# Jellyfish galaxies with the IllustrisTNG simulations – When, where, and for
how long does ram pressure stripping of cold gas occur?

Eric Rohr1, Annalisa Pillepich1, Dylan Nelson2, Elad Zinger3,1, Gandhali
Joshi4, and Mohommadreza Ayromlou2

1Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg,
Germany
2Zentrum für Astronomie der Universität Heidelberg, ITA, Albert Ueberle Str.
2, D-69120 Heidelberg, Germany
3Centre for Astrophysics and Planetary Science, Racah Institute of Physics,
The Hebrew University, Jerusalem 91904, Israel
4University College London, Gower St, London WC1E 6BT, UK
Contact e-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

Jellyfish galaxies are the prototypical examples of satellite galaxies
undergoing strong ram pressure stripping (RPS). We analyze the evolution of
535 unique, first-infalling jellyfish galaxies from the TNG50 cosmological
hydrodynamical galaxy simulation. These have been visually inspected to be
undergoing RPS sometime in the past 5 billion years (since $z=0.5$), have
satellite stellar masses $M_{\star}^{\rm sat}\sim 10^{8-10.5}\,{\rm
M}_{\odot}$ and live in hosts with $M_{\rm 200c}\sim 10^{12-14.3}\,{\rm
M}_{\odot}$ at $z=0$. We quantify the cold gas $(\leq 10^{4.5}$ K) removal
using the Lagrangian Monte Carlo tracer particles, confirming that for these
jellyfish, RPS is the dominant driver of cold gas loss after infall (Friends-
of-Friends halo membership). Half of these jellyfish are completely devoid of
cold gas by $z=0$, and these galaxies have earlier infall times and smaller
satellite-to-host mass ratios than those that still have some gas today. RPS
can act on jellyfish galaxies over long time scales of $\approx 1.5-8$ Gyr.
Jellyfish in more massive hosts are impacted by RPS for a shorter time span
and, at a fixed host halo mass, jellyfish with less cold gas at infall, lower
stellar masses at $z=0$, and fewer orbits by the time of total stripping have
shorter RPS time spans. While RPS may act for long periods of time, the peak
RPS period – where at least 50 per cent of the total RPS occurs – begins
within $\approx 1$ Gyr of infall and lasts $\lesssim 2$ Gyr. During this
period, the jellyfish are at host-centric distances between $\sim 0.2-2R_{\rm
200c}$, illustrating that much of RPS occurs at large distances from the host
galaxy. Interestingly, jellyfish continue forming stars until they have lost
$\approx 98$ per cent of their cold gas. For groups and clusters in TNG50
$(M_{\rm 200c}^{\rm host}\sim 10^{13-14.3}\,{\rm M}_{\odot})$, jellyfish
galaxies deposit more cold gas ($\sim 10^{11-12}\,{\rm M}_{\odot}$) into halos
than exist in them at $z=0$, demonstrating that jellyfish, and in general
satellite galaxies, are a significant source of cold gas accretion.

###### keywords:

galaxies: formation – galaxies: evolution – galaxies: haloes – methods:
numerical

††pubyear: 2023††pagerange: Jellyfish galaxies with the IllustrisTNG
simulations – When, where, and for how long does ram pressure stripping of
cold gas occur?–15

##  1 Introduction

At a fixed galaxy stellar mass, observations show that there are a number of
differences between field and satellite galaxies (satellites for short).
Namely with the Sloan Digital Sky Survey (SDSS), it has been shown that the
population of satellites has a higher quenched fraction, lower (specific)
star-formation rates (SFR, or sSFR), and redder colors compared to central
galaxies of the same stellar mass (Peng et al., 2010; Peng et al., 2012;
Wetzel et al., 2012). Moreover, satellite galaxies exhibit on average lower
neutral HI gas fractions, elevated gas metallicites, reduced circumgalactic
X-ray emission, and suppressed active galactic nucleus (AGN) activity compared
to their mass-matched analogs in the field (Giovanelli et al., 1985; Brown et
al., 2016; Maier et al., 2019a, b).

These observational trends suggest that, in addition to the secular processes
of galaxy evolution, satellite galaxies undergo additional environmental
phenomena. It is generally accepted that ram-pressure stripping (RPS) is one
of the most impactful among such environmental phenomena (Gunn et al., 1972,
see Boselli et al., 2022 for a recent review).

Ram pressure is proportional to $\rho v^{2}$, where $\rho$ is the density of
the surrounding ambient medium, and $v$ is the relative velocity of the
infalling galaxy (or a given parcel of gas) and the ambient medium. This
effect is expected to increase with host mass ($M_{\rm 200c}$) because
satellites in more massive hosts tend to fall in with higher velocities and
more massive hosts tend to have denser circumgalactic media (CGM)111Throughout
this paper, we define the CGM to be the entire multiphase gaseous medium
around central galaxies regardless of their stellar or total host mass, unless
explicitly referred to as intragroup medium (IGrM) for galaxy groups ($M_{\rm
200c}\sim 10^{13-14}\,{\rm M}_{\odot}$) or intracluster medium (ICM) for
clusters ($M_{\rm 200c}\sim 10^{14-14.3}\,{\rm M}_{\odot}$)., also depending
on the stellar and AGN feedback of the central galaxy. Moreover for a given
host, this pressure should increase with decreasing distance both because the
surrounding medium is denser at smaller radii, and galaxies move faster when
they are deeper into their hosts’ potential wells. These expected results are
broadly consistent with observations (e.g., Maier et al., 2019b; Roberts et
al., 2019) With respect to removing single parcels of gas from the infalling
satellite, RPS acts against the satellite’s gravitational restoring force,
dominated by the stellar body. Consequently the effectiveness of RPS is
expected to increase with decreasing satellite stellar mass.

For a given satellite galaxy, ram pressure first strips the hot or less
gravitationally-bound gas, a feature that has been inferred observationally
(Balogh & Morris, 2000) and assumed in semi-analytic models (Cole et al.,
2000; Somerville et al., 2008; Lagos et al., 2018; Ayromlou et al., 2019).
With respect to the satellite’s intersteller medium (ISM), RPS is thought to
work outside-in, as observationally inferred via truncated disks (Warmels et
al., 1988; Cayatte et al., 1990, 1994; Vollmer et al., 2001; Lee et al.,
2022a) and leading to outside-in quenching (Schaefer et al., 2017, 2019; Bluck
et al., 2020; Vulcani et al., 2020; Wang et al., 2023, contra: Wang, 2022).

Ram pressure is also thought to compress the satellite’s gas, especially on
the galaxy’s leading edge. This is inferred to cause temporary periods of
enhanced star formation (Gavazzi et al., 2001; Vulcani et al., 2018; Roberts &
Parker, 2020; Roberts et al., 2022) and AGN activity (Poggianti et al., 2017a;
Maier et al., 2022; Peluso et al., 2022, contra: Roman-Oliveira et al., 2019).
In turn, the feedback from star-formation and AGN may lower the binding energy
of the ISM gas, potentially facilitating RPS (Garling et al., 2022). Thus the
physical mechanism responsible for the loss of satellite ISM gas is likely a
combination of RPS and stellar/AGN-driven outflows. However, despite these
temporary periods of enhanced star-formation and AGN activity, RPS ultimately
leads to the removal of ISM gas and and to the quenching en masse of
satellites (e.g., Wetzel et al., 2013; Maier et al., 2019a; Boselli et al.,
2022, see Cortese et al., 2021 for a recent review). We note, however, that
the timescales related to environmental quenching are highly debated, ranging
from short $\lesssim 500$ Myr to long $\gtrsim 4$ Gyr times, typically but not
always measured from the first $R_{\rm 200c}$ crossing (Cortese et al., 2021,
and references therein).

Conversely, satellite galaxies are not only affected by their environment, but
they have the potential to perturb the ambient medium in a number of ways.
First, the bulk motion of the satellites is thought to affect the CGM
kinematics by inducing turbulence and by bringing in gravitational energy,
which heats the CGM via dynamical friction and shocks (e.g., Dekel & Birnboim,
2008). As the infalling galaxies may travel faster than the ambient medium’s
sound speed, some satellites are also expected to create bow shocks in CGM
(Yun et al., 2019). This shock and the induced turbulence may act as
perturbations, triggering the warm/hot $T\sim 10^{6-8}$ K CGM to cool into
$T\sim 10^{4-5}$ K clouds. Moreover, the gas that has been ram-pressure
stripped, namely the satellite’s cold ISM, is expected to be deposited into
the host’s halo. For groups and clusters with many satellite galaxies, there
could be a substantial amount of accreted halo gas originating from the
stripped satellites. However, this has never been quantified. Finally, while
currently still highly debated, such cold gas clouds in the CGM, regardless of
their origin, could be long-lived (e.g., Li et al., 2020; Sparre et al., 2020;
Gronke et al., 2022; Fielding & Bryan, 2022), and satellite-induced cold gas
clouds may be a source of cold gas found in the CGM today (Nelson et al.,
2020; Rodríguez et al., 2022).

Observed satellites that have been visually identified to be undergoing RPS
have been called jellyfish galaxies (from now on, jellyfish for short), where
their stellar bodies (the jellyfish heads) remain relatively unperturbed but
their gaseous disks are being stripped in the direction opposite of motion,
forming the jellyfish tails (e.g., Bekki, 2009; Ebeling et al., 2014;
McPartland et al., 2016). These jellyfish and their stripped tails are multi-
wavelength objects and have been observed in the X-ray, UV, optical, and radio
(e.g., Gavazzi & Jaffe, 1987; Gavazzi et al., 2001; Kenney et al., 2004; Sun
et al., 2006; Cortese et al., 2006; Smith et al., 2010; Jáchym et al., 2017;
Poggianti et al., 2019; Ignesti et al., 2022). However, many of these studies
have focused on single or a few objects. Observers have recently pushed for
systematic surveys of jellyfish galaxies, where the largest uniform samples
come from the GAs Stripping Phenomena in galaxies with MUSE (GASP; Poggianti
et al., 2017b, 51 galaxies), the LOw-Frequency ARray (LOFAR; Shimwell et al.,
2017; Roberts et al., 2021, 60), and the OSIRIS Mapping of Emission-line
Galaxies (OMEGA; Chies-Santos et al., 2015; Roman-Oliveira et al., 2019, 70).
The largest statistical studies of jellyfish galaxies come from Smith et al.
(2022), who use 106 jellyfish with radio continuum emission, and from Peluso
et al. (2022), who use 131 jellyfish with information on the central ionizing
mechanism.

Despite these recent efforts, unanswered questions still remain, such as: when
with respect to infall and where with respect to the host does RPS begin; for
how long does RPS act; did the quenched, low gas-fraction galaxies we see
today go through a jellyfish phase; what determines how long RPS will take to
totally remove a jellyfish’s gas; how does the RPS of jellyfish galaxies
compare to other satellites; where is the stripped gas being deposited, and
more generally, how much cold gas do satellites bring into their hosts’ halos?

The answers to these questions can provide both insights into environmental
quenching of satellites as well as important implications for the evolution of
massive hosts and their surrounding halo gas in the context of the cosmic
baryon cycle. While we have reached a general consensus that RPS is necessary
to remove satellite cold gas and reproduce the aforementioned environmental
trends, the timescales and locations of RPS and the associated satellite
quenching remain highly debated. Thus, we turn to numerical simulations with
temporal evolution to investigate the satellite-host interaction. Idealized
simulations have been able to reproduce jellyfish by imposing an external
wind, mimicking the RPS felt during infall through the CGM (e.g., Tonnesen &
Bryan, 2009; Lee et al., 2020; Choi et al., 2022). With the perspective of
satellite quenching, zoom-in and full cosmological hydrodynamical galaxy
simulations have studied more or less explicitly the RPS of satellites,
finding a wide range of quenching timescales that broadly agree with
observational inference (e.g., Bahé & McCarthy, 2015; Jung et al., 2018;
Wright et al., 2019; Yun et al., 2019; Oman et al., 2021; Rodríguez et al.,
2022; Samuel et al., 2022; Pallero et al., 2022; Wright et al., 2022).
However, quantitative and statistically-robust simulation predictions as to
the timings and modalities of RPS are still missing. And so, to understand
satellite quenching, we must first quantify the effects of perhaps its most
relevant process: RPS.

In this work, we use the high-resolution, $\sim 50$ Mpc magneto-hydrodynamical
simulation TNG50 from the IllustrisTNG project (TNG thereafter) to study the
satellite-host interaction in a realistic, cosmological context. In
particular, we aim at quantifying when, where, and for how long the RPS of
cold gas occurs. We focus on cold gas as this is the source of star formation
in galaxies and because its existence within the otherwise hot CGM of massive
halos is a compelling open question. Moreover, we focus on jellyfish galaxies
because these are satellites that, by identification and hence by
construction, are surely undergoing RPS. Among its advantages, the TNG50
simulation produces thousands of galaxies and hosts ranging over 5 orders of
magnitude in mass, and it naturally includes many environmental processes such
as pre-processing, tidal stripping, harassment, strangulation, starvation, and
RPS. The TNG simulations do not include possibly-relevant environmental
processes such as viscous momentum transfer or thermal evaporation, and there
is no explicit modelling of the multiphase ISM (Cowie et al., 1977; Nulsen et
al., 1982, see Zinger et al., 2018 and Kukstas et al., 2022 for discussions).
However, the TNG model has been shown to return satellite populations whose
quenched fractions and gas content are broadly consistent with observations
(e.g. Stevens et al., 2019; Donnari et al., 2021; Stevens et al., 2021).

In a companion paper, Zinger et al. (in prep.) visually inspect TNG satellites
to identify jellyfish galaxies using the citizen science Cosmological
Jellyfish project hosted on Zooniverse, yielding an unprecedented number of
more than 500 unique, first-infalling jellyfish galaxies in the TNG50 volume
alone. In another companion paper, Göller et al. (in prep.) study the star-
formation activity of these jellyfish both temporally and across populations.
In this paper, we employ the Monte Carlo Lagrangian tracer particles to follow
the flows of gas in and out of satellite galaxies, quantifying the cold gas
sources and sinks across cosmic time from when the galaxies were centrals,
through their jellyfish phases, and in some cases until they have been
completely stripped of all gas, existing as quenching, gas-poor satellites at
$z=0$.

We begin by introducing the methods (§ 2), namely by summarizing the TNG50
simulation (§ 2.1), the Cosmological Jellyfish project (§ 2.2), the tracking
of galaxies across cosmic time (§ 2.3), how we employ the tracer particles (§
2.5.1), and how we identify the onset and end of RPS (§ 2.5.2). We then
present our main results (§ 3). We start by comparing the jellyfish galaxy
population with that of the inspected and general $z=0$ satellite populations
(§ 3.1), and then comment on the origin of the jellyfish gaseous tails (§
3.2). After quantifying the strength of RPS post infall (§ 3.3) and
determining a subsample of jellyfish that are devoid of cold gas at $z=0$ (§
3.4), we answer when, where, and for how long RPS occurs (§ 3.5, 3.6). We then
discuss how we can generalize our jellyfish results with all $z=0$ satellites
(§ 4.1), connect the the cold gas loss via RPS with satellite quenching times
(§ 4.2, and illustrate how much and where cold gas is deposited via RPS into
halos (§ 4.3). We end by summarizing the main results and restating the
conclusions (§ 5).

Unless otherwise noted, all analysis including the TNG simulations adopt a
$\Lambda$CDM cosmology consistent with the Planck Collaboration et al. (2016)
results: $\Omega_{\Lambda,0}=0.6911,\Omega_{m,0}=\Omega_{\rm
bar,0}+\Omega_{\rm dm,0}=0.3089,\Omega_{\rm
bar,0}=0.0486,\sigma_{8}=0.8159,n_{s}=0.9667,{\rm and}\ h=H_{\rm 0}/(100\,{\rm
km\,s^{-1}\,Mpc^{-1}})=0.6774$, where $H_{0}$ is the Hubble parameter, and the
subscript “0" denotes that the quantity is measured today.

##  2 Methods and TNG50 jellyfish galaxies

###  2.1 The TNG50 simulation

The IllustrisTNG project222https://www.tng-project.org/ (Pillepich et al.,
2018b; Nelson et al., 2018; Naiman et al., 2018; Marinacci et al., 2018;
Springel et al., 2018) consists of a series of cosmological volume
$\Lambda$CDM simulations, including gravity + magneto-hydrodynamics (MHD) and
a galaxy formation model (see method papers for details: Weinberger et al.,
2017; Pillepich et al., 2018a). Here we briefly summarize the TNG simulations.

The TNG production simulations come in three volumes of side lengths $\sim
50,100,{\rm and}\ 300$ comoving Mpc, hereafter referred to as TNG50, TNG100,
and TNG300 respectively. The TNG galaxy formation model was designed at the
resolution of TNG100, which includes $2\times 1820^{3}$ resolution elements
with baryon mass resolution of $m_{\rm bar}=1.4\times 10^{6}\,{\rm
M}_{\odot}$. The large volume TNG300 has $2\times 2500^{3}$ resolution
elements with mass resolution $m_{\rm bar}=1.1\times 10^{7}\,{\rm M}_{\odot}$.
The high resolution TNG50 simulation has $2\times 2160^{3}$ resolution
elements with mass resolution $m_{\rm bar}=8.5\times 10^{4}\,{\rm M}_{\odot}$
(Nelson et al., 2019b; Pillepich et al., 2019). The minimum gas resolution in
TNG50 at $z=0$, i.e. the smallest non-vanishing gas mass in any given galaxy,
is $\approx 4\times 10^{4}\,{\rm M}_{\odot}$. These three simulations are
publicly available in their entirety (Nelson et al., 2019a). In this paper, we
work exclusively with the highest-resolution run TNG50.

The TNG simulations evolve gas, cold dark matter, stars, and super massive
black holes (SMBHs) within an expanding universe, based on a self-gravity +
MHD framework (Pakmor et al., 2011; Pakmor & Springel, 2013) using the Arepo
code (Springel, 2010). The fluid dynamics employ a Voronoi tessellation to
spatially discretize the gas. The TNG gas has a temperature floor at $10^{4}$
K, and the relationship between temperature and density for star-forming gas
is determined via an effective equation of state from Springel & Hernquist
(2003). For this analysis, we manually set the temperature of star-forming gas
to $10^{3}$ K. The TNG galaxy evolution models includes the following
processes: gas heating and cooling; star formation; stellar population
evolution + chemical enrichment from AGB stars and type Ia + II supernovae;
supernova driven outflows and winds (Pillepich et al., 2018a); formation,
merging, and growth of SMBHs; and two main SMBH hole feedback modes: a thermal
‘quasar’ mode, and a kinetic ‘wind’ mode (Weinberger et al., 2017). The TNG
simulations have reproduced many observational relations and properties across
orders of magnitude in mass and spatial scales.

The group and galaxy catalogs consist of the dark matter halos and the dark
matter plus baryonic galaxies. The dark matter halos are defined using the
Friends-of-Friends (FoF) algorithm with a linking length $b=0.2$, run only
using the dark matter particles (Davis et al., 1985). Then the baryonic
components are connected to the same halos as their closest dark matter
particle. Throughout this paper, we use “FoF", “group", “FoF group", “halo"
synonymously. The galaxies are identified using the subfind algorithm, which
connect together all gravitationally bound particles (Springel et al., 2001;
Dolag et al., 2009). We use the terms “subhalo" and “galaxy" synonymously even
though, in general, subfind objects may contain no stars and/or gas
whatsoever. Typically albeit not always, the most massive subhalo within a
halo is the “main" or “primary subhalo", also called the “central galaxy"; all
other subhalos within a halo are “satellites". In all cases, we only consider
subhalos of a cosmological origin as defined by the SubhaloFlag in Nelson et
al. (2019a).

###  2.2 The Cosmological Jellyfish project on Zooniverse

In this paper, we study jellyfish galaxies and identify them based on the
classification of the Zooniverse Cosmological Jellyfish
project333https://www.zooniverse.org/projects/apillepich/cosmological-
jellyfish.

The Zooniverse Cosmological Jellyfish project presented images of TNG50 and
TNG100 satellite galaxies on the Zooniverse platform for classification by
citizen scientists. Here several thousand volunteers underwent a training
session and classified whether the given galaxy resembles a jellyfish or not
(Zinger et al., in prep.).

Following the pilot project that visually classified a subset of TNG100
satellites (Yun et al., 2019), the term “jellyfish galaxy" was associated with
a satellite with a visually identifiable signature of RPS in the form of
asymmetric gas distributions in one direction. The visual inspection is based
on images of gas column density – i.e. all gas irrespective of phase,
temperature, etc. – with stellar mass contours, projected in random
orientations in a field of view of 40 times the 3D stellar half mass radius
$R_{\rm half,\star}$. Each image was classified by at least 20 inspectors
(trained volunteers) whose proficiency was measured when tallying the votes. A
galaxy image received a score between 0 and 1 based on these votes, whereby we
employ a threshold of 0.8 and above to identify jellyfish galaxies, as
recommended by Zinger et al. (in prep.).

Galaxies meeting the following criteria had their images posted for inspection
for the Zooniverse project:

  * •

non central, i.e. satellite;

  * •

of cosmological origin, as defined by the SubhaloFlag in Nelson et al.
(2019a);

  * •

$M_{\star}^{\rm sat}\equiv M_{\star}^{\rm sat}(<2\times R_{\rm
half,\star})>10^{8.3}$ or $10^{9.5}\,{\rm M}_{\odot}$ for TNG50 and TNG100,
respectively;

  * •

$f_{\rm gas}\equiv M_{\rm gas}^{\rm sat}/M_{\star}^{\rm sat}>0.01$, where
$M_{\rm gas}^{\rm sat}$ is the satellite’s total (i.e. gravitationally-bound)
gas mass.

For both TNG50 and TNG100, all galaxies satisfying the above criteria were
inspected at the following redshifts:
$z=0,0.1,0.2,0.3,0.4,0.5,0.7,1.0,1.5,2.0$, (every $\sim 1$ Gyr in cosmic
time), corresponding to TNG snapshots 99, 91, 84, 78, 72, 67, 59, 50, 40, 33.
Additionally, TNG50 galaxies were inspected at each available snapshot since
$z=0.5$ (every $\sim 150$ Myr in cosmic time), snapshots 67-99.

According to the results of the Zooniverse Cosmological Jellyfish project, for
TNG50, 4,144 of the total 53,610 (7.7 per cent) galaxy images are jellyfish;
for TNG100, 1,163 of the 27,094 (4.3 per cent) images are jellyfish. It is
possible that the jellyfish tails are more identifiable in higher resolution
simulations, potentially leading to the slightly higher percentage of
jellyfish in TNG50 compared to TNG100. In total, 80,704 galaxy images were
inspected, of which 5,307 (6.6 per cent) are jellyfish galaxies that we can
use for scientific analysis across TNG50 and TNG100.

###  2.3 Tracking galaxies along the merger trees

Based on the selection for the Zooniverse Cosmological Jellyfish project,
frequently an individual galaxy was inspected multiple times at different
points in time along its evolutionary track.

In this paper, we connect the galaxies that were inspected at multiple times
using sublink_gal (Rodriguez-Gomez et al., 2015). Briefly, sublink_gal
constructs the merger trees at the subhalo level by searching for descendant
candidates with common stellar particles and star-forming gas cells. Then
sublink_gal chooses the descendant by ranking all candidates with a merit
function that takes into account the binding energy of each particle/cell, and
choosing the candidate with the highest score as the descendant.

In this paper, we chiefly work with and follow the unique evolutionary tracks
of galaxies, branches, inspected in the Cosmological Jellyfish project. In
total, there are 5,023 (9,052) unique galaxy branches in TNG50 (TNG100) among
the inspected images. The analysis of these satellite galaxy populations along
their evolutionary tracks requires following the merger tree branches both of
the individual galaxies and their (sometimes temporary) hosts. We give results
on this in §3.1 and more details in Appendix A.

###  2.4 Galaxy sample selection of this analysis

In this paper, we focus exclusively on TNG50 galaxies. As we are interested in
ram pressure stripping quantitatively, using TNG50 ensures that we work with
the highest-resolution simulation available of the IllustrisTNG project.
Moreover, the tracer particle data and the jellyfish inspections are available
at every snapshot (compared to only at the full snapshots or a handful of
snapshots, respectively, in TNG100; more details in § 2.5.1).

With respect to the Zooniverse Cosmological Jellyfish project, we apply
additional selection criteria to be able to start from a sample of satellites
defined at $z=0$ that does not include backsplash and pre-processed galaxies.
Please see Appendix A for details regarding how we classify the galaxies as
backsplash and/or pre-processed.

Of the 5,023 inspected galaxy branches in TNG50, we apply the following sample
selection criteria. At each criterion, we list the number of remaining
branches in the simulation, and the number excised by this criterion in
parentheses444The number excised is the number from the previous criterion.
For example, criterion (ii) excises $x$ branches from the $y$ branches
remaining after applying criterion (i). This now leaves $y-x$ branches after
applying criterion (ii)..

  1. 1.

The galaxy must survive until the end of the simulation at $z=0$. That is, the
main descendant branch must track the subhalo until snapshot 99. TNG50: 3,018
(2,005 excised).

  2. 2.

There must be at least one snapshot since $z\leq 0.5$ when the galaxy was
inspected in the Zooniverse project (and therefore meeting the criteria
outlined in §2.2). TNG50: 2,398 (620).

  3. 3.

The galaxy must be a satellite galaxy at $z=0$, i.e., not a backsplash galaxy
at snapshot 99. TNG50: 2,062 (336).

  4. 4.

The galaxy must not have been pre-processed by a host group other than its
$z=0$ host. TNG50: 1,721 (341).

Thus our total number of cleaned, first-infalling inspected branches is 1,721
for TNG50. For TNG100, this number would be 3,307, due to the larger volume
and higher-mass hosts, but we focus hereafter only on TNG50 galaxies. Of these
branches, we separate them into those that have at least one jellyfish
classification since $z=0.5$, called “jellyfish" branches, and those without a
jellyfish classification since then, called “non-jellyfish" branches. The
numbers of jellyfish and of non-jellyfish branches in TNG50 are 535 and 1,186,
respectively555In TNG50, there are 12 cleaned, inspected branches that have a
jellyfish classification before $z=0.5$ but not afterwards. We exclude these
galaxies from the jellyfish sample. (see §3.1 for additional results).

###  2.5 On cold gas, infall time, tracer particles, and measuring ram
pressure stripping

In this work, we study the gravitationally-bound cold gas of TNG50 satellite
galaxies: by cold gas, throughout this paper, we mean gas with a temperature
$T_{\rm ColdGas}\leq 10^{4.5}$ K (including star-forming gas; see § 2.1 for
more details).

Throughout this paper, we define infall as the first time in cosmic history
that a galaxy becomes a satellite member of its $z=0$ FoF host, irrespective
of distance.

####  2.5.1 Following the gas with tracer particles

As TNG50 is based on a moving-mesh code to follow the evolution of the
underlying fluid field, we must employ the Monte-Carlo-Lagrangian tracer
particles to follow the history and evolution of individual gas parcels (Genel
et al., 2013; Nelson et al., 2013). Briefly, arepo treats the gas as a fluid
field through a Voronoi mesh. There is no innate method to follow the flow of
matter between the mesh elements and across time. Thus the tracers are
introduced, acting as test particles within the fluid. TNG50 was run with one
tracer per gas cell at the initial conditions. The tracers have a constant
identifying number (ID) throughout the simulation, and at each snapshot each
tracer has exactly one baryonic parent resolution element: a gas cell, a
stellar or wind particle, or a SMBH. This means that any given tracer
represents $8.5\times 10^{4}\,{\rm M}_{\odot}$ of baryonic mass with the
properties of its parent. For example, if a single tracer has a gas parent at
one time and a star parent at the next time, then the tracer represents
$8.5\times 10^{4}\,{\rm M}_{\odot}$ of gas mass being converted into stars. In
this way, one can track the flow of matter by following a given tracer and its
parent’s properties across cosmic time. In TNG50 the parents of the tracers
are output at each snapshot, describing the exchange of parcels of baryonic
material across resolution elements at time intervals of $\sim 150$ Myr. As
the tracer particles are Monte Carlo in nature, we make only statistical
statements about the behavior of thousands to millions of tracers.

In practice, at each snapshot and for each galaxy of interest, we find all
tracers whose parents are bound, cold gas cells. While not every gas cell
necessarily has an associated tracer and some gas cells may have multiple
child tracers, the total tracer cold gas mass (total number of tracers times
$m_{\rm bar}=8.5\times 10^{4}\,{\rm M}_{\odot}$) agrees with the total amount
of cold gas mass measured by gas cells (see Fig. 5 for an example). Then we
follow the tracers and their parents across snapshots in order to measure the
cold gas mass that is stripped or launched in an outflow, becomes hot,
participates in star formation, transforms into a wind particle, and gets
accreted into a SMBH.

We proceed as follows, on a galaxy by galaxy basis along its main descendant
branch (MDB). Starting from the first snapshot that the galaxy is identified
in the merger trees, we find all tracers whose parents are bound, cold gas
cells of this galaxy. Then at the next snapshot for the galaxy along its MDB,
we find which tracers belong to one of the following mutually exclusive and
completely exhaustive groups:

  1. 1.

are recorded in both snapshots: bound, cold gas that remains bound, cold;

  2. 2.

are recorded in the current snapshot but not in the previous one: currently
bound, cold gas that previously was either not bound or not cold;

  3. 3.

are recorded in the previous snapshot but not the current one: previously
bound, cold gas that no longer is;

Potential physical origins of tracers in group (ii) include inflows, cooling,
stellar mass return, or wind re-coupling. The group (iii) tracers could either
a) go from cold gas cells into one of the following: star particles (star
formation denoted SF, or SFR for star formation rate); SMBH sink particles
(i.e. SMBH accretion); bound, warm/hot gas cells (heating); or b) be no longer
bound gas cells (stripping + outflows). We denote the latter “RPS+outflows”
and will be focusing on this quantity throughout the paper. We include tracers
whose parents become unbound and hot in the same time step in this category.
We note that tidal stripping may be included in RPS+outflows, although visual
inspection shows that ram-pressure stripping is the dominant mechanism of
jellyfish galaxies, and a majority of galaxies do not reach host-centric
distances $\lesssim 0.2R_{\rm 200c}$. Moreover, the Zooniverse inspectors were
specifically asked not to classify an image as a jellyfish if there was a
close companion or gaseous tails were visible on both sides of the galaxy
(Zinger et al. in prep.).

####  2.5.2 Identifying the onset and end of ram pressure stripping

Throughout our analysis, prior to infall (host FoF membership; see above), we
assume that the RPS + outflows category is dominated by outflows, namely
outflows driven from stellar- and/or SMBH-feedback. As we further justify in §
3.3, for most jellyfish the amount of outflows before infall is approximately
constant (see Fig. 5 for an individual example). Immediately after infall,
there is commonly an increase in the RPS + Outflows category, indicating that
another physical process has become present, namely RPS. Moreover between
infall and pericenter, many satellites experience bursts of star formation
and/or AGN accretion, which has also been seen in observations, reproduced by
simulations, and thought to be caused by ram pressure compressing the ISM gas
(Gavazzi & Jaffe, 1987; Bahé & McCarthy, 2015; Mistani et al., 2016; Zoldan et
al., 2016; Vulcani et al., 2018; Roberts & Parker, 2020; Peluso et al., 2022;
Garling et al., 2022, Göller et al.,in prep.). These bursts of star formation
and/or AGN accretion would in turn induce turbulence in the ISM and drive
outflows, which then facilitate ram-pressure stripping (e.g., Bahé & McCarthy,
2015). Attempting to distinguish the relative contributions from outflows and
ram pressure becomes a chicken-and-egg problem. Thus, we consider the time of
infall to be the onset of RPS, and after infall relabel the quantity
“RPS+Outflows" as “RPS". We note that we have estimated the onset of RPS using
two alternative methods, and find that for most jellyfish the difference
between the various methods is $\lesssim 450$ Myr ($\lesssim 3$ snapshots):
see Appendix B and Fig. 14 for more details.

The end of ram-pressure stripping is either when the galaxy’s cold gas mass
falls below our resolution limit (namely below $\approx 4\times 10^{4}\,{\rm
M}_{\odot}$ i.e. $f_{\rm gas}\lesssim 5\times 10^{-4}$ for a galaxy at our
minimum stellar mass of $M_{\star}^{\rm sat}=10^{8.3}\,{\rm M}_{\odot}$), or
the end of simulation at $z=0$. In our sample, 261/535 ($\approx 50$ per cent)
galaxies lose all their cold gas at or before $z=0$.

We denote the onset of RPS as the infall time $\tau_{0}$ and its end as
$\tau_{100}$ (when 100 per cent of the RPS has occurred), so that the
difference between these two times returns in principle the maximum time span
over which RPS has acted on any given galaxy:

$\tau_{\rm RPS}=\tau_{100}-\tau_{0},$ (1)

where $\tau_{100},\tau_{0}$ are the ages of the universe at the given points.
This RPS time span is the longest duration over which RPS has acted for the
galaxies that have lost their cold gas prior to $z=0$. On the other hand, for
those satellites that still have some gas today, the above-defined timescale
of RPS is likely a lower estimate, while we speculate that these galaxies
would continue being stripped in the future. See §3.4 for differences between
these two ending states.

Throughout the paper, we will compare the times of RPS with estimates of the
quenching time, i.e. of the most recent and last time that a galaxy has fallen
1 dex below the star-forming main sequence (SFMS) for its mass and redshift,
as per definitions of Pillepich et al. (2019) and catalogs from Joshi et al.
(2021); Donnari et al. (2021).

##  3 Results

###  3.1 TNG jellyfish galaxies across their unique branches

Figure 1:  Selection of TNG50 galaxies studied in this work and the abundance
of jellyfish, along their unique branches. The Inspected sample (dark gray)
includes a subset of all satellite branches from TNG50, selected for the
identification of galaxies with clear signatures of RPS: this chiefly excludes
satellites with $M_{\star}^{\rm sat}<10^{8.3}\,{\rm M}_{\odot}$ and less than
1 per cent of gas mass fraction at the time of inspection, as well as pre-
processed and backsplash galaxies. The Jellyfish sample (green) also requires
at least one jellyfish-classified snapshot at $z\leq 0.5$. See §2.2 and 2.4
for more details. For comparison, we show all TNG50 $z=0$ satellites with
$M_{\star}^{\rm sat}>10^{8.3}\,{\rm M}_{\odot}$ (light gray). For the gas
properties in the bottom row, the galaxies with gas masses below our
resolution limit are placed manually at $\sim 10^{3}\,{\rm M}_{\odot}$. By
cold gas with mean temperatures $\leq 10^{4.5}$ K; hot gas has temperatures
$>10^{4.5}$ K.

According to the Cosmological Jellyfish project on Zooniverse, 4,144 of the
53,610 images from TNG50 are jellyfish galaxies (7.7 per cent, Zinger et al.
in prep.). Using the merger trees to identify when the same galaxies were
imaged at multiple points in cosmic time in TNG50 and applying our selection
criteria (§2.4), we now focus on our sample of 535 first-infalling unique
jellyfish galaxies, among 1,721 unique, inspected branches (31 per cent).

Fig. 1 shows our selection of jellyfish (green histograms) and inspected (dark
gray histograms) satellites at $z=0$. We now quote numbers in terms of unique
branches such that Fig. 1 is the branch- or merger tree-based counterpart of
similar histograms in Zinger+2023 (in prep., see their fig. 2). We include for
comparison the general population of $z=0$ satellites with $M_{\star}^{\rm
sat}(z=0)\geq 10^{8.3}\,{\rm M}_{\odot}$, which is generally similar to the
Zooniverse inspected sample, except that the general $z=0$ satellite
population includes pre-processed satellites. See § 4.1 for a more detailed
discussion on how representative the jellyfish sample is compared to all $z=0$
satellites above stellar mass.

Firstly, Fig. 1 shows, thanks to TNG50, that we can study satellite galaxies,
and hence jellyfish and RPS, in a rather extended range of stellar masses and
host masses. Namely we study satellites with stellar masses $\sim
10^{8-12}\,{\rm M}_{\odot}$ orbiting in hosts with total masses $\sim
10^{10.5-14.3}\,{\rm M}_{\odot}$ at $z=0$. However we cannot make statements
about satellites in the most massive clusters $M_{\rm 200c}\sim 10^{15}\,{\rm
M}_{\odot}$.

Figure 2:  The coldness of the ram-pressure stripped gas in TNG50 jellyfish
galaxies. We show gas temperature maps of 16 TNG50 jellyfish galaxies,
randomly chosen at $z=0$. Each image is $(40\times R_{\rm half,\star})^{3}$ in
size and depth, with $100\times 100$ pixels ($\sim$ kpc sized pixels) in the
same orientation as the jellyfish were posted to Zooniverse (i.e., random and
along the $z$-axis). Here, we measure the mass-weighted-average temperature
map of all (FoF i.e. ambient) gas within the cube, and overplot the jellyfish
(i.e. gravitationally-bound) gas. The white circle shows the galaxy stellar
radius ($R_{\rm gal}=2\times R_{\rm half,\star}$), and information about the
jellyfish galaxy and its host are in the top- and bottom- left corners. Star-
forming gas is placed at the nominal temperature of $10^{3}~{}$K, so all dark
blue locations represent active star-forming regions. The gas in the jellyfish
tails is typically and on average cold-cool $\sim 10^{4-5}$ K.  Figure 3:  The
metallicity of the ram-pressure stripped gas in TNG50 jellyfish galaxies.
Similar to Fig. 2 but here showing the mass-weighted gas metallicity rather
than the temperature. The tails of jellyfish are as enriched as the main body
of the satellites they stem from, but depending on the host, the tails may or
may not be more enriched than the ambient gas.

As shown in the top left panel, jellyfish galaxies (green) tend towards lower
stellar masses $M_{\star}^{\rm sat}(z=0)$ compared to the inspected galaxies
(dark gray), and especially to the non-jellyfish galaxies that have been
inspected (not shown, but would be dark gray minus green). Since the stellar
body is the primary foil to RPS, providing the gravitational binding energy
for the gas to remain in the galaxy, galaxies with a weaker restoring force
are naturally more susceptible to RPS, in line with other studies of TNG
jellyfish (Yun et al., 2019, Zinger et al., in prep.). Because we only inspect
galaxies with $M_{\star}^{\rm sat}>10^{8.3}\,{\rm M}_{\odot}$ at the time of
inspection, we see a decrease in the number of galaxies at lower masses. This
inspection criterion is only at the snapshot of inspection, so galaxies that
later lose stellar mass due to tidal stripping may have stellar masses below
this lower limit. The fact that only 2/535 (0.37 per cent) of jellyfish
branches compared to 24/1186 (2.0 per cent) of non-jellyfish branches have
stellar masses below the inspection criterion suggests that we are able to
separate galaxies undergoing tidal vs ram-pressure stripping. At the high-mass
end, $M_{\star}^{\rm sat}\gtrsim 10^{10.5}\,{\rm M}_{\odot}$ there are only a
few jellyfish galaxies. We speculate that this is a combination of two
effects: more massive satellites in hosts of this mass range better retain
their cold gas against stripping; at these stellar masses, the TNG kinetic
mode of SMBHs expels much of the galaxy’s gas (e.g. Terrazas et al., 2020;
Zinger et al., 2020), often at infall and before the peak effectiveness of
RPS.

In the top middle panel, we see that jellyfish typically live in more massive
hosts, and almost all inspected galaxies in massive hosts $M_{\rm 200c}^{\rm
host}\gtrsim 10^{13}\,{\rm M}_{\odot}$ have been classified at some point
since $z=0.5$ as jellyfish. The number of satellite galaxies increases with
the host halo mass due to hierarchical structure formation. With increasing
host mass the gravitational potential well deepens, which in turn leads to
both better retention of stellar- and SMBH-driven outflows from the central
and more cosmological gas accretion from the large scale structure. These
effects generally lead to a denser CGM or ICM. Moreover, deeper potential
wells increase the infall velocities of satellite galaxies, sometimes even to
supersonic speeds (Yun et al., 2019). The denser ambient medium and the
increased relative velocity both increase the strength of ram-pressure
stripping (e.g. Yun et al., 2019). However in the past five billion years, MW-
mass halos $M_{\rm 200c}^{\rm host}\sim 10^{12}\,{\rm M}_{\odot}$ have also
hosted a number of jellyfish galaxies.

In the top right panel Fig. 1, by combining the effects of satellite stellar
mass and host mass, we see that jellyfish galaxies typically have small mass
ratios $\mu\equiv M_{\star}^{\rm sat}/M_{\rm 200c}^{\rm host}$, and nearly
every inspected galaxy with a mass ratio $\mu\lesssim 10^{-4}$ is a jellyfish.

The satellite stellar mass distribution of the inspected galaxies (dark gray)
is slightly below but quite similar to that of the $z=0$ satellites (light
gray) for stellar masses $M_{\star}^{\rm sat}\sim 10^{8.3-10.5}\,{\rm
M}_{\odot}$ (top left), and the distributions are nearly identical for masses
$M_{\star}^{\rm sat}\sim 10^{10.5-11.8}\,{\rm M}_{\odot}$. Compared to the
$z=0$ satellites, the inspected galaxies have an under-population of high mass
hosts $M_{\rm 200c}^{\rm host}\sim 10^{13.5-14.3}\,{\rm M}_{\odot}$ (top
middle) and low mass ratios $\mu\lesssim 10^{-4}$ (top right). We speculate
that many of these $z=0$ satellites are pre-processed and therefore have been
excluded from this analysis, but they may also have had too low of gas masses
and their fractions to be inspected (bottom panels).

In the bottom panels of Fig. 1, we see that jellyfish galaxies typically
exhibit, at $z=0$ lower amounts of gravitationally-bound cold gas $M_{\rm
ColdGas}^{\rm sat}$ with temperatures $T_{\rm ColdGas}\leq 10^{4.5}$ K (or
star-forming; bottom left), hot gas $M_{\rm HotGas}^{\rm sat}$ with
temperatures $T_{\rm HotGas}>10^{4.5}$ K (bottom middle) and total gas $M_{\rm
TotGas}^{\rm sat}$ (bottom right) compared to the inspected branches. A
somewhat larger fraction of jellyfish ($\sim 50$ per cent) compared to non-
jellyfish ($\sim 45$ per cent) have gas masses below our resolution limit,
plotted here at $M_{\rm Gas}^{\rm sat}\sim 10^{3}\,{\rm M}_{\odot}$. We have
explicitly checked that the non-jellyfish inspected galaxies with large $z=0$
gas reservoirs are typically late-infallers and have higher mass ratios,
causing weaker ram-pressure stripping. Conversely, the non-jellyfish inspected
satellites without any gas at $z=0$ are typically early-infallers, namely they
joined their $z=0$ hosts when galaxies were inspected only every $\sim 1$ Gyr,
compared to every $\sim 150$ Myr after $z=0.5$. Additionally, there are a few
cases of massive galaxy mergers where the FoF-identified central galaxy
switches between the two galaxies; this means that these quasi-central
galaxies meet the inspection criteria but are not truly classical satellites.

###  3.2 Jellyfish tails stem from the stripped, cold ISM

Figure 4:  Time evolution of the total gravitationally-bound cold gas $M_{\rm
ColdGas}^{\rm subhalo}$ of a TNG50 jellyfish galaxy. The marker style denotes
the FoF membership of the galaxy, as either a central (triangle) or satellite
(circle), whereas the color indicates the distance to the $z=0$ host in
normalized units of [$R_{\rm 200c}(t)$]. The host-centric distance in physical
units (thin gray curve) uses the right y-axis, and we mark the pericentric and
apocentric passages with black “+" and “x" symbols respectively. The snapshots
when this galaxy has been visually inspected are outlined with a thick black
circle and the snapshot(s) when it has been classified as a jellyfish are
indicated with a central black dot. We place by hand the times after
$\tau_{100}$ when the cold gas mass $M_{\rm ColdGas}^{\rm subhalo}$ is below
our resolution limit ($\lesssim 4\times 10^{4}\,{\rm M}_{\odot}$) at the lower
y-limit (along the bottom x-axis). The thick black ticks denote the onset of
RPS as the infall time ($\tau_{0}$) and the end of RPS ($\tau_{100}$), in this
case when $M_{\rm ColdGas}^{\rm subhalo}(t)=0$. The red tick marks when the
galaxy quenches, defined as when the galaxy falls at least one dex below the
star-forming main sequence for the last time.

In this work, we study the ram-pressure stripping of cold gas because the
long-lived jellyfish tails originate mostly from the cold ISM of satellite
galaxies. We provide arguments for this as follows.

Firstly in Fig. 2 we show the gas temperature maps of 16 TNG50 jellyfish at
$z=0$. Each image is $(40\times R_{\rm half,\star})^{3}$ in size and depth,
with $100\times 100$ pixels ($\sim$ kpc sized pixels) in the same orientation
as the jellyfish were posted to Zooniverse (i.e., random and along the
$z$-axis). We measure the mass-weighted-average temperature map of all (FoF
i.e. ambient) gas within the cube, and overplot the jellyfish (i.e.
gravitationally-bound) gas. In each image, the jellyfish tails’ temperature
matches, or is at a similar temperature of, the ISM gas, which we roughly
denote as the gas enclosed by the white circles of radius $2\times R_{\rm
half,\star}$. In some cases, a bow shock is also present, which appears as a
stark contrast in temperature in the opposite direction of the tails (e.g.,
top left; see also other manifestations of bow shocks in front of TNG100
jellyfish galaxies in fig. 10 of Yun et al., 2019).

Fig. 3 showcases the metallicity maps for the same 16 TNG50 jellyfish
galaxies. Generally, the metallicity of the jellyfish tails is similar to that
of the main body of the galaxy (the jellyfish head). However, in some cases
(e.g., top right), the jellyfish tails have lower metallicities than the
stellar bodies, which is consistent with some observations and idealized
simulations suggesting that a portion of the stripped ISM mixes with the
surrounding medium (e.g., Franchetto et al., 2021; Choi et al., 2022; Lee et
al., 2022b). Unlike the temperature, the metallicity of the background halo
gas is not always so distinct from the jellyfish (e.g., bottom right), as it
depends on the satellite-to-host mass ratio.

Figure 5:  Time evolution of the tracer quantities for an example TNG50
jellyfish galaxy, the same galaxy as in Fig. 4. In the top panel, we see that
the total cold gas mass (thick black curve) and tracer mass with cold gas
parents (gray dashed curve) agree. The thin blue curve shows the the total new
cold gas, the tracer mass whose parents are now cold gas cells but previously
were not, such as cooling or inflowing gas. The thin red curve shows the
opposite, and we further separate the various physical mechanisms of cold gas
loss in the bottom panel, now normalized by the time between snapshots. The
contribution to net lost tracers from star formation (orange dotted) is small
at all times, while heating (pink dashed) is dominant only while the galaxy is
a central. The cold gas mass lost via SMBH accretion (black dot-dashed) is
shown here multiplied by 100 and negligible at all times. When the galaxy
becomes a satellite at infall (black tick mark, cosmic time 6.7 Gyr, $z\sim
0.8$), RPS (olive) becomes the dominant source of cold gas loss. Throughout
this paper and following the inspection of the evolutionary tracks of all
selected galaxies, we assume that before infall, RPS + Outflows is dominated
by outflows, and that after infall RPS is dominant.

These images exemplify that, at least at the time when a RPS tail is
identifiable in gas column density, the physical properties of the gas in the
tails are mostly similar to those in the ISM in the main body of the satellite
galaxy undergoing RPS. The tail gas is cold and is typically as metal enriched
as the jellyfish head.

Furthermore, we have checked that, at the time of infall, $\approx 75~{}(60)$
per cent of the gravitationally-bound gas mass is cold for jellyfish galaxies
with stellar masses at infall of $M_{\star}^{\rm
sat}(\tau_{0})=10^{8-9}~{}(10^{9-10})~{}{\rm M}_{\odot}$.

As a note, this ISM-origin of the RPS’ed gas does not preclude the jellyfish
tails to reveal themselves across a wide range of wavelengths (see § 1 for
references). Namely, although the bulk of the tail gas is cold or cool
according to TNG50, it can also manifest itself in e.g. soft X-ray (see fig.
12 from Kraft et al., 2022, for a mock 100 ks exposure from the Line Emission
Mapper for an example TNG100 jellyfish galaxy in the soft X-ray continuum and
at the OVII f line).

###  3.3 The majority of the cold gas loss after infall is due to ram-pressure
stripping

According to TNG50, RPS is the dominant source of cold gas loss after infall
for jellyfish galaxies. This is somewhat to be expected, given the jellyfish
nature of the selected galaxies under inspection. However, we have
demonstrated this for all 535 TNG50 jellyfish galaxies, using the tracer
particle analysis described in Section 2.5.1. We showcase this result with one
example galaxy below.

Fig. 4 shows the time evolution of the gravitationally-bound cold gas mass
$M_{\rm ColdGas}^{\rm subhalo}$ for one example TNG50 jellyfish galaxy. Prior
to infall (at cosmic times $\lesssim 6.5$ Gyr) and at large distances
($\gtrsim 2R_{\rm 200c}^{\rm host}\sim 10^{3}$ kpc), the cold gas associated
to the galaxy is approximately constant. After infall, $M_{\rm ColdGas}^{\rm
subhalo}$ decreases significantly through the first pericentric passage until
the satellite has effectively no cold gas remaining, which we denote as
$\tau_{100}$ (see §2.5.2 for more details). The galaxy quenches its star
formation for the last time shortly before $\tau_{100}$, at $\approx 3$ Gyr
after infall.

Fig. 5 graphs the evolution of cold gas mass and the associated tracers for
the same galaxy as in Fig. 4. In the top panel, $M_{\rm ColdGas}^{\rm
subhalo}$ measured using the gas cell data (thick black curve) is identical to
that in Fig. 4; moreover, $M_{\rm ColdGas}^{\rm subhalo}$ measured using the
tracers (dashed gray curve; the number of tracers with cold gas parents times
the baryonic mass resolution) closely matches the cold gas mass measured using
subfind at all times. This affirms that the tracers robustly measure the cold
gas mass (see § 2.5 and Appendix A for more details).

In the top panel of Fig. 5, while the galaxy is a central before infall at
cosmic times $\lesssim 6.7$ Gyr, the net new (thin blue curve) and lost (thin
red curve) cold gas tracers roughly balance each other, leading to the
approximately constant total $M_{\rm ColdGas}^{\rm subhalo}$. This likely
reflects a quasi-equilibrium galactic fountain scenario, where inflows and
outflows approximately cancel out to yield a constant $M_{\rm ColdGas}^{\rm
sat}$, at least for the depicted galaxy. At infall, there is an immediate drop
in new cold gas – the cold gas that the galaxy acquired via cold gas inflows
or gas cooling – which qualitatively agrees with the results from the EAGLE
simulation (Wright et al., 2019, 2022). The lost cold gas mass remains
approximately constant for $\sim 1$ Gyr after infall before eventually
declining. After infall, the lost cold gas is always similar to or higher than
the new cold gas, leading to the net decline in cold gas mass until $M_{\rm
ColdGas}^{\rm subhalo}<4\times 10^{4}\,{\rm M}_{\odot}$. However it is
interesting that the new cold gas remains nonzero for Gyrs after infall,
including during the pericenter passage.

In the bottom panel of Fig. 5, we show again the net new (thick blue curve)
and lost (thick red curve) tracers of cold gas, now shown as cold gas mass
rates normalized by the time between snapshots. Further, we split the lost
tracers into the various sinks of RPS+outflows (solid olive), gas heating
(dashed pink), star-formation (SF; dotted orange) and SMBH accretion
multiplied by 100 (BH acc.; black dot-dashed). See § 2.5.1 for additional
technical inputs.

Before infall, gas heating is the dominant mechanism of cold gas loss,
followed by RPS+outflows and SF. During this time, the shapes of the
RPS+outflows, SF and SMBH accretion$\times$100 are quite similar, suggesting
that SF and/or SMBH accretion are the primary drivers of outflows for this
galaxy. For the first $\sim$Gyr after infall, the SF remains roughly constant
while the RPS+outflows increases, confirming the onset of RPS. Moreover there
is a simultaneous net gas loss, translating into an increase in the
“efficiency" of RPS+outflows and SF, where efficiency here denotes
RPS+outflows or SF normalized by $M_{\rm ColdGas}^{\rm sat}$. During this
period, the cold gas lost via heating also decreases significantly. SMBH
accretion is the least dominant cold gas sink at all times, at least for this
galaxy. This galaxy has in fact experienced little to no kinetic AGN feedback,
though in general 47 of the 535 ($\approx 9$ per cent) of jellyfish galaxies
have $M_{\star}^{\rm sat}(z=0)>10^{10}\,{\rm M}_{\odot}$ and have experienced
kinetic AGN feedback. Through pericenter until the jellyfish has a gas mass
below our resolution limit, RPS+outflows remains the dominant physical
mechanism of cold gas removal.

As discussed and anticipated in §2.5.1, outflows and RPS are closely
intertwined, for example as outflowing gas is less gravitationlly bound and
therefore more susceptible to RPS. We hence avoid distinguishing between cold
gas that is lost (and becomes unbound) because of RPS or because of a
combination of RPS and high velocities, and we conclude that that RPS
(+outflows) is the dominant source of cold gas loss after infall for jellyfish
galaxies in TNG50.

###  3.4 Why do half of the TNG50 jellyfish have, or not have, cold gas today?

Figure 6:  Why do some jellyfish retain significant amounts of cold gas until
$z=0$, while others do not? The shaded regions and vertical lines mark the
16th/84th percentiles and medians of the distributions, respectively. The
TNG50 jellyfish with cold gas masses below our resolution limit at $z=0$
($M_{\rm ColdGas}^{\rm sat}<4\times 10^{4}\,{\rm M}_{\odot}$, red circles)
typically have earlier infall times $\tau_{0}$ and lower mass ratios
$\mu\equiv M_{\star}^{\rm sat}/M_{\rm 200c}^{\rm host}(z=0)$ than the
jellyfish with $M_{\rm ColdGas}^{\rm sat}>4\times 10^{4}\,{\rm M}_{\odot}$
today (blue circles).

The 535 jellyfish galaxies provided by TNG50 span orders of magnitude in their
$z=0$ stellar mass, host mass, and importantly their (cold) gas mass (Fig. 1).
Why do half of the jellyfish galaxies retain significant amounts of cold gas
until $z=0$, while the others do not?

As a reminder, in this paper we analyse TNG50 satellite galaxies that survive,
in terms of their galaxy stellar mass, through $z=0$ (see §2.2 and §2.4 for
more details). In Fig. 6, we show the satellite-to-host mass ratio $\mu$ vs.
the infall time for the population of jellyfish branches that end up with cold
gas masses above (blue circles) or below (red circles) our resolution limit
($4\times 10^{4}\,{\rm M}_{\odot}$) at $z=0$. Here, the 16/84th percentiles
and medians are marked with the shaded regions and vertical lines
respectively. We note that the results remain qualitatively similar when using
the satellite-to-host mass ratio at infall rather than at $z=0$.

The average infall occurs $\sim 2$ Gyr earlier for those jellyfish galaxies
with little to no cold gas remaining than for those that still retain some
cold gas at $z=0$. While the host halo masses might have not had as much time
to grow at earlier times and the $z=0$ mass ratios are $\mu\sim
10^{-3}-10^{-5}$, most of the early infallers (with infall times more than
$\sim$5 Gyr ago) have had enough time until $z=0$ to undergo secular and
environmental processes to lose their cold gas. Even if these galaxies
required multiple pericentric passages to lose their gas, they have had enough
time before $z=0$ to have done so. Conversely, the largest majority of late-
infalling jellyfish (i.e. with infall times as recent as a few Gyr ago) that
have lost their gas by $z=0$ exhibit very low $z=0$ mass ratios (in the range
$\mu\sim 10^{-4}-10^{-6}$), whereas those with cold gas today typically have
$\mu\sim 10^{-3}-10^{-5}$, either because they are more massive or because
they orbit in less massive hosts.

We speculate that the galaxies with non-vanishing cold gas masses that remain
satellites (i.e. do not become backsplash galaxies) would eventually lose all
their cold gas, i.e. if the simulation ran longer in time.

Whereas the characterization of Fig. 6 is not surprising, it reminds us that,
the longer a satellite has interacted with its host, the more time
environmental processes, such as RPS, have had to act upon it. And even though
some secondary effects may be in place – such as galaxy selection, orbital
trajectories, numbers of pericentric passages, edge-on vs. face-on orientation
of the satellite as it falls into the host, and/or satellite-satellite
interactions – this zeroth-order picture is in line with what has already been
quantified by Donnari et al. (2021); Joshi et al. (2021) for all TNG
simulations: satellites that have spent more time in their hosts are more
likely to be quenched compared to those that are still infalling or on their
first infalling trajectory.

###  3.5 For how long is ram pressure stripping in action?

Figure 7:  Distributions of onset, end, and duration of ram-pressure stripping
(RPS) for TNG50 jellyfish with cold gas masses at $z=0$ above (blue) and below
(red) our resolution limit. The onset of RPS $\tau_{0}$ (left panel) is
defined as cosmic time at infall. The end of RPS $\tau_{100}$ (center panel)
is defined as when the cold gas mass drops below our resolution limit ($M_{\rm
ColdGas}^{\rm sat}\lesssim 4\times 10^{4}\,{\rm M}_{\odot}$) or the end of the
simulation at $z=0$, if the galaxy always has $M_{\rm ColdGas}^{\rm
sat}\gtrsim 4\times 10^{4}\,{\rm M}_{\odot}$. The total RPS timescale
$\tau_{\rm RPS}$, i.e. the total duration of RPS (right panel), is the
difference between the end and onset of RPS. The jellyfish with substantial
cold gas masses at $z=0$ all have $\tau_{100}=0$ by definition, causing the
$\tau_{\rm RPS}$ distribution to be a reflection of the $\tau_{0}$
distribution. For the jellyfish with $M_{\rm ColdGas}^{\rm sat}(z=0)<4\times
10^{4}\,{\rm M}_{\odot}$, the $\tau_{\rm RPS}$ distribution appears bimodal,
with peaks at $\approx 1.5-2.0$ and $4.5-6.5$ Gyr. We examine this
distribution in detail in §3.5.1.  Figure 8:  Given that a given jellyfish
loses all of its cold gas, what determines how long it will take? Of all the
TNG50 jellyfish with $M_{\rm ColdGas}^{\rm sat}(z=0)<4\times 10^{4}\,{\rm
M}_{\odot}$, we bin the ram-pressure stripping (RPS) timescales $\tau_{\rm
RPS}$ by host mass $M_{\rm 200c}^{\rm host}(z=0)$ in the top panel. Here, only
cluster mass hosts (dark red, $M_{\rm 200c}^{\rm host}=10^{13.5-14.3}\,{\rm
M}_{\odot}$, 8 hosts in total) have many jellyfish with both short and long
RPS timescales. Then in the bottom panels we further bin the jellyfish
orbiting in cluster-mass hosts by satellite cold gas mass at infall $M_{\rm
ColdGas}^{\rm sat}(\tau_{0}$) (bottom left), satellite stellar mass today
$M_{\star}^{\rm sat}(z=0)$ (bottom middle), and the number of orbits by the
end of RPS $N_{\rm orbits}(\tau_{100})$, where the number of orbits is the
number of apocentric passages, and the end of RPS ($\tau_{100}$) is the first
time the satellite’s cold gas mass falls below our resolution limit (see text
for details). In all panels, the number of galaxies within each histogram is
in parentheses in the panel’s legend, and the vertical lines mark the medians
of each distribution.

We are hence ready to quantify for how long RPS acts or has acted on TNG50
jellyfish galaxies.

Fig. 7 shows the distributions of the onset of RPS ($\tau_{0}$ i.e. infall
time; left), the end of RPS ($\tau_{100}$; middle), and the duration of RPS
($\tau_{\rm RPS}$; right), for TNG50 jellyfish with cold gas masses below
(red) and above (blue) our resolution limit ($4\times 10^{4}\,{\rm
M}_{\odot}$) at the current epoch.

As we have seen in Fig. 6 and now again in the left panel, the jellyfish with
little to no cold gas at $z=0$ are typically early infallers, with a majority
falling in at $\tau_{0}\approx 4.5-7$ Gyr after the Big Bang and a tail of
late infallers at $\tau_{0}\gtrsim 10$ Gyr after the Big Bang. After the RPS
onset, the TNG50 jellyfish continue losing cold gas until sometime in the past
few billion years ($\tau_{100}\approx 9-14$ Gyr). We only select galaxies
which have been jellyfish since $z=0.5$, so we exclude galaxies that have been
totally stripped of cold gas before $z=0.5$.

Finally, according to TNG50, the distribution of the RPS duration ($\tau_{\rm
RPS}$, right panel) can be very wide, even for both subsets of jellyfish
galaxies. For the jellyfish with cold gas masses below our resolution limit at
$z=0$, the $\tau_{\rm RPS}$ distribution appears somewhat bimodal (see below
for more details). Among these jellyfish, a fraction have undergone RPS for
about $\approx 1.5-2.5$ Gyr and a larger fraction has undergone RPS for as
long as $\sim 4.5-7.5$ Gyr. As a reminder, these numbers represent the maximum
time span over which RPS has acted (§2.5.2); we see in the next Sections
whether RPS may be more effective on shorter timescales. Also for the
jellyfish with still some cold gas at the current epoch, environmental
effects, and hence also RPS, have acted on them for as many as billions of
years.

####  3.5.1 Physical origin of the diversity of RPS duration

What are the important factors in determining how long a given jellyfish needs
to be stripped of its cold gas? We focus from now on only on those jellyfish
that have cold gas masses below our resolution limit at the current epoch
$M_{\rm ColdGas}^{\rm sat}(z=0)<4\times 10^{4}\,{\rm M}_{\odot}$.

In the top panel of Fig. 8, we extract the distribution of the duration of RPS
$\tau_{\rm RPS}$ for the TNG50 jellyfish without substantial cold gas today,
stacked by halo mass $M_{\rm 200c}^{\rm host}$ of their current host. The
number of jellyfish (not the number of hosts) belonging to each host mass bin
is in parentheses in the upper right corner.

Jellyfish in clusters ($M_{\rm 200c}^{\rm host}=10^{13.5-14.3}\,{\rm
M}_{\odot}$, dark red histogram) exhibit the shortest median RPS duration
(vertical dark red line), although the distribution peaks at even shorter time
spans: $\tau_{\rm RPS}\sim 1.5-2$ Gyr. Then there is a valley at intermediate
stripping times $\tau_{\rm RPS}\sim 2.5-4$ Gyr, followed by a slight increase
from $\tau_{\rm RPS}\sim 4.5-6$ Gyr. The longest timescale for any jellyfish
in this host mass bin is 8 Gyr. The jellyfish in groups ($M_{\rm 200c}^{\rm
host}=10^{12.5-13.5}\,{\rm M}_{\odot}$, red histogram) show RPS timescales
that are single-peaked, with the median and mode coinciding at $\tau_{\rm
RPS}\approx 5.5$ Gyr. While not shown but explicitly checked, jellyfish in
group-mass hosts typically require at least 2 pericenteric passage to become
fully stripped of cold gas. There are only $\approx 10$ ($\approx 10$ per
cent) galaxies in this host mass bin with stripping times shorter than 4 Gyr.
This agrees with our earlier argument that RPS is more effective in higher
host masses. Moreover, the jellyfish in approximately Milky-Way mass halos
($M_{\rm 200c}^{\rm host}=10^{11.5-12.5}\,{\rm M}_{\odot}$, light red) require
at least 4 Gyr, or in some cases much longer, to be fully stripped of their
cold gas today. However, even for these MW-mass hosts, the satellite-to-
satellite variation is very large: there are TNG50 jellyfish that undergo RPS
for as long as 10 billion years in both group- and MW-mass hosts, and as long
as 8 billion years for cluster-mass hosts.

The trend whereby shorter RPS time spans occur, on average, for satellites in
more massive hosts is consistent with expectations described in § 1. However,
here we quantify it for the first time with a large number of jellyfish across
a wide range of host and satellite masses. Moreover, this trend is in place
(physically vs. hierarchical growth of structure) even though more massive
hosts exhibit in fact overall more recent infall times of their $z=0$
satellites than less massive hosts (not shown but explicitly checked).

In the bottom row of Fig. 8, we focus on the TNG50 jellyfish with no remaining
cold gas at $z=0$ in the 8 cluster hosts ($M_{\rm 200c}^{\rm
host}=10^{13.5-14.3}\,{\rm M}_{\odot}$) to investigate which additional
physical properties imprint secondary trends on the duration of RPS. In
practice, we show the $\tau_{\rm RPS}$ distributions binned by the satellites’
cold gas mass at infall (left), stellar mass at $z=0$ (middle), and number of
apocentric passages by $\tau_{100}$ (right).

In the bottom left panel of Fig. 8, we see that satellites with the smallest
(dark orange) and largest (light orange) amount of cold gas at infall are both
single peaked at $\tau_{\rm RPS}\approx 1.5$ and $5$ Gyr respectively.
Conversely, the intermediate bin (orange) has an approximately uniform
distribution from $\tau_{\rm RPS}\approx 1.5-6$ Gyr. Galaxies with less
strippable material at infall tend to have shorter stripping durations.

In the bottom middle panel, lower mass jellyfish (dark purple) are typically
stripped of all their cold gas faster, on average in $1-2$ Gyr, although a
non-negligible fraction of them still require $3-6$ Gyr to be fully stripped
of their cold gas. The intermediate (purple) and high (light purple) bins of
satellite’s stellar mass have similarly flat distributions with both a median
RPS duration of $\sim 5$ Gyr.

Lastly in the bottom right panel of Fig. 8, we show that satellites with the
shortest RPS durations are those with the fewest orbits by being totally
stripped $N_{\rm orbits}(\tau_{100})$, where $N_{\rm orbits}(\tau_{100})$ is
the number of apocentric passages before $\tau_{100}$. The jellyfish with the
shortest RPS duration are those that get stripped of all their cold gas before
or immediately after their first pericentric passage (dark green), whereas
satellites that require longer RPS time spans to be fully stripped of their
cold gas are characterized by more than one apocentric passage (green and
light green histograms).

###  3.6 When and where does ram pressure strip most of a jellyfish’s cold
gas?

Figure 9:  When after infall, where within the host halo, and for how long
does 50 per cent of RPS occur? Top panels: the cold gas mass loss due to RPS
at each snapshot normalized by the total amount of cold gas lost due to RPS
through the satellite’s life for two example galaxies (i.e. fractional RPS
loss as per equation (2)). The peak RPS period – the minimum amount of time
for 50 percent of the total RPS to occur (equation (3)) – is within the gray
box. The galaxy in the top left panel is the same as in Figs. 4, 5. Central
panel: Median stacking of the fractional RPS loss of all 261 TNG50 Jellyfish
with $M_{\rm ColdGas}^{\rm sat}(z=0)<4\times 10^{4}\,{\rm M}_{\odot}$. The
gray contour marks the phase space obtained when stacking only the peak RPS
periods of all 261 jellyfish. Bottom panels: Histograms detailing the peak RPS
period of the 261 TNG50 Jellyfish with $M_{\rm ColdGas}^{\rm sat}(z=0)<4\times
10^{4}\,{\rm M}_{\odot}$. Bottom left panel: the distributions of minimum
(dark red, ‘/’ hatch, solid outline) and maximum (light red, ‘\’ hatch, dashed
outline) host-centric distances [$R_{\rm 200c}(t)$] within the peak RPS
period. Bottom center panel: the distribution of onsets of the Peak RPS
periods [Gyr after infall]. Note that the total RPS time span $\tau_{\rm RPS}$
begins at infall which is at 0 Gyr on this plot. Bottom right panel: the
distributions of the peak RPS (dark red, ‘/’ hatch, solid outline) and total
RPS (light red, ’\’ hatch, dashed outline) time spans. Note that the total RPS
time span distribution is identical to that in the right panel of Fig. 7.

While most TNG50 jellyfish are stripped of their cold gas within $\sim 1-7$
Gyr after infall (Fig. 7 right panel; Fig. 8 top panel), the amount of gas
being stripped is not constant throughout this period. The halo gas is denser
at closer distances to the central galaxy, and jellyfish move faster at closer
distances while they are deeper in their host’s potential well. Both of these
effects increase the ram pressure stripping acting on satellites at closer
distances. We quantify this increase in RPS with decreasing distance in Fig.
9. In the top panels for two example galaxies, we show the fractional RPS loss
$f_{\rm RPS}(t_{i})$ – the amount of cold gas lost due to RPS since the last
snapshot $\Delta t_{{\rm snap}_{i}}=t_{i}-t_{i-1}$, normalized by the total
amount of cold gas lost due to RPS in the satellite’s life:

$f_{\rm RPS}(t_{i})=\frac{\displaystyle\int_{t_{i-1}}^{t_{i}}{\rm RPS}(t)\
{\rm d}t}{\displaystyle\int_{\tau_{0}}^{\tau_{100}}{\rm RPS}(t)\ {\rm d}t}$
(2)

where $\tau_{0}$ and $\tau_{100}$ define the total time span of satellite RPS.
In general, the fractional RPS increases as a jellyfish approach pericenter,
followed by a decrease as it approaches apocenter. For galaxy with SubhaloID
439110 (right panel), the fraction of total gas lost is higher during the
first pericentric passage compared to the second because $M_{\rm ColdGas}^{\rm
sat}$ – the total amount of cold gas able to be stripped – is an order of
magnitude higher at infall than after its first orbit (at apocenter). There is
still an increase in fractional RPS during the second pericentric passage
compared to at apocenter, but the majority of RPS for this jellyfish occurs
during the first infall through pericentric passage.

We characterize the period of most effective RPS, the peak RPS period, by
finding the minimum amount of time required for 50 per cent of the total cold
gas loss via RPS to occur. That is, we minimize the difference in bounds
$(t_{\rm stop}-t_{\rm start})$ such that the integral of the fractional RPS
$f_{\rm RPS}(t)$ is at least 50 percent:

$\text{peak RPS}\coloneqq\text{MIN}\left(t_{\rm stop}-t_{\rm
start}\right)\left|\int_{t_{\rm start}}^{t_{\rm stop}}f_{\rm RPS}(t)\ {\rm
d}t\geq 0.5\right..$ (3)

We highlight the peak RPS periods for the two examples galaxies in Fig. 9,
top, with gray boxes, which in both cases occur during the first infall
towards pericenter.

In the central panel of Fig. 9, we stack all 261 TNG50 jellyfish with $M_{\rm
ColdGas}^{\rm sat}(z=0)<4\times 10^{4}\,{\rm M}_{\odot}$, taking the median
fractional RPS loss in the bins with more than one galaxy. Additionally, the
gray contour denotes the phase space region obtained when stacking only the
peak RPS periods of the jellyfish. Based on the fractional RPS loss (color of
bins) and the peak RPS contour, a majority of the RPS occurs within the first
few Gyrs after infall and over a wide range of host-centric distances. At a
fixed time since infall (single column), there is a higher fractional RPS loss
at closer distances. However and especially at times $\lesssim 2$ Gyr after
infall, there is a significant amount of RPS occurring at large host-centric
distances, up to $\approx 3R_{\rm 200c}$. At later times $\gtrsim 2.5$ Gyr
after infall, the peak RPS only occurs at closer host-centric distances
$\approx 0.2-1.0R_{\rm 200c}$. The smallest host-centric bin $<0.1R_{\rm
200c}$ is largely unpopulated (no color) or with few galaxies (not shown but
checked). This means that their pericentric passages are at distances $\gtrsim
0.1R_{\rm 200c}$, and that they are being stripped of their cold gas in the
halos rather than directly into central galaxy. This again supports the claim
that tidal stripping is likely not a significant mechanism for cold gas
removal for this jellyfish sample.

In the bottom panels of Fig. 9, we show the distributions of the peak RPS
period quantities. In the bottom left panel, we show the minimum (dark red,
’/’ hatch, solid outline) and maximum (light red, ’\’ hatch, dashed outline)
host-centric distances during the peak RPS periods. The minimum peak RPS
distance distribution has its peak (mode) at $0.3R_{\rm 200c}$, and the median
(16, 84 percentiles) are $0.43\,(0.22,\ 1.1)\ R_{\rm 200c}$. The maximum peak
RPS distance distribution peaks at $R_{\rm 200c}$, with median (16, 84
percentiles) at $1.2\,(0.75,\,1.9)\ R_{\rm 200c}$. These distributions reflect
that the peak RPS period starts at large distances in the halo (which has been
discussed in, e.g., Bahé et al., 2013; Zinger et al., 2018) and continues
until approximately the pericentric approach, which for our sample of
jellyfish that lose all cold gas by $z=0$ tends to be at $\gtrsim 0.2R_{\rm
200c}$ (see Zinger et al. in prep., for more details about TNG jellyfish at
large distances $d_{\rm sat}^{\rm host}>R_{\rm 200c}$). The cold gas is being
stripped in, and thereby deposited into, the host halos; we extensively expand
on this in § 4.3 and Fig. 10.

In the bottom center panel, we see that the onset of the peak RPS occurs at or
just after ($\lesssim 1$ Gyr) infall. Only $\approx 15$ per cent of these
jellyfish galaxies begin their peak RPS period $>1$ Gyr after infall,
suggesting that the infall time is a reasonable definition for the start of
the total RPS time span.

In the bottom right panel, we show the peak RPS (dark red, ’/’ hatch, solid
outline) and total RPS (light red, ’\’ hatch, dashed outline) time spans. The
two distributions here have different times of onset; the peak RPS onset is
that given in the bottom center panel, while the total RPS onset is the infall
time, which would be 0 in the bottom center panel. The total RPS time span is
identical to that in Fig. 7. While the total RPS duration spans a broad range
of times $\approx 1-7$ Gyr, the peak RPS period is much narrower, spanning
only $\lesssim 2$ Gyr after onset. Thus while the total RPS time span may be
quite long, a majority of the RPS occurs in a relatively short period. While
not shown here, the distribution of peak to total RPS time span ranges from
$\approx 0.1-0.4$, with the mode and median at $\approx 0.15$ and $0.20$
respectively.

We note an alternative method for characterizing the effectiveness or peak RPS
as the specific RPS (sRPS): RPS / $M_{\rm ColdGas}^{\rm sat}$. Typically for
the TNG50 jellyfish without cold gas at $z=0$, the specific RPS + outflows is
approximately constant before infall. At infall, the sRPS typically increases
through pericenter and near apocenter either plateaus or decreases, sometimes
to its pre-infall value. For the galaxies that lose all their cold gas only at
or after second pericenter (subfindID 439110 in the top right panel of Fig. 9
for example), the sRPS increases again and always reaches its maximum value at
or shortly before $\tau_{100}$. See Appendix B for more details and Fig. 13
for an example.

##  4 Discussion

###  4.1 How do these jellyfish-based results generalize to all satellite
galaxies?

Throughout this paper, we have focused on jellyfish galaxies, as these are
satellites with manifest signs of ongoing RPS. In particular, we have followed
satellites along their evolutionary tracks through cosmic epochs and dubbed
the inspected galaxies as jellyfish only if they have at least one jellyfish
classification since $z=0.5$. This is when the temporal sampling of the images
on Zooniverse transitioned from every $\approx 1$ Gyr to every $\approx 150$
Myr. We also restate that the images posted to Zooniverse used a fixed gas
column density colorbar in the range $\Sigma_{\rm gas}\in 10^{5-8}\,{\rm
M}_{\odot}\,{\rm kpc}^{-2}$ and did not include background subtraction,
mimicking a surface brightness limited sample. Hence to have been classified
as a jellyfish galaxy, the stripped tails must have been dense enough to have
been distinguishable against the background. Lastly, we expect that at any
given snapshot, we miss $\approx 30-40$ per cent of jellyfish galaxies due to
projection effects (Yun et al., 2019, Zinger et al., in prep.).

As shown in Fig. 1 and discussed in § 3.1, jellyfish galaxies tend towards
lower stellar masses $M_{\star}^{\rm sat}$, higher host masses $M_{\rm
200c}^{\rm host}$, and lower satellite-to-host mass ratios $\mu$ at $z=0$
compared to the inspected galaxies and the general $z=0$ population of
satellites. Conversely, the inspected galaxies that were not identified as
jellyfish tend towards the opposite. First 187 of the 1,186 ($\approx 15$ per
cent) non-jellyfish galaxies have stellar masses $M_{\star}^{\rm
sat}>10^{10}\,{\rm M}_{\odot}$, and may have experienced phases of kinetic AGN
feedback, ejecting much of their gas. Of the lower mass non-jellyfish galaxies
$M_{\star}^{\rm sat}<10^{10}\,{\rm M}_{\odot}$, we affirm that many of these
satellite galaxies are still undergoing or have undergone RPS (see Figure 11),
and the question becomes why have they not been identified as jellyfish. These
non-jellyfish galaxies have a median satellite-to-host mass ratio
$\mu=7.8\times 10^{-4}$, $\approx 15$ times higher than that for the jellyfish
$\mu=5.0\times 10^{-5}$. We generally expect and have shown in Fig. 8 that
with increasing satellite-to-host mass ratio RPS is weaker and acts over
longer time spans. Accordingly, some of the gaseous tails may not have been
identifiable in gas column density compared to the ambient medium. Moreover,
the non-jellyfish galaxies have an average infall time at 9.2 Gyr, $\approx
1.7$ Gyr later than that of the jellyfish galaxies at 7.5 Gyr. So it is also
possible that these late-infalling non-jellyfish have not yet had enough time
to undergo enough RPS to form the recognizable tails, although the timescales
associated with the appearance and disappearance of the jellyfish tails is
largely unconstrained (Smith et al., 2022).

While 261/535 ($\approx 50$ per cent) of the jellyfish galaxies have cold gas
masses below our resolution limit at $z=0$, this is only the case for 141/1186
($\approx 12$ per cent) of the non-jellyfish galaxies. Then how can these 141
gas-less satellites have lost all of their cold gas without being identified
as jellyfish? Of the galaxies with $M_{\rm ColdGas}^{\rm sat}(z=0)<4\times
10^{4}\,{\rm M}_{\odot}$, the RPS duration for jellyfish includes both short
and long time spans $\tau_{\rm RPS}\approx 1.5-8$ Gyr, while for the non-
jellyfish the time spans are only long $\tau_{\rm RPS}\approx 3.5-7.5$ Gyr.
Again, this demonstrates that the RPS for the non-jellyfish with higher mass
ratios is slower, potentially causing the gaseous tails to be unidentifiable.
Furthermore, these $z=0$ gas-less non-jellyfish tend to have even earlier
infall times than their jellyfish counterparts. In fact, almost all of the
non-jellyfish have infall times before $z=0.5$, before the temporal sampling
of the images on Zooniverse transitioned from every $\approx 1$ Gyr to every
$\approx 150$ Myr. So before $z=0.5$, we may be missing some jellyfish simply
by not inspecting their images often enough. Based on the statistical,
physical differences between the general jellyfish and non-jellyfish galaxies,
and that we may be missing some high-redshift, jellyfish-like galaxies, we
conclude that our primary sample of jellyfish galaxies is pure, but perhaps
not complete. And when generalizing the results of the RPS time spans from the
jellyfish to all satellites, the time spans would only increase. We have also
checked that the peak RPS periods are slightly longer and still occur in the
halos for the non-jellyfish galaxies. However, our results only apply to
first-infalling galaxies, i.e not to pre-processed galaxies, which is more
significant for less massive satellites in more massive hosts. Extending this
analysis to pre-processed galaxies would require distinguishing how much RPS
occurs in each host, and when the infalling group’s intragroup medium gets
stripped.

Figure 10:  TNG jellyfish deposit a significant amount of cold gas into their
host halos. For the most (left) and second most (right) massive clusters in
TNG50 ($M_{\rm 200c}^{\rm host}(z=0)\approx 10^{14}\,{\rm M}_{\odot}$), we
plot the cold gas column density at $z=0$ (top panels; all halo gas with
temperatures $\leq 10^{4.5}$ K, including gas gravitationally bound to
satellites) and the total amount of cold gas deposited in the host halos from
all TNG50 jellyfish in bins of host-centric distance and time since infall
(bottom panels). For both halos, the total amount of cold gas deposited into
the halos from ram-pressure stripped jellyfish is $\sim 10^{12}\,{\rm
M}_{\odot}$ over the last 5 billion years. In the insets (bottom panels), we
compare the radial distributions of the cold gas deposited via RPS from all
jellyfish at $z\lesssim 0.5$ (green) with the cold gas that exists in and
around the halos at $z=0$ (gray, excluding cold gas bound to satellite
galaxies). Together with Fig. 11, this shows that jellyfish, and more
generally satellites, contribute a significant amount of cold gas into their
host halos.

###  4.2 The connection between ram pressure stripping and quenching
timescales

The 261 TNG50 jellyfish that are gas-poor at $z=0$ were star forming galaxies
– on the star forming main sequence (SFMS) – before infall and are instead
quenched at $z=0$ – at least than one dex below the SFMS. The question
becomes, when between the RPS onset at infall and its end at $\tau_{100}$ do
these jellyfish quench. We calculate the amount $f_{\rm RPS}(<\tau_{\rm
quench})$ of RPS that has already occurred by the time of last quenching
$\tau_{\rm quench}$ via

$f_{\rm RPS}(<\tau_{\rm quench})=\int_{\tau_{0}}^{\tau_{\rm quench}}f_{\rm
RPS}(t)\,{\rm d}t$ (4)

where the fractional RPS $f_{\rm RPS}(t)$ is defined in equation (2), and
$\tau_{\rm quench}$ is the last time that the galaxy falls at least 1 dex
below the SFMS for the last time (Pillepich et al., 2019; Joshi et al., 2021;
Donnari et al., 2021).

On average, the jellyfish do not quench until $\gtrsim 99$ per cent of the
total RPS has occurred. Only 3/261 ($\approx 1$ per cent) of the jellyfish
quench before $f_{\rm RPS}(<\tau_{\rm quench})=97$ per cent. Moreover, these
jellyfish have already lost $\gtrsim 98$ per cent of their cold gas by the
time they quench. Of the 267 jellyfish galaxies with $M_{\rm ColdGas}^{\rm
sat}(z=0)>4\times 10^{4}\,{\rm M}_{\odot}$), only 83 ($\approx 30$ per cent)
have quenched whereas the others are still forming stars (see also Göller et
al. in prep). These quenched jellyfish also have already lost $\approx 98$ per
cent of their cold gas before quenching. While the peak period of RPS
typically occurs during the first infall through pericenter, lasting $\lesssim
2$ Gyr, the jellyfish do not quench for the last time until nearly all of
their cold gas has been stripped on time spans that can be $\gtrsim 5$ Gyr
after infall. This does not necessarily imply that the galaxies are on the
SFMS for the entire duration between infall and $\tau_{100}$, but instead that
they quench for the last time only after being stripped of almost all of their
cold gas. Jellyfish galaxies are able to continue forming stars well after
infall and after they have lost almost all of their cold gas due mostly to
RPS.

To define a quenching timescale, one also needs to define the onset of
quenching (See Cortese et al., 2021, for a review of various definitions used
in the literature). If we assume the infall time $\tau_{0}$ as the onset of
quenching, then the distribution of quenching timescales is approximately the
same as the RPS timescale distribution in Fig. 7 (right panel, red histogram).
Thus, the quenching timescales for the TNG50 jellyfish studied here and
without cold gas at $z=0$ range from $\approx 1-7$ Gyr after infall. However,
we note that many of these jellyfish undergo brief ($\lesssim 1$ Gyr) bursts
of star-formation between infall and first pericentric passage (Göller et al.,
in prep.). While it may seem counter-intuitive for the onset of quenching – in
this case, infall – to be directly before a burst of star-formation, this
starburst coincides exactly with the time span that most jellyfish incur their
peak gas loss due to RPS (Fig. 9). Thus the RPS and burst of star-formation
may act together, perhaps even enhancing each other, to remove the cold gas
from the jellyfish, eventually quenching them.

###  4.3 RPS deposits satellites’ ISM into the halo

Figure 11:  TNG satellites (both jellyfish and inspected satellite branches)
are a significant source of cold gas for their host halos. We extend the
analysis from Fig. 10 now to include all inspected branches (gray circles) and
hosts down to mass $M_{\rm 200c}^{\rm host}=10^{11.5}\,{\rm M}_{\odot}$. We
compare the the cold circumgalactic medium (CGM) gas (black circles, excluding
cold gas bound to satellites) to the amount of cold gas brought in over the
past $\sim 5$ Gyr by jellyfish (green circles) and all inspected galaxies
(jellyfish + non-jellyfish, gray circles) from ram pressure stripping (RPS).
We place by hand the halos without any cold gas deposited from inspected
galaxies at $M_{\rm ColdGas}=10^{6}\,{\rm M}_{\odot}$. At host masses $\gtrsim
10^{13}\,{\rm M}_{\odot}$, the inspected galaxies have brought more cold gas
into the halos over the past $\sim 5$ billion years than exists in the CGM
today.

In addition to being stripped of their cold gas, jellyfish galaxies, and more
generally satellite galaxies, also provide a source of cold gas to the halo.
For a given host, how much cold gas comes directly from ram-pressure stripped
jellyfish galaxies?

In Fig. 10, we combine all jellyfish galaxies (regardless if they have
substantial cold gas at $z=0$ or not) in the the two most massive clusters
$M_{\rm 200c}^{\rm host}(z=0)\approx 10^{14}\,{\rm M}_{\odot}$ in TNG50, and
show the total amount of deposited cold gas in the time since infall – host-
centric distance space, similarly to Fig. 9. We also show the cold gas column
density maps for these clusters for reference. Please see Fig. 15 for similar
maps of group-mass hosts ($M_{\rm 200c}^{\rm host}\sim 10^{13-14}\,{\rm
M}_{\odot}$), where there are clear features of cold gas clouds that are not
gravitationally bound to any satellites. The two clusters have hosted 54 and
55 total contributing jellyfish since approximately $z\sim 0.5$, depositing a
total of $\sim 10^{12}\,{\rm M}_{\odot}$ of cold gas mass into the hosts. This
is a substantial amount of cold gas: it is about one tenth of the total amount
of gas in these halos at any given time, it is of a similar order of magnitude
as the stellar mass of the central galaxy of the host at $z=0$, and it is
orders of magnitude more than the amount of ionized and molecular gas that
have been recently observed around several brightest central galaxies (e.g.
McNamara et al., 2014; Russell et al., 2019).

We can integrate the contributed cold gas along the host-centric distance,
yielding the 1D distribution of the deposited cold gas from RPS in time since
infall. For the most massive halo in TNG50, the median (16/84 percentiles) of
the cold gas from RPS distribution occurs $0.9\,(0.3/1.8)$ Gyr after infall;
for the second most massive halo, $1.2\,(0.4/2.1)$ Gyr after infall. Again,
while some jellyfish contribute cold gas to the hosts over long periods
$\gtrsim 3$ Gyr, a majority of the RPS occurs shortly after infall,
qualitatively agreeing with the results discussed in § 3.6 shown in Fig. 9
(bottom center and right panels).

Additionally, we can integrate the contributed cold gas along the time since
infall, yielding the host-centric radial distribution of deposited gas from
RPS. For the most massive halo in TNG50, the median (16/84 percentiles) of the
cold gas from RPS distribution occurs at $1.0\,(0.6/2.2)\,R_{\rm 200c}$; for
the second most massive halo, $0.8\,(0.2/1.8)\,R_{\rm 200c}$. Thus, we see
that the majority of contributed cold gas from RPS is deposited into the
outskirts of the gaseous halos (i.e., CGM or ICM) of these most massive hosts
in TNG50. In the figure insets, we compare the radial-density distributions of
cold gas deposited via RPS from all jellyfish (green) with the cold gas that
exists in the halos at $z=0$ (gray, excluding cold gas bound to satellites).
For these two cluster-mass hosts, more cold gas has been brought into their
halos via RPS over the last many billion years than exists in their intra-
cluster media today.

We extend this analysis in Fig. 11 now to include all inspected satellite
branches (gray circles) and group- ($M_{\rm 200c}^{\rm host}(z=0)\sim
10^{13}\,{\rm M}_{\odot}$) and Milky-Way-mass ($M_{\rm 200c}^{\rm
host}(z=0)\sim 10^{12}\,{\rm M}_{\odot}$) hosts. According to our analysis and
to TNG50, over the past $\sim 5$ billion years satellite galaxies have
deposited more than $10^{10}\,{\rm M}_{\odot}$ of cold gas mass via RPS in the
CGM of halos more massive than $10^{12.5\,{\rm M}_{\odot}}$. The amount of
cold gas in the CGM at $z=0$ (black circles) increases with halo mass until
$\sim 10^{13.5}\,{\rm M}_{\odot}$, and afterwards is approximately constant.
The amount of cold gas deposited by inspected galaxies in low mass hosts
$M_{\rm 200c}^{\rm host}\lesssim 10^{12}\,{\rm M}_{\odot}$ is bimodal, where
many hosts have 0 inspected branches. Of the low-mass hosts with inspected
branches, the amount of cold gas deposited by ram pressure stripping (RPS)
increases with halo mass, which continues with all studied halo masses.

Of the amount of cold gas deposited by RPS of the inspected galaxies, the
relative contribution of jellyfish galaxies increases with halo mass,
reflecting the trend that a higher percentage of inspected galaxies are
jellyfish at the higher host masses (see Fig. 1). At host masses $\gtrsim
10^{13}\,{\rm M}_{\odot}$, the inspected galaxies have brought more cold gas
into the halos over the past $\sim 5$ billion years than exists in the CGM
today.

Thereby, we claim that jellyfish, and the more generally inspected or
satellite galaxies, bring a significant amount of cold gas in the CGM/ICM of
massive halos. The question then becomes, what happens to the stripped cold
gas between being deposited and $z=0$. We speculate that this gas could either
(i) remain cold in the CGM, (ii) remain cold and rain down on the central
galaxy, (iii) mix and heat up with the surrounding hot medium, and/or (iv) be
heated up and/or pushed outside of the halo by kinetic AGN feedback (e.g.,
Ayromlou et al., 2022). Conversely, one could start with the cold CGM clouds
at $z=0$ and follow their histories back in time, quantifying how much came
from satellites (Nelson et al., 2020). We postpone the task of quantifying the
fate of the cold gas brought by satellites into the CGM around galaxies for a
future work.

##  5 Summary and Conclusions

In this work, we use the high-resolution, $\sim 50$ Mpc magneto-hydrodynamical
simulation TNG50 from the IllustrisTNG project to study the satellite-host
interaction in a cosmological context for $\approx 5$ orders of magnitude in
satellite stellar and host total mass. In particular, we quantify when, where,
and for how long the ram pressure stripping (RPS) of cold gas occur, by
focusing on jellyfish galaxies, i.e. satellites with manifest signs of RPS.

We use the results from Zinger et al. (in prep.), which is a follow up from
the pilot Zooniverse study from Yun et al. (2019), to identify jellyfish
galaxies via visual inspection. Namely, Zinger et al. (in prep.) report and
discuss the visual inspection via Zooniverse of 53,610 and 27,094 satellite
galaxies from TNG50 and TNG100 with $f_{\rm gas}<0.01$ and $M_{\star}^{\rm
sat}>10^{8.3},\,10^{9.5}\,{\rm M}_{\odot}$, for TNG50 and TNG100 respectively.
For this paper, in TNG50, we track the 53,610 inspected images across cosmic
time, finding a total of 5,023 unique galaxy branches. In the main analysis of
this work, we focus on the galaxy branches that survive until $z=0$, were
inspected in the Zooniverse project since $z\leq 0.5$, are satellites at
$z=0$, and have not been pre-processed; this returns a pure sample of 1721
galaxies. 535 of these 1721 branches ($\approx 30$ per cent) are jellyfish
galaxies, meaning that they were classified as a jellyfish galaxy for at least
one snapshot since $z\leq 0.5$.

Compared to the inspected galaxies and general $z=0$ satellites with
$M_{\star}^{\rm sat}>10^{8.3}\,{\rm M}_{\odot}$, the TNG50 jellyfish galaxies
tend to have lower stellar masses, higher host masses, lower satellite-to-host
mass ratios, and less gas (Fig. 1). The tails of the jellyfish galaxies are
made up of mostly cold gas $(\leq 10^{4.5}$ K) stemming from the jellyfish
interstellar media (Figs. 2, 3), though the jellyfish tails may also be
observable in, e.g., soft x-rays.

We employ the Monte-Carlo-Lagrangian tracer particles to quantify the relative
importance of each cold gas sink, namely SMBH accretion, star-formation (SF),
gas heating, and ram-pressure stripping (RPS) + outflows. As individual galaxy
tracks suggest, we assume that before infall, i.e. the first time the galaxy
becomes a member of its Friends-of-Friends (FoF) host group, RPS+outflows
category is dominated by outflows, and after infall RPS is dominant. Then we
define the onset $\tau_{0}$ of RPS as the infall time and the end $\tau_{100}$
of RPS as either when the galaxy’s cold gas mass falls below our resolution
limit of $m_{\rm gas}\approx 4\times 10^{4}\,{\rm M}_{\odot}$ ($f_{\rm
gas}\lesssim 5\times 10^{-4}$) or at the end of the simulation at $z=0$; then
the total RPS time span $\tau_{\rm RPS}$ is the difference between
$\tau_{100}-\tau_{0}$. With this sample of 535 jellyfish and method to measure
RPS, here are main results:

  * •

For an individual example, we show that a single jellyfish branch loses all of
its cold gas between infall and apocenter (Fig. 4), and during this period RPS
is the dominant channel of cold gas loss (Fig. 5). We check and find that RPS
dominates the post-infall cold gas loss for all other jellyfish in the sample.

  * •

Approximately half (261/535) of the jellyfish have been stripped of all cold
gas by $z=0$. The jellyfish without cold gas at $z=0$ (i.e. with cold gas
$<4\times^{4}\,{\rm M}_{\odot}$) tend to have smaller satellite-to-host mass
ratios and earlier infall times than the jellyfish that retain some cold gas
at $z=0$ (Figs. 6, 7).

  * •

For the 261 jellyfish galaxies without cold gas at $z=0$, the total RPS
durations span $\tau_{\rm RPS}\approx 1-7$ Gyr (Fig. 7). The dominant factor
for determining the RPS time span is the host mass, whereby jellyfish in
higher-mass hosts have shorter RPS durations (Fig. 8, top panel). Secondarily,
RPS durations decrease with satellite cold gas mass at infall, the stellar
mass at $z=0$, and the number of orbits by $\tau_{100}$ (Fig. 8, bottom panels
respectively).

  * •

While the total RPS duration may be quite long, most jellyfish incur a
majority of their cold gas mass loss via RPS within a short peak RPS period,
beginning $\lesssim 1$ Gyr after infall and lasting $\lesssim 2$ Gyr (Fig. 9
top, bottom center, and bottom right panels). Typically this peak RPS period
occurs within $\approx 0.2-2R_{\rm 200c}$ of the host and during the first
infall.

  * •

jellyfish galaxies continue forming stars for billions of years after infall,
until they have lost $\approx 98$ per cent of their cold gas mass. They quench
for the last time only after $\approx 99$ per cent of the RPS has occurred (§
4.2).

  * •

In the two most massive $\sim 10^{14}\,{\rm M}_{\odot}$ halos in TNG50,
jellyfish galaxies contribute $\approx 10^{12}\,{\rm M}_{\odot}$ of cold gas
into the intra-cluster medium over the past $\sim 5$ billion years (ICM; Fig.
10). The radial distribution of cold gas brought in via jellyfish RPS is
significantly higher than the amount of cold gas existing in the ICM today. In
fact, satellite galaxies deposit over the past $\sim 5$ billion years $\gtrsim
10^{10}\,{\rm M}_{\odot}$ of cold gas in the CGM of $\gtrsim 10^{12.5}\,{\rm
M}_{\odot}$ TNG50 halos (Figs. 11 and 15). For massive hosts, this cold gas
contribution is of the same order of magnitude as the stellar mass in the
central galaxy today. Therefore, jellyfish galaxies, and the more general
population of satellites, bring a significant amount of cold gas into the
CGM/ICM of massive hosts.

In summary, we have shown that, according to TNG50, RPS is the dominant cause
of loss of cold gas in satellites after they start to interact with their
$z=0$ hosts and that satellite galaxies are significant contributors of cold
gas to the CGM and ICM. RPS acts on infalling galaxies for very long periods
of time, i.e. many billion years on average, even though the majority of the
cold gas mass loss occurs faster, with half of the cold gas of satellites
being stripped in the span of about 2 billion years or less. This cold gas is
typically deposited by the satellites all the way from intermediate host-
centric distances to beyond the virial radii of their hosts.

We note that these results apply only to the satellite stellar and total host
masses studied in this work, within the TNG model of galaxy formation. For the
most massive satellites, $M_{\star}\sim 10^{10-11}\,{\rm M}_{\odot}$, it is
possible that their stellar potential is deep enough to retain some of their
own CGM post-infall, shielding some of their ISM gas. At these masses, the TNG
kinetic mode of SMBH feedback also becomes important, and is thought to
dominate, along with RPS, the quenching of these satellites (Donnari et al.,
2020). In a future work, we extend these results to more massive satellite and
host masses using the upcoming TNG-Cluster project that focuses on massive
hosts $M_{\rm 200c}^{\rm host}\sim 10^{14-15}\,{\rm M}_{\odot}$ using the TNG
galaxy formation model.

## Acknowledgements

ER is a fellow of the International Max Planck Research School for Astronomy
and Cosmic Physics at the University of Heidelberg (IMPRS-HD). ER would like
to acknowledge the following friends and colleagues (in alphabetical order)
for useful comments and discussions that improved the quality of the
manuscript: Luca Cortese, Morgan Fouesneau, Junia Göller, Max Häberle, Iva
Momcheva, Nhut Truong, Hans-Walter Rix, Nico Winkel.

DN and MA acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG)
through an Emmy Noether Research Group (grant number NE 2441/1-1).

GJ acknowledges funding from the European Union’s Horizon 2020 research and
innovation programme under grant agreement No. 818085 GMGalaxies.

The TNG50 simulation was run with compute time granted by the Gauss Centre for
Supercomputing (GCS) under Large-Scale Projects GCS-DWAR on the GCS share of
the supercomputer Hazel Hen at the High Performance Computing Center Stuttgart
(HLRS).

This publication uses data generated via the Zooniverse.org platform,
development of which is funded by generous support, including a Global Impact
Award from Google, and by a grant from the Alfred P. Sloan Foundation. We wish
to extend our thanks to the team at Zooniverse with their advice and
assistance in building and running this project. We also thank the thousands
of volunteers who invested their time and effort to assist us in this project.

## Data Availability and Software Used

All TNG simulations are publicly available at https://www.tng-project.org/ and
described in Nelson et al. (2019a). The data underlying this article will be
shared on reasonable request to the corresponding author. The underlying code
will be made publicly available on GitHub upon paper acceptance.

Software used: Python (Van Der Walt et al., 2011); IPython (Pérez & Granger,
2007); Numpy (Van Der Walt et al., 2011; Harris et al., 2020); Scipy (Virtanen
et al., 2020); Matplotlib (Hunter, 2007); Jupyter (Kluyver et al., 2016).

This work made extensive use of the NASA Astrophysics Data System and
arXiv.org preprint server.

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##  Appendix A Tracking individual galaxies across epochs

In the Cosmological Jellyfish project, whose results we use here and are
summarized and discussed by Zinger (et al., in prep.), gas-map images were
posted of TNG50 (and TNG100, though not discussed here) satellite galaxies
meeting the criteria summarized in § 2.2 at all 33 snapshots since $z=0.5$
(snapshots 99-67), plus at four additional snapshots corresponding to
redshifts $z=0.7,1.0,1.5,2.0$ (snapshots 59, 50, 40, 33). Many galaxies were
inspected at multiple snapshots. However, for this work we focus on the unique
evolutionary tracks, or branches, of individual galaxies using sublink_gal
(Rodriguez-Gomez et al., 2015). In practice, we load the main progenitor
branch (MPB) of every galaxy inspected at $z=0$ (snapshot 99), saving the
galaxies’ subfindIDs at all previous snapshots. Then at each earlier
inspection snapshot (98, 98, …, 67, 59, 50, 40, 33), we check which galaxies’
MPB have already been saved. If not, then we save the MPB and continue to the
next snapshot. Within the 53610 inspected galaxy images in TNG50, there 5023
are unique branches.

As summarized in § 2.4, throughout this work we exclude galaxies if they do
not exist at $z=0$, are backsplash galaxies at $z=0$, or have been pre-
processed. For each branch that is not inspected at $z=0$ (snapshot 99), we
load the main descendent branch (MDB). With the MDB, we find the last snapshot
at which the galaxy exists, typically either when the galaxy merges with
another more massive galaxy (subhalo coalescence) or at $z=0$. We are
interested only in galaxies that exist as satellites at $z=0$, so we exclude
2341 branches that do not exist as satellites at $z=0$. To determine whether
the remaining galaxies that are satellites at $z=0$ are have been pre-
processed, we examine both the galaxies’ and their $z=0$ hosts’ MPBs
(technically the MPBs of their $z=0$ hosts’ main subhalos). Then at each
snapshot, we classify each galaxy’s Friends-of-Friends (FoF) group membership
into exactly one of three categories:

  * •

the main (central) subhalo of the group;

  * •

a satellite of its $z=0$ host;

  * •

a satellite of a group other its $z=0$ host.

Then using these categorizations across the snapshots, we can determine if the
galaxy was pre-processed; if the galaxy spent at least $N_{\rm snaps}=5$
consecutive snapshots as a satellite in a host – other than its $z=0$ host –
of mass $M_{\rm 200c}^{\rm host}>M_{\rm LowLim}=10^{11}\,{\rm M}_{\odot}$. If
the galaxy instead spent these $N_{\rm snaps}$ snapshots in its $z=0$ host,
then spent some snapshots as a central galaxy, before eventually being a
satellite in the same group, then we do not consider the galaxy to be pre-
processed and include the galaxy in the analysis. We exclude a total of 341
TNG50 pre-processed galaxies666The most massive TNG50 halo accretes a group
$M_{\rm 200c}\sim 10^{13}\,{\rm M}_{\odot}$ at $z\approx 0.7$, and we
therefore exclude many of this group’s pre-processed jellyfish..

In general, we classify all TNG50 $z=0$ galaxies at all snapshots as one of
the three above categories. Then for all $z=0$ systems, we further flag and
exclude current backsplash galaxies, i.e. galaxies that have spent $N_{\rm
snaps}=5$ consecutive snapshots in a host of mass $M_{\rm 200c}^{\rm
host}>M_{\rm Lowlim}=10^{11}\,{\rm M}_{\odot}$ before eventually being a
central galaxy at $z=0$. This definition is nearly equivalent to that used by
Zinger et al. (2020), except that they use $N_{\rm snaps}=3$ and $M_{\rm
Lowlim}=0$ (i.e., no criterion for host mass). Additionally we note that,
especially during massive mergers, subfind may confuse which galaxy is
actually the central and which is the satellite. Consequently some galaxies
(such as central galaxy of the most massive cluster in TNG50) may be
classified as a backsplash galaxy due to this “swapping problem", so we
recommend using caution when physically interpreting these backsplash
galaxies.

Further, we check whether each $z=0$ satellite subhalo was pre-processed or
previously a backsplash galaxy. The pre-processing definition is above. A
galaxy was previously a backsplash galaxy if it spent at least $N_{\rm
snaps}=5$ in any host of mass $M_{\rm 200c}^{\rm host}>M_{\rm
LowLim}=10^{11}\,{\rm M}_{\odot}$, then spent at least $N_{\rm snaps}$
consecutive snapshots as a central, before eventually being a satellite at
$z=0$. A given $z=0$ satellite may be a previous backsplash but not pre-
processed if the previous host is also the $z=0$ host; pre-processed but not
previously a backsplash if the galaxy falls into a pre-processing host and
this pre-processing host falls into the $z=0$ host; both a previous backsplash
and pre-processed; or neither. Previously Donnari et al. (2021) combined these
two flags – pre-processed and previous backsplash – as one general “pre-
processing" flag, while in this analysis we include previous backsplash
galaxies. Additionally, Donnari et al. (2021) use $N_{\rm snaps}=3$ and
$M_{\rm LowLim}=10^{12}\,{\rm M}_{\odot}$, and the catalogs utilize sublink
rather than sublink_gal (Rodriguez-Gomez et al., 2015).

##  Appendix B Comparisons of the Onset of RPS

Figure 12:  Time evolution of the cold gas content and associated tracers of a
TNG jellyfish galaxy that is stripped of all cold gas between first and second
pericentric passage. The marker style denotes the FoF membership and the color
the distance to the $z=0$ host in units of [$R_{\rm 200c}^{\rm host}(t)$]. The
inspected snapshots are outlined, and the jellyfish classified have a black
dot. We plot snapshots where the y-axis quantity is below our resolution limit
at the lower y-limit (along the bottom x-axis). The thick black ticks denote
the fiducial start (infall) and end (when $M_{\rm ColdGas}^{\rm subhalo}=0$)
times of ram-pressure stripping (RPS), while the purple (“RPS est") and olive
(“RPS sRPS") ticks denote two alternative methods of measuring the start of
RPS. See the text for additional details regarding the definitions of RPS est
and RPS sRPS. For this galaxy, the three definitions yield similar results for
the onset of RPS. Top panel: the total gravitationally-bound cold gas mass
$M_{\rm ColdGas}^{\rm subhalo}$. Middle panel: RPS+outflows and instantaneous
SFR as the thin gray curve. Bottom Panel: sRPS = RPS+outflows / $M_{\rm
ColdGas}^{\rm subhalo}$, in addition to SFR / $M_{\rm ColdGas}^{\rm subhalo}$
as the thin gray curve; the dashed line denotes $1/t_{H}$, where $t_{H}$ is
the Hubble Time; the dotted line denotes the approximate inverse time between
snapshots $1/\Delta t_{\rm snap}\approx 1/150\,{\rm Myr}\approx 7\times
10^{9}\,{\rm yr}^{-1}$.  Figure 13:  Time evolution of the cold gas content
and associated tracers of a TNG50 jellyfish galaxy that is stripped of all
cold gas only after its second pericentric passage. Similar to Fig. 12, except
for this example the onset of RPS $\tau_{0}$ for “RPS est" (purple) and “RPS
sRPS" (olive) are significantly later than the infall time (black, fiducial).
This represents a non-common case, so that we can safely take the infall time,
i.e. the first time a galaxy becomes part of the Fof of its $z=0$ host, as the
onset of RPS.  Figure 14: Alternative methods for measuring the onset of RPS
compared to the fiducial choice of infall time.

Throughout the paper, we define the onset $\tau_{0}$ of ram-pressure stripping
(RPS) as the infall time, as the first time a galaxy becomes a member of its
$z=0$ Friends-of-Friends (FoF) host group. The FoF algorithm decides group
membership based on the relative positions of dark matter particles, and there
are a-priori no constraints on the shape or total size of the halo; that is,
we do not assume spherical halos where galaxies become satellite members upon
crossing the virial radius $R_{\rm 200c}$. Consequently, there is a range of
infall distances $d_{\rm sat}^{\rm host}(\tau_{0})$, ranging from $\approx
1-4R_{\rm 200c}$ (see Fig. 9, middle panel). Throughout this paper, we
consider this FoF infall time to be onset of environmental effects for these
first-infalling, not pre-processed jellyfish. Moreover, using this infall time
as the onset and the time when galaxies lose all cold gas as the end of RPS
allows us to measure the entire RPS time span $\tau_{\rm
RPS}=\tau_{100}-\tau_{0}$, at least for the galaxies that lose all cold gas by
the end of the simulation at $z=0$.

We have checked our results using two additional definitions for measuring the
onset of RPS $\tau_{0}$. First, we assume that the pre-infall outflows are
primarily star-formation-driven outflows. Then we measure each galaxy’s cold
gas loading factor $\eta_{\rm ColdGas}$ as the median cold gas loss due to
outflows divided by the star-formation rate (SFR)

$\eta_{\rm ColdGas}=\text{median}\left(\frac{\rm Outflows(<infall\,time)}{\rm
SFR(<infall\,time)}\right)$ (5)

where “Outflows" is the (RPS + Outflows) total cold gas mass loss from RPS +
outflows directly measured using the tracer particles, before infall assumed
to be entirely outflows. Then after infall, we estimate the amount of star-
formation-driven outflows as the product of $\eta_{\rm ColdGas}$ and the SFR.
Thus, we attempt to separate the measured RPS+Outflows into the two
components:

$\begin{split}\text{RPS est}(t)&=(\text{RPS +
outflows})(t)-\text{outflows}(t)\\\ &=(\text{RPS + outflows})(t)-\eta_{\rm
ColdGas}\times\text{SFR}(t)\end{split}$ (6)

where “RPS est" is the estimated RPS and (RPS + outflows) is the quantity
measured using the tracer particles. Then we find the peak of “RPS est" and go
backwards in time until the this estimated RPS vanishes, i.e. until the (RPS +
outflows) can be fully estimated by just star-formation-driven outflows. In
practice, we calculate the running median of the estimated outflows and total
RPS + outflows over $N_{\rm snaps}=7$ consecutive snapshots ($\sim 1$ Gyr),
and find where the difference, the estimated RPS, peaks. Then we go backwards
in time until the running median of the estimated RPS vanishes, where this
time marks the onset of RPS. In Fig. 12, the onset of RPS using this “RPS est"
is shown with a purple tick, where this estimated onset of RPS is 2 snapshots
($\approx 300$ Myr) after the infall time. In the middle panel after infall,
there is an increase in the total RPS + outflows (triangles and circles) while
the SFR (gray) remains approximately constant. Thus, the “RPS est"-onset is
similar to the infall time.

While this method attempts to separate the relative amounts of cold gas loss
via (RPS + outflows) into RPS and outflows, there are a number of
disadvantages. First, this method assumes that the cold gas mass loading
factor $\eta_{\rm ColdGas}$ – which varies with galaxy stellar mass, cold gas
mass, and SFR – is approximately constant before and after infall. Then as a
number of galaxies experience a burst of star-formation between infall and
first pericentric passage, the approximations of $\eta_{\rm ColdGas}$ may
break down. For the example jellyfish in Fig. 13, there is both an increase in
the RPS + outflows and in the SFR after infall, delaying the “RPS est"-onset
of RPS by 7 snapshots ($\sim 1$ Gyr). In fact, for 137/535 ($\approx 25$ per
cent) jellyfish galaxies, the $\eta_{\rm ColdGas}$-estimated outflows account
for the entire budget of cold gas mass loss via (RPS + outflows), meaning that
the estimated RPS is null. For this sample of visually-inspected galaxies to
be undergoing RPS, we conclude that this method overestimates the contribution
from outflows and thereby may inaccurately determine the onset of RPS.

The increased star-formation in jellyfish galaxies during infall may be caused
by the RPS-induced compression of gas, especially on the leading side (e.g.,
Roberts et al., 2022). Then the star-formation-driven outflows fight against
the gravity of the stellar body, making the gas more susceptible to RPS (e.g.,
Garling et al., 2022). In this context, separating the total RPS and outflows
may be futile. Both outflows and RPS work to remove the galaxy’s cold gas, the
fuel for star-formation, and deposit the galaxy’s ISM into the halo.

In spite of this, for the remaining 391 jellyfish galaxies with non-vanishing
estimated RPS contributions, the “RPS est" onset of RPS is typically $\lesssim
3$ snapshots ($\lesssim 450$ Myr) later than the infall time (Fig. 14, top
panel). The difference between this and the fiducial onset is significantly
shorter than the total RPS time span, where the end of RPS is the same for
both definitions (when $M_{\rm ColdGas}=0$). Thus, when attempting to capture
the entire duration of RPS, we choose infall time over the “RPS est" start as
the fiducial onset of RPS.

As a second alternative, we use the specific RPS and outflows (sRPS+O), namely
the cold gas mass loss due to RPS and outflows (RPS+O) divided by the total
amount of cold gas $M_{\rm ColdGas}^{\rm subhalo}$. Here, the units are $[{\rm
time}^{-1}]$, where the inverse yields the timescale to lose all cold gas to
RPS+O (at constant RPS+O). We calculate the median pre-infall sRPS+O, and find
where the sRPS+O peaks. For all 261 jellyfish without cold gas at $z=0$, the
maximum sRPS+O (shortest timescale) occurs at one of the last 3 snapshots that
the galaxy has some cold gas. This peak value is typically $\sim 10^{-8}\,{\rm
yr}^{-1}$, which is approximately the inverse time between snapshots $1/\Delta
t_{\rm snap}\approx 1/(150\,{\rm Myr})$. Then we go backwards from this peak
until the running median of the sRPS+O over $N_{\rm snaps}=7$ ($\sim 1$ Gyr)
returns to the pre-infall average, and this time defines the onset of RPS. In
Figs. 12, 13, this “RPS sRPS" onset is marked with olive ticks.

For galaxies that lose all cold gas approximately within the first orbit or by
the first pericentric passage, such as the example in Fig. 12, the “RPS sRPS"
onset is typically $\approx\pm 2$ snapshots ($\approx\pm 300$ Myr) of the
infall time (Figure 14, top panel). While the sRPS+O generally increases
between infall and pericentric passage, the sRPS+O may plateau or decrease
near apocenter. Sometimes this apocentric decrease may bring the sRPS+O back
to the pre-infall average. In these cases, then the “RPS sRPS" onset occurs on
the second infall, such as for the example in Fig. 13. This leads to a number
of galaxies with “RPS sRPS" onsets significantly after infall, shown in Fig.
14 (bottom panel). This definition estimates that these galaxies actually
undergo multiple periods of ram-pressure stripping. However for determining
the entire duration of RPS, splitting the entire RPS process into multiple
periods is not helpful. Thus, we choose the infall time as the onset of RPS.

##  Appendix C Cold gas in massive groups of TNG50 today

Fig. 15 shows the column density maps of the cold gas (all halo gas with
temperatures $\leq 10^{4.5}$ K, including gas gravitationally bound to
satellites) in the twenty most massive halos of TNG50 at $z=0$ beyond those