 (Weisz et
al., 2014; Skillman et al., 2017), with individual SFHs in each mass bin shown
as thin dashed/dotted lines, while the median is shown as a thicker black
dashed line. We find that our simulated LMC satellites have SFHs that are
broadly consistent with observed MW/M31 satellites at fixed stellar mass,
especially in the two highest mass bins. The observed galaxies in the lowest
bins tend to form later than their simulated counterparts. This is perhaps due
to observational uncertainty, since the majority of stars in each observed
galaxy are formed in early times, consistent with our simulated galaxies.
Constraining the exact time of quenching can be a challenge with observational
data. For example, see Weisz et al. (2015) for a discussion of the impact of
blue straggler stars on the estimation of SFHs via color-magnitude diagram
fitting.

The similarity of SFHs between observed & simulated LG/MW/M31 satellites and
our simulated LMC satellites suggests that quenching of satellites may not be
restricted to high-mass systems, and that dwarf-dwarf quenching could proceed
likewise to quenching in LG-type environments, an effect that may be impactful
on the interpretation of future observational missions categorizing satellites
of LMC-mass hosts. There is evidence that the CGM of the MW is dense and
structured enough to affect the evolution of its intermediate-mass dwarf
satellites (e.g. Grcevich & Putman, 2009; Peek et al., 2009; Nakashima et al.,
2018). The apparent similarity in SFHs between Local Group and LMC satellites
suggests this may be true of LMC-like environments as well.

Figure 3 shows the quiescent fraction (i.e., the portion of galaxies which
have not formed a star particle within the last 500 Myr) versus stellar mass
of LMC satellites and centrals along with additional observed and simulated LG
satellites. Due to the fact that not all of our simulated LMC-mass systems
contain satellites in each mass bin, our error bars are derived from Poisson
scatter. Error bars on W15 were calculated from the difference in reported
quenching fractions when considering morphological dTrans galaxies as either
star-forming or quiescent.

Largely, our population of LMC satellites quenches similarly to LG satellites.
Consistent with observations of satellites within the LG (McConnachie, 2012;
Wetzel et al., 2015b) as well as with simulated MW/LG satellites (in FIRE-2 –
Samuel et al. in prep; and CHANGA – Akins et al., 2021), we find that LMC
satellites with $M_{\star}<10^{7}$ M⊙ are universally quenched, and satellites
with $M_{\star}\gtrsim 10^{8}$ M⊙ are predominantly star-forming. This is also
in agreement with semi-analytical models of the LG population (Fillingham et
al., 2016, 2019). The interim region of $10^{7}$ M⊙$<M_{\star}<10^{8}$ M⊙
consists of satellites that are either presently star-forming or quenched
within the last $\sim$2 Gyr. In contrast, we find that 90 per cent of nearby
centrals are quenched below $M_{\star}<10^{6}$ M⊙, while 91 per cent of
centrals with $M_{\star}>10^{6}$ M⊙ are star-forming by $z=0$, with outliers
in quenching status also being outliers in halo mass. This indicates that
satellites of $M_{\star}=10^{6-7}$ M⊙ are ideal probes of environmental
quenching, while satellites with $M_{\star}>10^{8}$ M⊙ are difficult to
quench, in agreement with previous work on the quenching of satellites of
higher mass hosts (Wheeler et al., 2014; Weisz et al., 2015; Fillingham et
al., 2015; Wetzel et al., 2015b). We therefore find that LMC-mass hosts,
though they possess a reduced amount of substructure, may in fact be able to
quench their satellites in a similar manner as MW/LG-like environments.

Figure 4: (top panels) Stellar mass versus the 90 per cent star-formation
time-scale ($\tau_{90}$) for simulated satellites and field galaxies,
including observed local-group dwarf galaxies from Weisz et al. (2015). We use
$\tau_{90}$ here as a comparable analog to the observational data,
representing the cosmic time at which 90 per cent of the present-day stellar
mass was formed. The yellow shaded region denotes the range in $\tau_{90}$ for
environmentally quenched satellites, which are plotted as yellow diamonds (see
text for identification criteria). We find that our simulated environmentally
quenched satellites fall within the distribution of observed galaxies in the
local group. We find fewer low-mass late-forming satellites, though that could
be due to the high variance of quenching times and our small sample size.
(bottom panels) Stellar mass versus $z=0$ distance to the host galaxy,
normalized by $R_{\text{200m}}$. Star forming galaxies are shown as solid
markers, while quenched galaxies are shown as open markers.

Figure 4 shows the stellar mass and 90 per cent quenching time-scale
($\tau_{90}$) for our simulated centrals and satellites, as well as observed
LG satellites as reported by Weisz et al. (2015). The highlighted region
indicates the $\tau_{90}$ range of environmentally quenched satellites (see
Section 3.3). Note that $\tau_{90}$ and $t_{\text{quench}}$ are not identical
quantities; $t_{\text{quench}}$ indicates the formation time of the youngest
star particle, while $\tau_{90}$ represents the time at which 90 per cent of
the $z=0$ stellar mass was formed. The environmentally quenched satellites of
LMC-mass hosts are consistent with the trend of observed LG satellites in
their stellar masses and $\tau_{90}$, further supporting the case that
isolated LMC-mass hosts can environmentally influence their satellites
similarly to the LG. The bottom panel of Figure 4 shows the $z=0$ distance to
the primary central normalized to its $R_{\text{200m}}$ versus stellar mass
for both satellites and centrals, also marking their star-forming state. Note
that there are far fewer star-forming satellites than centrals. Consistent
with Figure 3, we define quenched satellites as those with
$t_{\text{quench}}$$<$ 13.2 Gyr (i.e. not having formed a star particle in the
last $\sim$500 Myr).

Figure 5: (top panels) Orbits of environmentally quenched satellites (EQSs),
with the virial radius of the host shown as a red dashed line, infall times
marked as circles, and quenching times (i.e. the formation time of the
youngest star particle) marked as triangles. This population of satellites was
selected as having been quenched between 4 and 13 Gyr, and that were quenched
at a distance of less than twice the virial radius of their host at that time.
Five of the eight galaxies were quenched before th,eir first pericenter,
suggesting the gaseous halos of these simulated LMC-mass hosts are rich enough
to affect satellite evolution as far away as their virial radii. (bottom
panel) Quenching time-scales for EQSs, defined as
$t_{\text{quench}}-t_{\text{infall}}$ such that galaxies which were quenched
after infall appear above the horizontal line. The grey bar indicates the
quenching time-scale due to stripping and feedback predicted for satellites of
MW-like hosts (Fillingham et al., 2016). In addition, we plot the time-scales
corresponding to the time of closest approach for subhalos 44820 & 82233
(partially transparent cyan & yellow markers) of m11h due to the fact that
they come within the vicinity of the host halo around their respective
quenching time, but splash back on wider orbits before later falling into the
host halo.

###  3.3 A Closer Look at Environmental Quenching

Here we investigate the specific circumstances of quenching for the 8
identified environmentally quenched satellites (EQSs). Satellites were
identified as being environmentally quenched by requiring intermediate to late
quenching times such that 4 Gyr $<$ $t_{\text{quench}}$ $<$ 13.2 Gyr, and
proximity to the host halo
$d_{\text{host}}(t_{\text{quench}})<2r_{\text{200m}}(t_{\text{quench}})$. We
allow for objects to be located outside the host virial radius at quenching
time due to previous works highlighting the consistency of galaxies within
2$R_{\text{200m}}$ of the MW with environmental quenching (Fillingham et al.,
2018), indicating that the sphere of influence of the primary central is not
strictly limited to such a radius. Three satellites quench outside their host
$R_{\text{200m}}$: 44820 at 1.7$R_{\text{200m}}$, 56887 at
1.06$R_{\text{200m}}$, and 82233 at 1.05$R_{\text{200m}}$.

Figure 5 shows the orbits of these objects, as well as the evolution of the
host virial radius, whose intersection with each orbit defines the infall
times (marked as circles). Halo ID numbers are shown on the figure in
corresponding colors, and consistent coloring will be used in further plots
that highlight this sample. Five of the eight EQSs were quenched at or near
the host virial radius, often with infall times shortly before or after their
quenching times, suggesting that the circumgalactic medium (CGM) of the hosts
are dense enough to influence satellites of this mass. These galaxies are
lower mass, with stellar masses of $M_{\star}\approx 10^{5-6}$ M⊙ and peak
halo masses of $M_{\text{halo,peak}}\approx 2\times 10^{9}$ M⊙. The other
three EQSs are more massive, with $M_{\star}\approx 10^{7}$ M⊙, and
$M_{\text{halo,peak}}\approx 10^{10}$ M⊙. These satellites fell into their
host halos later, and quenched after first pericenter.

This trend can be seen in the bottom panel of Figure 5, which shows the
stellar mass of EQSs compared to their quenching time-scales, defined as
$\tau_{\text{quench}}=t_{\text{quench}}-t_{\text{infall}}$. The three most
massive EQSs have $\tau_{\text{quench}}\approx 2-4$ Gyr, while the lowest mass
EQSs have $-1$ Gyr $\lesssim\tau_{\text{quench}}\lesssim 1$ Gyr. Objects 44820
(cyan) & 82233 (yellow) undergo a pericentric passage around the host before
falling within $R_{\text{200m}}$ (note the difference between quenching time
and infall time markers on Figure 5, and the orbital minima that occur near
quenching time). We therefore include a secondary $\tau_{\text{quench}}$ based
on the time of this pericenter rather than the infall time, as the boundary of
the DM halo is somewhat arbitrarily defined, especially when considering the
baryonic effects of the central galaxy. These points are shown as partially
transparent markers connecting to the original point based on
$t_{\text{infall}}$ via a dashed line, and bring them into stronger agreement
with the other low-mass EQSs, with faster quenching time-scales such that
$t_{\text{infall}}\approx t_{\text{quench}}$.

The distinction in quenching time-scales and infall times of low-mass versus
intermediate-mass satellites suggests that there may be further stellar-mass
dependence within the quenching model of Fillingham et al. (2016). We have
indicated their predicted quenching time-scales due to feedback and ram-
pressure/turbulent stripping for satellites of MW-mass hosts in the bottom
panel of Figure 5 as a grey bar. We find that intermediate-mass
($M_{\star}\approx 10^{6.5-7.5}$ M⊙) satellites of LMC-mass hosts have
somewhat longer quenching time-scales than predicted for satellites of MW-mass
hosts. This makes sense in light of the lower stellar-mass to halo-mass ratio
for dwarf galaxies like the LMC, and the predicted lower level of disruption
for such systems when compared to MW-mass hosts (Jahn et al., 2019). We also
find that low-mass satellites ($M_{\star}\lesssim 10^{6}$ M⊙), which are not
resolved in the analysis of Fillingham et al. (2016), have somewhat lower
quenching time-scales than predicted for intermediate mass satellites. This is
likely due to the lower binding energy of their less massive DM halos, leading
to higher susceptibility to ram-pressure stripping and therefore quenching
earlier in the infall process from less dense gas in the outer parts of the
parent halo. In principle, this mechanism should apply to hosts of any mass,
suggesting fast quenching time-scales (perhaps within -0.5 to 0.5 Gyr) for
low-mass satellites of MW-like hosts. It is unclear at this point whether
these two subtypes (i.e. $M_{\star}\approx 10^{6.5-7.5}$ M⊙ with
$t_{\text{infall}}>8$ Gyr and quenching time-scales of $2-4$ Gyr versus
$M_{\star}\lesssim 10^{6}$ M⊙ with $t_{\text{infall}}<7$ Gyr and quenching
time-scales of -0.5 to 0.5 Gyr) lie on a continuous distribution of satellite
quenching behavior, or if there is a stellar mass cutoff between distinct
populations.

Figure 6: (Top 4 panels) Total (thin) and cold (thick) gas mass within
2$R_{50\star}(t)$ for each EQS, excluding gas with high relative velocity to
the satellite such that
$|v_{\text{gas}}-v_{\text{sat}}|<10\times$max$[v_{\text{circ}},\sigma_{v}]$,
all measured within the satellite. Each galaxy experiences a steep drop in
$M_{\text{gas}}$ at or near its quenching time, indicating a removal of gas
through either star formation bursts or ram pressure (or both) rather than
starvation or gravitational stripping, which are characterized by slower
reductions in $M_{\text{gas}}$ (on the order of several Gyr or longer).
(Bottom 2 panels) Cold gas mass within 2$R_{50\star}(t)$ for star-forming
(left) and quenched (right) centrals. We find a consistent presence of cold
gas throughout the history of centrals that are star-forming at $z=0$, while
quenched centrals cease to contain cold gas after their quenching times.
Figure 7: Properties of the gaseous halos of all LMC-mass halos at $z=0$. The
left column shows a projection of gas temperature, the second column shows a
projection of gas density (with points marking the locations of satellites),
and the right column shows the phase-space diagram for halo gas, defined as
$2R_{50\star}<r_{\text{gas}}<R_{\text{200m}}$ and outside $2R_{50\star}$ of
luminous satellites. Each host exhibits a significant mass in a hot
(T$=10^{4.5-6}$K) corona with $M=3-6$$\times 10^{9}$ M⊙, with highly
structured regions of hot (rarefied) versus cold (dense) gas, with visible
shock fronts.

The top panels of Figure 6 show the total and cold gas mass within
$2r_{50\star}(t)$ for each EQS as a function of time. We find universally
steep drop-offs in gas content near the quenching time for each galaxy,
suggesting some form of hydrodynamic gas removal, which operates on much
faster time-scales than gravitational stripping or starvation (Fillingham et
al., 2015; Emerick et al., 2016).

While some galaxies retain or even re-accrete some amount of gas, none re-
ignite their star formation after the initial gas-loss event. Take, for
example, the m11h satellite 56887 (bottom left panel, green line), which is on
a splashback trajectory before settling permanently in an orbit within the
virial radius of the host halo at $t\sim 9$ Gyr. This satellite loses its gas
and quenches after first infall, but is able to regain some gas on its
trajectory back out of the host halo. It is possible that some or all of this
gas is not tightly bound to the satellite, as our velocity cuts are somewhat
liberal, but $\sim 10^{6}$ M⊙ of gas remains within that radius during the
object’s splashback orbit for another few Gyr before it infalls again and
fully loses all remaining gas content. Interestingly, the re-accretion of gas
to pre-quenching levels is not sufficient to reignite star formation in the
satellite. The correlation of multiple infall and subsequent gas removal
events is an encouraging suggestion that the environment of the host halo is
responsible for stripping away any gas bound to the satellite. We therefore
turn our attention to the CGM properties of the LMC-mass host galaxies.

The bottom 2 panels of Figure 6 show the history of cold gas within star-
forming and quenched centrals. There are no obvious signatures in this data
that distinguish the gas content of centrals from EQSs. We therefore look into
further details of the gas content of EQSs in Section 3.3.2. For now, we turn
our attention to the gas content of the host halos.

####  3.3.1 Characterizing the Gaseous Halos of LMC analogs

Figure 7 shows the temperature and density projection for each LMC-mass host
in out sample, as well as the phase diagram, with each pixel colored according
to the total mass of gas contained within it. Projections are constructed by
selecting gas with $|x|<1.1$$R_{\text{200m}}$, $|y|<1.1$$R_{\text{200m}}$, and
$|z|<0.2$$R_{\text{200m}}$, where $z$ is perpendicular to the plane of the
figure. We choose no particular orientation with respect to the host galaxy.
This gas is then divided into evenly spaced 2-D ($x,y$) pixels, giving a
column of gas with length 0.4$R_{\text{200m}}$. The pixel is then colored
according to the median physical value of temperature or density for all
particles within its boundary. If there are no particles within the pixel, it
is colored according to the median value of nearest non-empty pixels.

We find that a hot, richly structured gaseous halo is present around all LMC-
mass hosts to varying degrees. While not all centrals are host to EQSs, the
ubiquity of the rich gaseous halo suggests that the presence of such galaxies
relies more on varying cosmological abundances of structure than it does on
the ability any particular LMC-analog to quench its satellites. We identify
two primary components of the CGM based on features in the phase diagrams: the
hot corona, found in the upper left quadrant, and the horizontal feature of T
$\sim 10^{4}$ K gas with $10^{-4}\lesssim\rho/$cm${}^{-3}\lesssim 10^{0}$.
Some runs also contain a small component of cold, dense gas in the lower right
quadrant. Star-forming gas in FIRE is restricted to densities above $10^{3}$
cm-3, and is not abundant enough outside of $2R_{50\ast}$ compared to the halo
gas to appear on this figure.

Quantifying the hot corona as gas with $10^{4.5}<$ T/K $<10^{6}$, and
10${}^{-6}<\rho/$cm${}^{-3}<10^{-4}$, as well as being located outside
2$r_{50\star}$ of the host galaxy and all satellite galaxies, we find that
LMC-mass halos have 3$\sim$6$\times 10^{9}$ M⊙ of gas in their hot coronas.
Additionally, we find mean gas densities of $\sim$5$\times 10^{-4}$ cm-3 and
mean temperatures of $\sim$1$\times 10^{5}$ K, both quantities volume-
weighted. These predictions are in good agreement with the detection of a hot
ionized component in the LMC (Wakker et al., 1998; Lehner & Howk, 2007)
suggesting the presence of hot gas around the Magellanic clouds as well as
with recent theoretical arguments of a need of a hot corona in the LMC to
fully explain the morphology of the Magellanic stream Lucchini et al. (2020).
While there are differences in the presence and radial distribution of hot and
cold CGM components between LMC and MW-mass galaxies in the FIRE-2 simulations
(Stern et al., 2021), the existence of a relatively massive hot component out
to $\gtrsim$ 100 kpc is consistent between host mass scales.

Figure 8: Gas density (color) and velocity (arrows; in the reference frame of
the satellite) projection of two satellites of m11h as they fall into the host
halo, demonstrating two primary modes of quenching: SF-driven stripping (top)
and standard ram pressure stripping (bottom). Each image is centered on the
halo location as provided by ROCKSTAR at the shown snapshot, despite the lack
of obvious structure in later panels. The satellites are similar in size, with
$M_{\text{200m}}$($t_{\text{quench}}$)2$\approx$3$\times 10^{9}$ M⊙ and
$M_{\star}$($t_{\text{quench}}$)1$\approx$2$\times 10^{6}$ M⊙, and they both
quench around $t\sim 4.5$ Gyr. The first panel of both rows shows the
satellite near $t_{\text{quench}}$, with visible tails due to its motion
through ambient halo gas. The top row demonstrates quenching due to a
combination of feedback and ram pressure, which is a common mechanism among
our population satellites. The bottom row demonstrates a classic example of
ram pressure stripping of a satellite.

####  3.3.2 Quenching via Ram-Pressure and Feedback

There are many possible sources for gas removal in satellites, for example,
energetic feedback from star formation which can be induced by the increased
pressure of the host environment, interactions with other galaxies, or ram-
pressure stripping from the ambient halo gas. The time-scale of gas removal
seen in Figure 6 is short enough to rule out starvation, which occurs on
longer time-scales as gas reservoirs within the satellite are depleted
(Fillingham et al., 2015). Interactions such as fly-by events and ram-pressure
stripping are functions of environmental properties (abundance of satellites,
density of gas), while feedback-driven, self-induced quenching only depends on
the star formation history of each galaxy (though the SFH may also be
dependent on host environment). It is likely that a combination of these
effects simultaneously occurs in orbiting satellites.

Although the energetic feedback of the FIRE simulations is certainly enough to
strongly affect the ISM of dwarf galaxies (El-Badry et al., 2016, 2017, 2018;
Hopkins et al., 2018b), the general lack of isolated dwarfs with
$t_{\text{quench}}$$=4-13$ Gyr and $M_{\star}=10^{5-7}$ M⊙ makes self-
quenching alone an unlikely cause for the halting of star formation in these
satellites. However, one could not rule out environmentally induced starbursts
(i.e. from compression of gas at orbital pericenters), or removal of low
density gas blown out by feedback, which may have cooled and fallen back into
the satellite if it were in isolation, but is easily swept away by the high
density of the host’s ISM. Such effects, which may not neatly be described as
strictly environmental or strictly self-induced, seem to drive the evolution
in some of the satellites in our sample, as illustrated by the two case
studies presented here.

Figure 8 shows a series of density projections at four sequential time stamps
of two low-mass satellites of m11h that were quenched near its virial radius.
Also shown is the normalized gas velocity field in the reference frame of each
satellite. Time stamps were chosen simply to highlight the state of the gas in
and around each satellite as it is quenched, with the first panel being chosen
as the snapshot immediately prior to the formation of its last star particle.
The stellar half mass radius of each satellite is also shown as a red circle.
Each frame is centered on the satellite’s position at the given time.

The top row shows a satellite with $M_{\text{200m}}$($t_{\text{quench}}$)
$\approx$ 3 $\times 10^{9}$ M⊙, $M_{\star}(t_{\text{quench}})\approx$ 2$\times
10^{6}$ M⊙, and $M_{\text{gas}}(t_{\text{quench}})\approx 10^{6}$ M⊙, where
$t_{\text{quench}}$ $=4.75$ Gyr. It demonstrates trails characteristic of ram
pressure in the first panel, but the gas is sufficiently dense in its core as
to resist stripping. The velocity field reveals turbulence around the galaxy
as well, though there is a clear front of gas moving downwards from the top of
the figure. The second panel shows a burst of star formation that moves this
gas out of the central region, heating and rarefying it. This enables the gas
to be pushed out of the halo by the pressure from ambient halo gas in the
third panel, resulting in no clear gaseous component to the halo in the fourth
panel, where the velocity field has become more uniform. This process is
generally consistent with ram pressure stripping, though it requires
sufficient stellar feedback to ‘loosen’ the gas within the satellite before
the ambient halo pressure is capable of stripping and quenching it.

The bottom row shows a second similar mass satellite, with $t_{\text{quench}}$
$\approx$ 4.5 Gyr, this time with a much more uniform velocity field. This
object demonstrates a more standard picture of ram pressure characterized by a
gas stream extending from the satellite opposite the direction of motion.
There is no feedback event that processes the gas prior to stripping - the
pressure from ambient halo gas is sufficient to strip away the dense, bound
gas within the satellite. Note that the second panel shows an increased amount
of dense gas within $r_{50\star}$ due to compression via the ambient velocity
field. The time-scale for each galaxy to go from possessing dense,
concentrated, star-forming gas to possessing virtually no gas is $\sim 300$
Myr in both cases, though it is slightly faster in the case where feedback is
involved.

An important qualification to this analysis is that both satellites come from
the same parent halo – m11h. This halo is host to an unusual abundance of
satellites: 10 in total (12 including all subhalos with assigned star
particles, forgoing the cuts described in Section 3.1). As seen on the left
side of the first panel of the top row in which an additional locus of dense
gas is present, satellite-satellite interactions can also be a source of
environmental quenching. This particular event seems to have compressed the
gas in the satellite shown, leading to a strong burst of star formation,
rarefying the gas and making it more susceptible to ram-pressure stripping via
the halo gas. These objects were chosen for the case study due to their high
resolution and obvious visual features. We have done a similar analysis of all
EQSs and find ram-pressure alone or in combination with feedback from star
formation to be the quenching mechanism for all EQSs.

Interactions can also be seen in the orbits of the above objects in Figure 5
which appear to have pericenter with some object other than the host prior to
final infall. We have checked this explicitly, though the other satellites are
not shown on the figure for visual clarity. We include this type of
interaction under the umbrella of environmental quenching, though it does
require the presence of sufficiently many companion galaxies for satellite-
satellite interactions to take place. It is unclear how cosmologically common
this is for LMC-mass hosts, but in our set of 6 centrals and 30 satellites, we
identified 1 host with 2 instances of interactions.

The pre-processing of satellites prior to infall is a natural prediction of
$\Lambda$CDM (Li & Helmi, 2008; Wetzel et al., 2015a; Benavides et al., 2020),
with part of the aim of this study to understand how the environment of the
LMC could have affected its satellites prior to the group’s infall into the
halo of the MW. We expect that pre-processing – whether due to prior group
association or individual fly-by events – before to infall into LMC-mass halos
will perhaps be less common than for systems like the MW, simply due to the
relative abundance of structure in each. However, this example demonstrates
that pre-processing on much smaller scales than the MW is indeed possible, and
perhaps contributes to the relatively high amount of environmentally quenched
satellites within m11h.

##  4 Effects of Tides on Satellites of LMC Analogs

It has been shown that MW-mass galaxies are hosts to rich tidal features
including coherent stellar streams and kinematically mixed stellar halos
(Helmi & White, 1999). These features result from interactions between dwarf
satellites on close orbits with their more massive hosts that tidally strip
mass (both dark and luminous) from their companions. Similar processes are
expected to occur also for satellites of lower mass hosts, with a handful of
observations confirming the presence of tidal streams in satellites of dwarf-
mass centrals (e.g., Martínez-Delgado et al., 2012).

Figure 9: (Top) Dark matter and (bottom) stellar mass of all satellites versus
time, with environmentally quenched satellites highlighted in color. All such
satellites experience significant tidal stripping of their DM halos after
infall, with anywhere from $82-99.9$ per cent of the DM mass being lost by
$z=0$. Stellar masses shown here are from ROCKSTAR, and do not necessarily
reflect all stellar mass loss due to stripping as streams are not detected and
removed. However, stripping of the stellar component can still be seen in
objects 82233 & 66354 (yellow and orange, respectively).

To investigate the tidal stripping of simulated satellites around LMC-mass
hosts, the top panel of Figure 9 shows the dark matter mass of satellites as a
function of time, with the previously described population of environmentally
quenched satellites (EQSs) highlighted. We find that the majority of
satellites experience tidal stripping of their dark matter halos, beginning at
or near their infall times onto the host, with EQSs generally experiencing the
largest decreases in halo mass, losing $82-99.9$ per cent of the peak halo
mass ever obtained.

In one case (halo ID 82233, yellow), the satellite appears to have its halo
mass reduced by a factor of $\sim 5\times$ prior to quenching, and by $\sim
10\times$ prior to first infall. This is the galaxy shown on the bottom panel
of Figure 8. It is clear from our previous analysis that ram-pressure plays an
important role in its quenching, but here we demonstrate that it is also
subjected to severe tidal stripping. This object also experiences the highest
magnitude of halo mass loss by $z=0$ due to its short orbital period, early
infall time, and apparent interaction with other satellites prior to infall,
as seen in Figure 5.

The bottom panel of Figure 9 shows the evolution of stellar mass of all
satellites. Most satellite galaxies do not experience significant stripping of
their stellar components as they are deeply segregated in the inner regions of
their dark matter subhalos (Peñarrubia et al., 2008), but we do find a handful
of objects that appear to have had various degrees of stellar mass loss due to
stripping. Significant halo mass loss is not necessarily a guaranteed
indicator of stellar stripping, but the two halos which lost the highest
fraction of halo mass also lost the highest fraction of stellar mass (82233 &
66354). This makes sense as the DM component is far more extended than the
stellar component, and would therefore be first to be stripped away when tidal
forces begin to take hold.

Figure 10: Tidal streams at $z=0$ originating from stripped satellites around
LMC-mass hosts. Small, black dots are star particles belonging to the host
galaxy at $z=0$, while thicker dots are the present-day locations of star
particles belonging to satellites of the corresponding color at their first
infall times. We find coherent stellar streams in all hosts with EQSs
(Environmentally Quenched Satellites), and none in hosts without EQSs. It is
unlikely that the quenching is a direct result of this tidal stripping, but
these could be correlated as a result each effect’s individual dependence on
satellite mass (coherent tidal streams require sufficiently many stars to
strip - as well as late(r) infall times - while environmental quenching
requires sufficiently high mass as to not by quenched by reionization
heating).

Since gravitational interactions with satellites are known to be a source of
stellar tidal streams in MW-mass galaxies, we plot the $z=0$ locations of star
particles that were assigned to any satellite galaxies at their infall times
in Figure 10. Streams were then identified by examining the evolution of the
spatial distribution of such star particles. Streams became apparent when star
particles were pulled from their original locations within satellites as they
made close approaches to the host, forming extended stellar structures. We
find four hosts with tidal streams originating from five satellites.

We find no tidal features that arose from any satellite galaxy that was not
environmentally quenched, be it a low-mass early quenched galaxy, or a high
mass star-forming galaxy. This may be a result of mass selection: low-mass
satellites, while occurring frequently and infalling early, do not possess a
large population of stars that can be striped with a well-resolved stream in
our runs, meanwhile, high mass satellites, while having an abundant stellar
component, are less common and infall late, thus not having sufficient time to
interact with the host. Tidal stripping is not the dominant factor in
quenching these galaxies (see the location of triangle symbols in Fig. 9
mostly not correlated to stripping events). It is simply that satellites that
were quenched due to ram-pressure stripping seem to be also those experiencing
significant tidal stripping not only of their dark matter but also of their
stars.

The streams depicted in Figure 10 are highly extended, containing stars
located within $\sim$1-4 kpc of the primary host galaxy out to $\sim$80 kpc.
All stripped stars are on highly radial orbits, in agreement with previous
theoretical predictions (Abadi et al., 2006). The amount of stellar mass
contained in the streams ranges from $10^{6-7}$ M⊙, with a median value of
$2\times 10^{6}$ M⊙. The streams around our simulated LMC-mass galaxies are
quite substantial, and may be observable around dwarf centrals through deep
photometry.

We find that tidal structures result from the highest mass environmentally
quenched satellites, having a stellar mass range of
$M_{\star,\text{max}}=10^{6\sim 7}$ M⊙. Most satellites in this mass range
though are star-forming and late-infallers. The tidal structures from later-
infalling satellites (such as those around m11c, m11d, and m11q) are
morphologically distinct from those formed by early-infalling satellites, as
they have not experienced enough dynamical times to become kinematically
mixed. The streams around m11h originate from satellites that fell in around 9
Gyr ago, and have undergone many pericenters as seen on Figure 5. This results
in streams that are more diffuse, though still retaining clear spatial
cohesion along the orbital path.

We note that we only investigate stellar streams that form directly as the
result of tidal forces that strip stars from satellite galaxies as they orbit
the central. Extended stellar structures also exist in the form of in-situ
stellar streams and stellar halos, which have been investigated in the FIRE
simulations (e.g. Yu et al., 2020). El-Badry et al. (2016) showed that
galaxies with $M_{\star}=10^{6.3-10.7}$ M⊙ experience radial migration of
stars on both short and long time-scales due to star-forming clouds that are
driven to high radial velocities from bursty feedback, as well as due to
energy transfer from the fluctuation of the galactic potential. This migration
can result in stars located $\gtrsim 10$ kpc from the radial position of their
formation, contributing to wide variations in half light radius over time.
They note that the stellar mass range $M_{\star}=10^{7-9.6}$ M⊙ is optimal for
maximizing the physical effects that cause stellar migration, suggesting that
LMC-mass centrals may have a significant in-situ stellar halo as well.

Recently, Panithanpaisal et al. (2021) investigated the formation of stellar
streams around MW-mass galaxies in the FIRE-2 simulations. They find that
present-day satellites are good proxies for the progenitors of stellar
streams. They further show that low mass ($M_{\star}<2.25\times 10^{6}$ M⊙)
stream progenitors are likely to have their star formation quenched prior to
infall, while progenitors above that stellar mass threshold are quenched by
the host environment. This is consistent with our analysis of EQSs, though we
find that low-mass stream progenitors may be environmentally quenched as well.

While tidal features arising from satellite interactions have been observed
around dwarf galaxies (Martínez-Delgado et al., 2012; Carlin et al., 2019),
their frequency in as of yet unknown. The presence of resolved stellar streams
in configuration space around four of our six LMC-mass hosts is an encouraging
sign that satellite-host interactions may result in observable tidal
structures in a substantial fraction of LMC-mass dwarf galaxies in the field.

##  5 Summary & Conclusions

We investigate various properties of the satellite population of six LMC-mass
hosts in the FIRE simulations. By comparing their star formation histories
(SFHs) to those of other centrals of similar stellar mass in Figure 1, we find
that LMC satellites have more diverse SFHs and quenching times than central
galaxies, which are strongly bimodal – either forming all their stars before
$t=4$ Gyr, or continuing active star formation at $z=0$. We further compare to
simulated satellites of Local Group pairs from Garrison-Kimmel et al. (2019b)
(also in the FIRE simulations), and find that satellites of LMC-mass hosts
have similar SFHs to LG-satellites at fixed mass. LMC satellites retain the
general mass-dependence of quenching times: low mass satellites
($M_{\star}<10^{6}$ M⊙) quench early, while high mass satellites
($M_{\star}>10^{7}$ M⊙) quench late or continue forming stars, as shown in
Figure 3. Intermediate mass satellites have the greatest diversity of
quenching times (Figure 4).

We identified 8 environmentally quenched satellites, selected as having
intermediate quenching times ($t_{\text{quench}}$$=4-13$ Gyr) and located
within twice the virial radius of the host at the time of quenching. By
examining their orbits and quenching time-scales (Figure 5), we identify two
subtypes: higher mass, late infalling satellites that quench after first
pericenter; and lower mass, early infalling satellites that quench near the
host virial radius. It is unclear whether these subtypes are distinct
populations, or if quenching time-scale and infall time are continuous
functions of stellar mass. Encouragingly, early data from the LBT-SONG survey
also hints at environmental quenching occurring in satellite dwarfs of two
observed LMC-like hosts, NGC 628 (Davis et al., 2021) and NGC 4214 (Garling et
al., 2020).

All our simulated galaxies experience a stark drop in their gas content after
quenching (Figure 6), indicating hydrodynamic rather than gravitational
effects. We find that the LMC-mass hosts contain hot, richly structured
gaseous halos, with $3\sim 6\times 10^{9}$ M⊙ of gas in their hot (T
$=10^{4.5-6}$K), diffuse ($\rho=10^{-6}$ – $10^{-4}$cm-3) coronas, as shown in
Figure 7. We further demonstrate that this rich environment is able to strip
gas from satellites via ram-pressure, halting their star formation. This
process can be made more efficient through internal burst of feedback within
the satellite, moving its gas to a higher energy state and expediting the
effects of ram-pressure. Case studies of two satellites that illustrate
quenching due to SF-aided ram-pressure stripping versus pure ram-pressure are
shown in Figure 8

By examining the evolution of the dark and stellar mass components of
satellites, we find that all 8 environmentally quenched satellites have lost
$82-99.9$ per cent of their peak DM mass via tidal stripping, with other
satellites undergoing varying amounts of DM loss, some losing almost none due
to their late infall times, as shown in Figure 9. Stellar mass loss greater
than $\sim$10 per cent due to tidal stripping is rare, happening in only 2
satellites in our sample.

We investigate vestigial structures of host-satellite interaction by
identifying the $z=0$ location of stars that were assigned to satellites at
their infall times, and find extended stellar streams around 4 of 6 LMC-mass
hosts, seen in Figure 10. All originated from environmentally quenched
satellites. Three formed from $M_{\star}=10^{6.5-7.5}$M⊙ satellites infalling
within the last $\sim$2 Gyr, while the two streams around the fourth host
originated from (pre-infall) $M_{\star}\approx 10^{6}$ M⊙ satellites with
infall time $\sim$8 Gyr ago, around $z\sim 1$.

Our findings have strong implications for current and upcoming observational
missions targeting LMC analogs in the field. We suggest that such objects may
be host to 1-4 intermediate mass ($M_{\star}=10^{5-7}$ M⊙) satellites which
are likely to be environmentally quenched at intermediate – late times
($t_{\text{quench}}$$=4-13$ Gyr), depending on mass. This satellite population
would be present along side a potential bright, star-forming satellite, as
well as $\gtrsim$3 ancient ultrafaint satellites with $10^{4}\leq M_{*}\leq
10^{5}$ M⊙, though not all of our runs resolve this scale. LMC-mass galaxies
in the field can additionally host tidal streams due to past interactions with
their satellites.

## Acknowledgements

LVS acknowledges support from the NASA ATP 80NSSC20K0566 and NSF CAREER
1945310 grants. AW and JS received support from NASA through ATP grants
80NSSC18K1097 and 80NSSC20K0513; HST grants GO-14734, AR-15057, AR-15809, and
GO-15902 from STScI; a Scialog Award from the Heising-Simons Foundation; and a
Hellman Fellowship. MBK acknowledges support from NSF CAREER award
AST-1752913, NSF grant AST-1910346, NASA grant NNX17AG29G, and HST-AR-15006,
HST-AR-15809, HST-GO-15658, HST-GO-15901, HST-GO-15902, HST-AR-16159, and HST-
GO-16226 from the Space Telescope Science Institute, which is operated by
AURA, Inc., under NASA contract NAS5-26555. JSB was supported by NSF grants
AST-1910346. We ran simulations using: XSEDE, supported by NSF grant
ACI-1548562; Blue Waters, supported by the NSF; Pleiades, via the NASA HEC
program through the NAS Division at Ames Research Center.

## Data Availability

The data supporting the plots within this article are available on reasonable
request to the corresponding author. A public version of the GIZMO code is
available at http://www.tapir.caltech.edu/~phopkins/Site/GIZMO.html.
Additional data including simulation snapshots, initial conditions, and
derived data products are available at https://fire.northwestern.edu/data/.

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