11institutetext: Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la
Astronomía s/n, 18008 Granada, Spain 22institutetext: ASTRON, the Netherlands
Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
33institutetext: Kapteyn Astronomical Institute, University of Groningen, P.O.
Box 800, 9700 AV Groningen, The Netherlands 44institutetext: Department of
Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI,
53706, USA 55institutetext: Research Center for Intelligent Computing
Platforms, Zhejiang Laboratory, Hangzhou 311100, China 66institutetext:
Department of Astronomy, University of Cape Town, Private Bag X3, 7701
Rondebosch, South Africa 77institutetext: Purple Mountain Observatory and Key
Laboratory of Radio Astronomy, Chinese Academy of Sciences, 10 Yuanhua Road,
Qixia District, Nanjing 210033, PR China 88institutetext: Département de
physique, Université de Montréal, Complexe des sciences MIL, 1375 Avenue
Thérèse-Lavoie-Roux, Montréal, Qc, Canada H2V 0B3 99institutetext: Laboratoire
de Physique et de Chimie de l’Environnement, Observatoire d’Astrophysique de
l’Université Ouaga I Pr Joseph Ki-Zerbo (ODAUO), BP 7021, Ouaga 03, Burkina
Faso 1010institutetext: Inter-University Institute for Data Intensive
Astronomy (IDIA), University of Cape Town, Rondebosch, Cape Town, 7701, South
Africa 1111institutetext: Western Sydney University, Locked Bag 1797, Penrith
South DC, NSW 1797, Australia

# NGC 3314a/b and NGC 3312: Ram pressure stripping
in Hydra I Cluster substructure

Kelley M. Hess Corresponding author<EMAIL_ADDRESS>3314a/b and NGC 3312: Ram
pressure stripping  in Hydra I Cluster substructureNGC 3314a/b and NGC 3312:
Ram pressure stripping  in Hydra I Cluster substructureNGC 3314a/b and NGC
3312: Ram pressure stripping  in Hydra I Cluster substructureNGC 3314a/b and
NGC 3312: Ram pressure stripping  in Hydra I Cluster substructureNGC 3314a/b
and NGC 3312: Ram pressure stripping  in Hydra I Cluster substructureNGC
3314a/b and NGC 3312: Ram pressure stripping  in Hydra I Cluster substructure
Ralf Kotulla NGC 3314a/b and NGC 3312: Ram pressure stripping  in Hydra I
Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure stripping  in Hydra
I Cluster substructure Hao Chen Corresponding author:
<EMAIL_ADDRESS>3314a/b and NGC 3312: Ram pressure stripping  in
Hydra I Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure stripping
in Hydra I Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure
stripping  in Hydra I Cluster substructureNGC 3314a/b and NGC 3312: Ram
pressure stripping  in Hydra I Cluster substructureNGC 3314a/b and NGC 3312:
Ram pressure stripping  in Hydra I Cluster substructureNGC 3314a/b and NGC
3312: Ram pressure stripping  in Hydra I Cluster substructure Claude Carignan
NGC 3314a/b and NGC 3312: Ram pressure stripping  in Hydra I Cluster
substructureNGC 3314a/b and NGC 3312: Ram pressure stripping  in Hydra I
Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure stripping  in Hydra
I Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure stripping  in
Hydra I Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure stripping
in Hydra I Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure
stripping  in Hydra I Cluster substructure John S. Gallagher NGC 3314a/b and
NGC 3312: Ram pressure stripping  in Hydra I Cluster substructureNGC 3314a/b
and NGC 3312: Ram pressure stripping  in Hydra I Cluster substructure T. H.
Jarrett NGC 3314a/b and NGC 3312: Ram pressure stripping  in Hydra I Cluster
substructureNGC 3314a/b and NGC 3312: Ram pressure stripping  in Hydra I
Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure stripping  in Hydra
I Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure stripping  in
Hydra I Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure stripping
in Hydra I Cluster substructureNGC 3314a/b and NGC 3312: Ram pressure
stripping  in Hydra I Cluster substructure Renée C. Kraan-Korteweg NGC 3314a/b
and NGC 3312: Ram pressure stripping  in Hydra I Cluster substructureNGC
3314a/b and NGC 3312: Ram pressure stripping  in Hydra I Cluster substructure

(Received —; accepted —)

Cluster substructure and ram pressure stripping in individual galaxies are
among the primary evidence for the ongoing growth of galaxy clusters as they
accrete galaxies and groups from their surroundings. We present a multi-
wavelength study of the center of the Hydra I galaxy cluster, including
exquisite new MeerKAT H i and DECam H$\alpha$ imaging which reveal conclusive
evidence for ram pressure stripping in NGC 3312, NGC 3314a and NGC 3314b
through compressed H i contours, well-defined H i tails, and ongoing star
formation in the stripped gas. In particular, we quantify the stripped
material in NGC 3312 and NGC 3314a, which makes up between 8% and 35% of the
gas still in the disk, is forming stars at $\sim$ 0.5 M⊙ yr-1, and extends
$\sim$30-60 kpc from the main disk. The estimated stellar mass in the tails is
an order of magnitude less than the H i mass. A fourth “ring” galaxy at the
same velocity does not show signs of ram pressure in H i. In addition, we use
the H i and stellar morphologies, combined with a Beta model of the hot
intracluster medium, to constrain the real distances of the galaxies to the
cluster center, and we use the chance alignment of NGC 3314b behind NGC 3314a
to break the degeneracy between whether the galaxies are in front or in back
of the cluster. The drag seen in the H i tails supports our preferred scenario
that NGC 3312 and NGC 3314a are moving towards us as part of a foreground
substructure which has already passed its pericenter and is on “out fall” from
the cluster. The high surviving H i content of the galaxies may suggest that
the substructure/intragroup medium can protect them from the harshest effects
of ram pressure, or that in fact the galaxies are on more tangential orbits.

###### Key Words.:

Galaxies: clusters: individual: Hydra I – Galaxies: clusters: intracluster
medium – Galaxies: evolution – Galaxies: individual: NGC 3312, NGC 3314a, NGC
3314b Galaxies: ISM – Galaxies: star formation –

##  1 Introduction

Galaxy transformation happens rapidly at the outskirts of clusters. Newly
arrived galaxies fall into clusters on predominantly radial orbits (Colless &
Dunn, 1996; Ghigna et al., 1998; Vollmer et al., 2001b; Biviano & Katgert,
2004; Mamon et al., 2019) and experience a variety of hydrodynamical and
gravitational mechanisms that disrupt their equilibrium (Cowie & Songaila,
1977; Larson et al., 1980; Nulsen, 1982; Valluri, 1993; Moore et al., 1996),
driving changes in their morphology and composition (Moore et al., 1998;
McIntosh et al., 2004; Boselli & Gavazzi, 2006; Moran et al., 2007). The most
visually striking among these mechanisms is ram pressure stripping (RPS) in
which the interstellar medium (ISM) of a galaxy is impacted and removed by its
interaction with and motion through a hot intracluster medium (ICM; Gunn &
Gott 1972).

The impact of ram pressure on galaxy disks was first recognized in optical
images of cluster member galaxies: some late-type galaxies could be seen with
asymmetric dust lanes–having been swept away from the leading edge of the
galaxy–and trailing condensations of star formation (e.g. Gallagher 1978 in
NGC 3312; Kenney & Koopmann 1999 and sources therein). However, tidal
interactions could not be ruled out as being responsible for these features.

The neutral atomic hydrogen (H i) component of galaxies has been critical to
confirming episodes of RPS, showing for example how the edges of the
kinematically cold gas disk can be clearly displaced from the stellar disk in
a sort of bow-shock as galaxies move through the ICM (Kenney et al., 2004;
Crowl et al., 2005). Resolved surveys of nearby clusters have shown there is a
large-scale anti-correlation between the orientation and location of extended
H i tails, swept-back disks, and truncated disks compared to the X-ray
emitting ICM within clusters (Chung et al., 2007, 2009). RPS can explain the
well known H i deficiency of galaxies in the center of clusters (Giovanelli &
Haynes, 1985; Haynes & Giovanelli, 1986; Bravo-Alfaro et al., 2000; Solanes et
al., 2001; Chung et al., 2009; Hess et al., 2015; Loni et al., 2021), and
reveal the sequence of evolution as the multi-phase gaseous component is
removed from galaxies by the ICM wind (Cayatte et al., 1994; Tonnesen & Bryan,
2009). These observations have been widely shown to be consistent with
increasingly sophisticated simulations (Abadi et al., 1999; Brüggen & De
Lucia, 2008; Kapferer et al., 2009; Yun et al., 2019; Lee et al., 2020).

Due to the sensitivity of H i to its environment (Jones et al., 2020), the
relatively short life time for H i in the ICM, and that H i detected galaxies
tend to reside on the outskirts of clusters (Bravo-Alfaro et al., 2000; Chung
et al., 2009; Hess et al., 2015; Loni et al., 2021; Molnar et al., 2021), H i
in cluster members is a signpost for recent accretion in the cluster
environment. Combined with a measure of the cluster substructure (e.g.
Dressler & Shectman 1988), H i may be valuable in estimating what fraction of
recently accreted galaxies fell into clusters as individuals or as members of
galaxy groups, and their degree of pre-processing (Jaffé et al., 2013, 2016;
Hess & Wilcots, 2013; Hess et al., 2015; Healy et al., 2020; Kleiner et al.,
2021; Bakels et al., 2021). Substructures with multi-wavelength coverage may
also allow a reconstruction of the merger history of clusters (Colless & Dunn,
1996; Hess et al., 2015).

Ongoing accretion in clusters has been observed in at least three major ways
(1) evidence for ram pressure stripping which is most easily and dramatically
identified in H i or H$\alpha$, (2) through cluster substructure identified
through statistical tests or phase space diagrams (Hou et al., 2012; Hess et
al., 2015; Jaffé et al., 2013, 2016; Sampaio et al., 2021), or (3) through
X-ray substructure (e.g. Flores et al. 2000; Ferrari et al. 2006; Mann &
Ebeling 2012). The Hydra galaxy cluster has largely avoided being identified
strongly with any of these features.

Indeed, the Hydra I Cluster (also known as Abell 1060) is unusual compared to
well-known and well-studied nearby clusters such as Coma, Virgo, and Antlia.
Hydra has been identified as relatively spiral-rich (Wirth & Gallagher, 1980),
with several large spirals seen in close projection to the cluster center. It
is also unusually gas-rich, hosting a number of gas-rich dwarfs, and its
central spiral galaxies are not particularly H i deficient (Richter &
Huchtmeier, 1983; McMahon et al., 1992; Duc et al., 1999; Wang et al., 2021).
Despite this apparently young persona, Hydra has a dynamically relaxed X-ray
halo centered on the cD galaxy, NGC 3311 (Fitchett & Merritt 1988; Tamura et
al. 2000; Hayakawa et al. 2004; although see also Ventimiglia et al. 2011),
and a fairly Gaussian distribution of cluster member velocities (Fitchett &
Merritt, 1988). Ventimiglia et al. (2011) and Arnaboldi et al. (2012) identify
a collection of dwarf galaxies and planetary nebulae at 5000 km s-1 and tidal
features around NGC 3311, which suggest a history of mergers within the core.
However, the best case to support recent infall in Hydra, is that many studies
find evidence for 2-3 galaxy groups or substructures projected along the line-
of-sight to the cluster core (Fitchett & Merritt, 1988; McMahon et al., 1992;
Lima-Dias et al., 2021). Hydra thus presents an interesting and challenging
environment to study galaxy cluster assembly which ties together ram pressure,
cluster substructure, and the limitations of our observing capabilities to
disentangle three dimensional structures.

###  1.1 Ram pressure in clusters

The ram pressure felt by a galaxy moving through its environment is
parameterized as a wind, whose strength depends on the density of the medium
and the relative velocity of the galaxy through that medium:
$\rho_{ICM}v_{ICM}^{2}$. In the cluster environment, the typical ICM densities
are low, but the orbital or infall velocity of the galaxies are high.
Stripping occurs when the pressure from the ICM wind overcomes the local
gravitational restoring force of the galaxy disk (Gunn & Gott, 1972), at which
point, gas is pushed out of the disk (for example through turbulent/viscous
processes; Nulsen 1982). Although the maximum effect of RPS may be evident
only after the peak of $\rho_{ICM}v_{ICM}^{2}$, depending on the strength of
the ram pressure and the angle at which it impacts the disk (Jáchym et al.,
2009; Roediger et al., 2015; Tonnesen, 2019).

The signatures of ram pressure are seen in X-ray (Sun et al., 2006), optical
(Poggianti et al., 2016; Roberts & Parker, 2020), H$\alpha$ (Kenney &
Koopmann, 1999; Sun et al., 2007; Fumagalli et al., 2014), dust (Cortese et
al., 2010; Abramson & Kenney, 2014; Kenney et al., 2015; Abramson et al.,
2016), CO (Kenney et al., 1990; Vollmer et al., 2001a, 2008a), H i (Kenney et
al., 2004; Oosterloo & van Gorkom, 2005; Reynolds et al., 2021b), radio
continuum (Gavazzi et al., 1995; Chen et al., 2020; Roberts et al., 2021),
polarized emission (Vollmer et al., 2008b), and combinations thereof (e.g.
Crowl et al. 2005; Vollmer et al. 2009; Abramson et al. 2011; Ramatsoku et al.
2019; Longobardi et al. 2020; see also Boselli et al. 2022 for a review). H i
is particularly valuable as a tracer of RPS, because interferometric
observations resolve both the spatial and kinematic morphology of the gas in
the process of being stripped. Similar analysis is becoming increasingly
common in H$\alpha$ using Fabry-Perot (Chemin et al., 2005) or integral field
spectrographs (MUSE, Fumagalli et al. 2014; Poggianti et al. 2017; Sheen et
al. 2017). The disrupted velocity fields, in addition to the morphology,
provide information about not only the projected motion of the galaxy in the
plane of the sky, but the relative motion of galaxy material along the line of
sight.

The ultimate impact of ram pressure on the evolution of galaxies is complex:
it both compresses the interstellar medium of the galaxy driving star
formation in dense material; and heats and strips loosely bound material
eroding the gas reservoir. The leading edge of galaxies experiencing RPS can
be bluer than the trailing edge as dust is removed, and leaves linear dust
filaments where the dust has been eroded by the wind around denser cores
(Abramson et al., 2016). On the trailing edge of galaxies, new stars can form
out of the stripped gas in “fireballs” (Kenney et al., 2014). The most extreme
examples are known as “jellyfish” galaxies (e.g. Owers et al. 2012; Ebeling et
al. 2014; McPartland et al. 2016; Poggianti et al. 2019). Star formation in
both the compressed gas on the leading edge and in trailing gas can also be
reproduced in simulations (Bekki & Couch, 2003; Kronberger et al., 2008; Lee
et al., 2020). Whether this sequence leads first to a star formation
enhancement (Roberts & Parker, 2020) before the galaxies are ultimately
quenched – from the outside in (Koopmann et al., 2006) – is unclear (Vollmer
et al., 2012). However, by combining H i, various measures of star formation
in the stripped gas, and statistical studies, one can estimate the timescales
over which stripping is occurring (e.g. Cortese et al. 2021).

It is perhaps worth mentioning that our understanding of ram pressure
discussed above is for galaxies falling into clusters as individuals, however
many galaxies fall in as groups (McGee et al., 2009; Hess et al., 2015).
Falling into a cluster as a member of a group may significantly modify the
details of how a galaxy experiences ram pressure: for example, a galaxy may
experience pre-processing in the group environment (Bahé et al., 2013;
Vijayaraghavan & Ricker, 2013), or they may experience less RPS as a result
being shielding from the ICM by the bulk motion of the group and the
intragroup medium (IGM) through the cluster. Present-day simulations may be
able to provide some insight. On the observing side, correlating infalling
groups with the bulk motions of substructure in clusters will only be possible
with the next generation of X-ray telescopes (Ettori et al., 2013).

The presence of RPS in the Hydra Cluster has been a subject of uncertainty for
the last four and a half decades. In particular, NGC 3312 and NGC 3314a/b are
three spiral galaxies, seen in close projection to the core of Hydra.
Gallagher (1978) suggested NGC 3312 as a stripping candidate based on
asymmetric optical morphology alone, showing sharp dust lanes on the east side
of the galaxy and trailing clumps of blue stars (“condensations”) off the disk
to the southwest. Its disk is seen at a steep angle while H$\alpha$ data show
HII regions asymmetrically extended on the “downwind” side, and likely out of
its plane (see also Ho et al. 2011). NGC 3314a is a face-on galaxy, at the
same recessional velocity as NGC 3312, which has a trail of stellar material
extending to the southwest (most recently described by Iodice et al. 2021).
NGC 3314a is backlit by the highly inclined NGC 3314b (Richter et al., 1982;
Schweizer & Thonnard, 1985; Keel & White, 2001) which is also a Hydra cluster
member. The two are separated by about 1850 km s-1.

H i observations of NGC 3312 and NGC 3314a with the Very Large Array suggested
disturbed gas in the outskirts of the disks, but the interpretation favored
that these galaxies were perhaps undergoing tidal interactions in the
foreground group, rather than ram pressure stripping (McMahon et al., 1992).
This is because X-ray data showed that if the galaxies were well within the
Hydra cluster, RPS should have stripped the gas down to column densities of
$6\times 10^{20}$ cm-2, well above the detection threshold of the H i
observations. Instead, it was suggested that NGC 3312, NGC 3314a, and a gas-
rich ring galaxy, LEDA 753342 (first noted by Wirth & Gallagher 1980) may be
part of a foreground cluster substructure: NGC 3312 and NGC 3314a have almost
the same systemic velocity, and tidal interactions could be responsible for
their disturbed outer H i morphology. Recent studies as part of WALLABY
(Koribalski et al., 2020) early science operations, have quantified the amount
of strippable gas in NGC 3312 (Wang et al., 2021), or argue based on the
galaxy’s position close to the center in phase space that it is most likely
undergoing ram pressure stripping (Reynolds et al., 2021b), but the H i
morphology of the ASKAP maps are still inconclusive.

More recently, Iodice et al. (2021) described the stellar streams in NGC 3312
and NGC 3314a in deep VLT Survey Telescope (VST) broadband images, as part of
the VST Early-type Galaxy Survey (VEGAS). They report the detection of an
ultra diffuse galaxy, UDG 32, in the stellar tail of NGC 3314a and discuss the
possibility of its formation due to RPS. If confirmed to be at the distance of
NGC 3314a, UDG 32 would be the first such object to be attributed to a ram
pressure origin.

###  1.2 Substructure in clusters

Substructure in galaxy clusters is a natural consequence of hierarchical
structure formation (Press & Schechter, 1974; McGee et al., 2009; Fakhouri et
al., 2010), and significant substructure in groups or clusters is believed to
be correlated with relatively young systems that have recently merged or
accreted bound collections of galaxies to make a larger halo (Hou et al.,
2012). Cluster substructure can be apparent in X-ray observations (e.g. Briel
et al. 1992; Schuecker et al. 2001; Zhang et al. 2009), or detected using
statistical methods to measure the spatial and velocity deviations from the
parent halo (e.g. Dressler & Shectman 1988; Colless & Dunn 1996; Hou et al.
2012). In the cluster environment, kinematically identified substructures have
been correlated with an enhancement in the number of H i detections (Hess et
al., 2015; Jaffé et al., 2013, 2016).

The degree of substructure within the Hydra I Cluster is uncertain and
difficult to quantify. For example, Baier & Oleak (1983) suggested an
enhancement in the galaxy number density to the south of the cluster center,
but it lacks the signatures of substructure that are often identified with
statistical tests within 45′ of the cluster core (Stein, 1997; Lima-Dias et
al., 2021). Lima-Dias et al. (2021) only identify substructure in this
particular manner near the virial radius of the cluster. The X-ray halo is
smooth and symmetric about the cD galaxy NGC 3311 suggesting there have been
no major mergers in the last few gigayears (Tamura et al., 2000). It is
considered to be the archetype of a relaxed system (Yamasaki et al., 2002).
Finally, a histogram of the cluster velocities out to the virial radius
suggest it is close to Gaussian (Fitchett & Merritt, 1988). On face value,
this is difficult to reconcile with the relatively gas-rich nature of the
cluster.

The explanation for Hydra which may unite these observations was already
suggested by Fitchett & Merritt (1988) who found that, considering the
velocity distribution of galaxies only within $40^{\prime}$ of the cluster
core, Hydra appears to break up into 2-3 velocity substructures along the line
of sight (see also Bird 1995). Valluri et al. (in preparation) assess the
history of substructure studies in Hydra and use a mixture modeling algorithm
(McLachlan & Basford, 1988) which favors three structures along the line of
sight. They also show that Hydra galaxies at low velocity lie in a group which
is systematically less H i deficient than the main Hydra cluster,
strengthening the argument that they belong to a foreground group and are not
a highly blue-shifted cluster moving through the cluster center. Substructure
along the line of sight would account for the under-luminous nature of Hydra
for its mass on the $L_{X}$-$\sigma$ relation (Fitchett & Merritt, 1988).

###  1.3 Outline

In this paper we present new deep H i, H$\alpha$, and optical broadband
observations of the Hydra Cluster. The H i observations were taken with the
MeerKAT Radio Telescope (Jonas & MeerKAT Team, 2016) during the first period
of 4K Open Time. These observations represent the deepest and highest spatial
resolution H i images of the Hydra I Cluster to date. We show conclusively
that NGC 3312 and NGC 3314a are experiencing RPS. We also present the first
resolved H i detection of NGC 3314b which is in an even more advanced state of
RPS due to its truncated H i disk. We complement the H i imaging with new
broad- and H$\alpha$ narrow-band DECam imaging and archival multi-wavelength
data from X-rays to infrared to estimate the total extent and amount of gas
and stars in the stripped material. Finally, based on these exquisite data
sets, we discuss the location of these galaxies within Hydra, in the context
of Hydra substructure, and attempt to constrain the galaxy orbits and
evolutionary history.

Throughout the paper we assume a $\Lambda$CDM cosmology with
$\Omega_{m}=0.27$, $\Omega_{\Lambda}=0.73$ and $H_{0}=70$ km s-1 Mpc-1. In
addition, we assume a distance of 58.6 Mpc to the Hydra Cluster center based
on Tully (2015) which uses a combination of cosmic flow models and measured
distances. This is consistent with the broad range of other estimates from the
literature, which have trended with time towards larger distances (e.g.
Fitchett & Merritt 1988; McMahon et al. 1992; Kourkchi & Tully 2017111The high
redshift cutoff for Kourkchi & Tully (2017) is 3500 km s-1 so this particular
distance estimate for Hydra is strongly biased due to excluding more than half
of the cluster in the calculation (Figure 7)., Lima-Dias et al. 2021).
Fundamental plane measurements for 11 E/S0 Hydra cluster galaxies give a
redshift-independent distance of $56.6\pm 4$ Mpc (Jorgensen et al., 1996).
While a pure luminosity distance of $D=59$ Mpc, taking into account bulk
motions relative to the 3K CMB, was adopted by Reynolds et al. (2021b) and is
consistent with that reported in the NASA Extragalactic Database
(NED)222http://ned.ipac.caltech.edu. Thus, our assumed value of 58.6 Mpc is
consistent with the best recent estimates. All distances discussed above are
the original measurements scaled to our preferred cosmology.

The paper is organized as follows. In Section 2 we describe the radio and
optical observations and data reduction, and processing of archival data. In
Section 3 we present our results and describe them in the context of ram
pressure stripping simulations. In Section 4 we discuss our results in the
context of the cluster as a whole and what they mean for cluster substructure,
cluster assembly, and cluster driven galaxy evolution. In Section 5 we
summarize our conclusions.

##  2 Data

###  2.1 H i observations

The 1.4 GHz spectral line data were observed as part of a MeerKAT-64 4K Open
Time project to mosaic the Hydra Cluster (SCI-20190418-CC-01; PI: C.
Carignan). The observations consisted of a 13 pointing mosaic that was
observed in two eight hour intervals on 5 July and 13 July 2019 using the
SKARAB correlator in 4k mode with full polarization. The MeerKAT L-band
receivers cover a frequency range from approximately 900–1670 MHz. In 4K mode,
the data are divided into 4096 channels with a channel width of 208.9 kHz
which equates to roughly 44 km s-1 at $z=0$. (See Jonas & MeerKAT Team 2016
for a full description of the MeerKAT system.)

To maintain consistent UV coverage across the mosaic, we cycled between
pointings, interspersed with regular visits to the gain calibrator. We
observed six mosaic pointing for three minutes each, followed by the gain
calibrator for two minutes (J1051-2023), and then the remaining seven
pointings before another visit to the gain calibrator. This cycle was repeated
10 times in each eight hour period for a total of 60 minutes on source per
mosaic pointing. The mosaic pointings were arranged in a hexagonal grid
overlapping by $\Theta_{FWHM}/\sqrt{2}$ for nearly uniform sensitivity (Condon
et al., 1998), where $\Theta_{FWHM}$ is the primary beam width which we
assumed to be $54.8^{\prime}$. The bandpass calibrators (PKS 0408-65) was
visited once for 8 minutes at either the beginning or middle of the
observation.

The data of each 8 hours observation were reduced individually using the
CARACal pipeline (Józsa et al., 2020a, b) on the ilifu computer cluster hosted
by the Inter-University Institute for Data Intensive Astronomy
(IDIA333https://www.idia.ac.za). For the H i data cube, only the horizontal
(HH) and vertical (VV) polarisations of the sub-band 1370 $\sim$ 1418 MHz (509
$\sim$ 11030 km s-1) covering the Hydra I cluster velocity were reduced at the
full frequency resolution (209 kHz, 44 km s-1 at $z=0$). The CARACal pipeline
performs the data reduction by making use of STIMELA, a radio interferometry
scripting framework based on container technologies and Python (Offringa et
al., 2010), to run many open-source radio interferometry software packages,
such as Cubical, CASA, WSClean and Montage, etc. Generally, we flagged the
radio frequency interference (RFI), did cross-calibration, self-calibration,
and continuum subtraction, created the H i cubes with CARACal pipeline for
each mosaic pointing. Before the cross-calibration, the data were flagged for
geometric shadowing by nearby dishes. Possible RFI in the calibrator data was
flagged using tricolour with the builtin strategy of
‘calibrator_soft_flagging.ymal’. While AOflagger was used to flag the possible
RFI in the target data with the builtin strategy of ‘firstpass_Q.rfis’, which
only inspects the Stokes Q amplitudes of the visibilities. Cross-calibration
was done with CASA in the standard way and all solutions were then applied to
the target field. The calibrated visibilities were imaged with WSClean and
self-calibration was done with CubiCal for three times. Before imaging the HI
spectral line, the continuum model visibilities were subtracted from the field
visibilities, then continuum was fitted and subtracted with 3 orders of
polynomials from the individual real and imaginary visibility spectra with
CASA mstransform task. The Doppler-tracking correction is included in the run
of CASA mstransform at the same time. The pure H i spectral line cube is
created with WSClean.

The data were combined in the image plane, by smoothing each pointing for both
days to the smallest common beam and then mosaicking. The final image has a
spatial resolution of $11.8^{\prime\prime}\times 18.0^{\prime\prime}$ and rms
noise of 0.13 mJy beam-1 channel-1, or a 1$\sigma$ H i column density
sensitivity of $N_{HI}=3.0\times 10^{19}$ cm-2 channel-1. In addition, we made
a second version of the mosaic by smoothing the data to $40^{\prime\prime}$
resolution which resulted in a cube with 0.31 mJy beam-1 channel-1, or an H i
column density of $N_{HI}=9.4\times 10^{18}$ cm-2 channel-1. The
$40^{\prime\prime}$ cube may have picked up up to $\sim 10$% more diffuse mass
around the galaxies, but the resolution is too poor to separate gas in the
galaxies from gas that is stripped, so in this work we only present results
from the high spatial resolution cube.

We conducted spectral line source finding using the well tested Source Finding
Application (SoFiA-2; Westmeier et al. 2021). SoFiA-2 generates masks around
each detected source, from which are derived moment maps and source
properties. For this paper we only consider the H i detections within 15′ of
the cluster center. The four sources presented here are H i bright and so the
characterization of the sources is not very sensitive to the exact input
parameters to SoFiA-2. A detailed description of the source finding will be
presented in a future paper on the full MeerKAT Hydra Cluster mosaic. See also
Appendix A for additional discussion of the H i masses calculated for each
object.

###  2.2 Optical broad- & narrow-band imaging

The optical data for the Hydra cluster were obtained with the Dark Energy
Camera installed on the CTIO Blanco 4m telescope (DECam; Flaugher et al. 2015)
over 3 nights on DECam (2021-04-09 to 2021-04-11; project 2021A-0117; PI: R.
Kotulla), covering the entire cluster in 6 bands (u, g, r, i, z, and N662: a
narrow-band H$\alpha$ filter). Observations were dithered to fill in gaps
between detectors to yield a complete sky coverage without holes, with
individual and cumulative exposure times as follows: u-band: $38\times 300$
seconds; g: 19x180; r: 19x180; i: 38x180; z: 19x120s; N662: 20x600s.
Observations in r and N662 were interleaved to minimize image depth and
quality differences and yield a better narrow-band continuum subtraction. Data
reduction for all DECam data was performed using the obs_subaru package in the
LSST science pipeline444https://pipelines.lsst.io, which performs overscan-
and bias subtraction, flat-fielding, astrometric calibration relative to GAIA
as reference, and photometric calibration relative to photometry obtained from
PanSTARRS. For the background subtraction we adapted the algorithm developed
initially for Hyper Suprime-Cam for use with DECam: data from multiple fields
obtained as part of our observing campaign were first normalized and then
combined to generate a sky-template across the full focal plane. In a second
step, this global sky template was then intensity scaled to each individual
frame and subtracted off. Compared to the standard detector-by-detector sky
estimation this approach yields slightly larger small-scale background
residuals, but – critically important for this project – does preserve
extended galaxy structures that otherwise are modeled as part of the
background and removed from the images. Table 1 summarizes image resolutions
and limiting surface brightnesses for all UV, optical, and near-IR datasets
used here.

Bandpass | Resolution | 1-$\sigma$ surface | scale†
---|---|---|---
| [arcsec] | brightness limit | [arcsec]
FUV | 4.2 | 28.4 mag/arcsec2 | 10.5
NUV | 5.3 | 28.7 mag/arcsec2 | 10.5
u | 1.4 | 27.0 mag/arcsec2 | 1.5
g | 1.5 | 28.2 mag/arcsec2 | 1.5
r | 0.9 | 27.8 mag/arcsec2 | 1.5
i | 0.9 | 27.4 mag/arcsec2 | 1.5
z | 0.9 | 26.4 mag/arcsec2 | 1.5
N662 | 0.9 - 1.1 | 27.2 mag/arcsec2 | 1.5
IRAC 3.6 | 1.8 | 0.33 $\mu$Jy/arcsec2 | 3.6
Table 1: Summary of image resolution and photometric depth for the UV,
optical, and near-IR datasets.
† Spatial scale used to estimate the 1-$\sigma$ surface brightness limit (more
smoothing or binning would allow to increase the surface brightness
sensitivity).

To calculate the continuum-subtracted H$\alpha$images we scale both N662 and
r-band images to the same instrumental zeropoint, and in a first step subtract
the narrow-band image from the broad-band image, accounting for the different
filter widths, to obtain a line-free image. Using this line-free image we then
subtract the underlying continuum from the narrow-band data to finally yield
the pure nebular emission map of the entire region. As the narrowband and
r-band filters have different filter widths and are not centered on the same
central wavelength, there remains a small, systematic trend of the required
scaling factor on the optical color of the region. Based on results from
stellar population modeling using GALEV (Kotulla et al., 2009) we estimate
that ignoring this trend adds uncertainties in the narrow-band flux of $<\sim
10\%$ for typical galaxy colors ($g-r$ in the range between 0.2 mag to 0.7
mag). Other sources of uncertainties in the narrow-band are small-scale
photometric uncertainties due to pixel-noise (equivalent to $6.5\times
10^{-4}$ M⊙ yr-1 kpc-2) and large-scale background variations due to galactic
cirrus and scattered star-light around bright stars (equivalent to $\sim
10^{-3}$ M⊙ yr-1 kpc-2, measured on a spatial scale of $\sim 1$ arcmin). For
all galaxies at the heart of this study this latter large-scale variation is
the dominating factor in the measured uncertainty for derived star formation
rates.

The resulting data, focused on the central part of Hydra, is shown as a multi-
band color composite in Figure 1.

###  2.3 Infrared imaging

We retrieved archival 3.6 $\mu$m Spitzer Space Telescope (Werner et al., 2004)
Infrared Array Camera (IRAC; Fazio et al. 2004) imaging for NGC 3312 and NGC
3314a/b, which we use to calculate the stellar mass surface density,
$\Sigma_{*}$, of the galaxies. The native units of the calibrated images are
in units of Jy steradian-1 which we convert to M⊙ kpc-2 assuming a mass-to-
light ratio of 0.47 (McGaugh & Schombert, 2014). In the case of NGC 3314 we
also have a stellar surface density map derived from stellar population fits
(see Figure 5 and Section 3.1.3) to our optical data detailed above that
provides an excellent confirmation (scatter $<$ 0.2 dex) to our results
derived from the Spitzer infrared data. The data quality is summarized in
Table 1.

The 3.6 $\mu$m Spitzer maps are then used to infer the $\Sigma_{*}$ where ram
pressure stripping is occurring (see Section 3.1). The H i contours where we
infer the hot ICM is interacting with the cold ISM span a range of values.
Therefore, we use maps of binned $\Sigma_{*}$ to estimate the approximate
stellar mass surface density where ram pressure is occurring and include the
bin widths in the error budget. These maps are presented in Appendix B.

For comparison across our multi-wavelength data we also report the global
measurements of the stellar mass and star formation rates from the WISE
Extended Source Catalogue (WXSC; Jarrett et al., 2013, 2019) which include the
scaling relations of Cluver et al. (2014, 2017). The WXSC was specifically
created to characterize resolved galaxies using specially constructed native
resolution stacked mosaics derived from the classic WISE mission (Wright et
al., 2010) and the ongoing NEOWISE mission (Mainzer et al., 2011) imaging
products. The star formation rate and stellar mass of each galaxy reported in
Tables 5 and 6, respectively.

###  2.4 UV imaging

We retrieved archival GALEX (Martin et al., 2005; Morrissey et al., 2007)
near- and far-ultraviolet (NUV/FUV) imaging from the GALEX Science Archive,
covering all three systems of interest here. The Hydra region was observed as
part of the GALEX All-Sky Imaging Survey, with short exposure times of only
210s in each NUV and FUV, resulting in relatively shallow data with a
comparably low image resolution of $\sim 5$ arcsec. Nevertheless, it provides
sufficient image depth to detect even low-level star formation within the
stripped material as shown in Figure 6. The data quality is again summarized
in Table 1 while UV derived star formation rates are in Table 5.

Figure 1: Three galaxies in the foreground group are detected in HI: the ring
galaxy (top left), NGC 3312 (top right), NGC 3314a (center bottom). The
contours show that NGC 3312 and NGC 3314a are moving across the front of the
cluster. H i contours (grey lines) are plotted at (1.2, 2.4, 4.8, 9.6) $\times
10^{20}$ cm-2 which corresponds to (4, 8, 16, 32) times the rms in a single 44
km s-1 channel. H i contours for NGC 3314b are excluded for clarity (Figure
2). This false color image has been made from a composite of all optical DECam
data, including H$\alpha$. The small inset plot in the bottom right shows
x-ray contours based on ROSAT All-Sky Survey data Voges et al. (1999). The
position of our three galaxies of interest is shown with the black stars. The
blue box shows the size and location of the field presented in the large
panel. In both panels North is up and East is left, parallel to the figure
axes.

##  3 Results

Figure 2: NGC 3314b. _Left_ : DECam false color image with H i total intensity
contours at (1.2, 2.4, 4.8, 9.6) $\times 10^{20}$ cm-2 which corresponds to
(4, 8, 16, 32) times the rms in a single 44 km s-1 channel. North is up and
east is left, parallel to the figure axes. MeerKAT beam is the ellipse in the
top right. _Top right_ : H i total intensity gray scale. Red dashed and orange
dot-dashed lines indicate the direction of motion/wind through the ICM as
implied by the steep H i contours and the H i tail, respectively. Purple arrow
indicates direction to center of the Hydra Cluster (NGC 3311). _Middle right_
: H i intensity-weighted velocity map. _Bottom_ : Integrated H i profile. NGC
3314b is at $\sim$4700 km s-1; the emission below $\sim$3000 km s-1 is H i
associated with NGC 3314a in the foreground (see Figure 3).

Figure 3: _Top_ set of images is NGC 3312; _bottom_ is NGC 3314a. _Left_ :
DECam false color with H i contours at (1.2, 2.4, 4.8, 9.6) $\times 10^{20}$
cm-2 which corresponds to (4, 8, 16, 32) times the rms in a single 44 km s-1
channel. North is up and east is left, parallel to the figure axes. MeerKAT
beam is the ellipse in the top right. _Top right_ : H i total intensity gray
scale. Red dashed and orange dot-dashed lines indicate the direction of
motion/wind through the ICM as implied by the steep H i contours and the H i
tail, respectively. Purple arrow indicates direction to center of the Hydra
Cluster. _Middle right_ : H i intensity-weighted velocity map. _Bottom_ :
Integrated H i profile. The ultra diffuse galaxy, UDG 32, recently reported by
Iodice et al. (2021) is visible to the lower right of NGC 3314a, outside the H
i contours. Figure 4: LEDA 753342. _Left_ : DECam false color image with H i
total intensity contours at (1.2, 2.4, 4.8, 9.6) $\times 10^{20}$ cm-2 which
corresponds to (4, 8, 16, 32) times the rms in a single 44 km s-1 channel.
North is up and east is left, parallel to the figure axes. MeerKAT beam is the
ellipse in the top right. _Top right_ : H i total intensity gray scale.
_Middle right_ : H i intensity-weighted velocity map. _Bottom_ : Integrated H
i profile.

Figure 1 provides an overview of the gas rich galaxies southeast of the Hydra
Cluster core. NGC 3311, the central cD galaxy of the cluster is on the top
right edge of a false-color u,g,r,i,z, H$\alpha$ DECam image. H i detected
galaxies NGC 3312, NGC 3314a, and LEDA 753342 are shown with their H i
contours. In addition, we also detect NGC 3314b in H i (Figure 2), the close
to edge-on spiral galaxy seen in nearly perfect projection behind NGC 3314a.
The detected X-ray halo of Hydra, shown in the inset, is centered on NGC 3311
and extends symmetrically to a radius beyond NGC 3314a/b. (Fitchett & Merritt,
1988; Hayakawa et al., 2004).

The H i and optical imaging show some of our key results: (1) NGC 3314a is a
classic “jellyfish” galaxy with bright star forming regions coincident with an
extended H i tail; (2) NGC 3312 has a sharply swept-back disk with the H i
disk edges now trailing behind the galaxy as it moves through the cluster; (3)
the ring galaxy, LEDA 753342, is not obviously experiencing ram pressure; and
(4) NGC 3314b has already lost a significant amount of its H i gas through its
interaction with the ICM. As the deepest H i observations to date, with
excellent spatial resolution, combined with deep optical imaging, we report
the conclusive observation of active ram pressure stripping in NGC 3312, NGC
3314a, and NGC 3314b. The first two galaxies show not only gas loss through
truncated H i disks and extended H i tails (Figure 3), but active star
formation in H$\alpha$ streams and clumps in and around the stripped gas. NGC
3314b reveals a late-type galaxy at a more advanced stage of RPS.

The alignment of NGC 3314a and NGC 3314b is critical to interpreting the
relative location of the galaxies in Hydra and their motion through the
cluster. NGC 3314a is in the foreground at lower systemic velocity. NGC 3312
and LEDA 753342 share roughly the same velocity as NGC 3314a. At some 1850 km
s-1 higher velocity, NGC 3314b is both behind NGC 3314a – as evidenced by the
foreground dust lanes of NGC 3314a in optical imaging (Keel & White, 2001) –
and at higher redshift. This is unusual because it is expected that foreground
galaxies falling into the cluster should be red-shifted with respect to the
cluster velocity, and background galaxies should be blue-shifted: the opposite
to what we see here. The H i systemic velocity of NGC 3314b agrees with
previous identifications of the H$\alpha$ in long slit spectra (W. Keel,
private communication).

At projected distances of 110-220 kpc, all four of these galaxies are peculiar
for their H i-richness seen in close projection to the center of the Hydra
Cluster. By comparison, the closest H i-detected galaxy to the cluster center
in Virgo–a similar mass cluster–is NGC 4388 ($\log(M_{HI}/M_{\odot})=8.56$;
Oosterloo & van Gorkom 2005) which sits at 370 kpc and hosts a severely
truncated H i disk (Yoon et al., 2017). The closest H i detection in the
relatively lower mass Fornax Cluster is NGC 1427A
($\log(M_{HI}/M_{\odot})=8.32$), which sits at a projected distance of
$\sim$280 kpc from the center while the second closest H i detection is more
than twice as far from the cluster center (Loni et al., 2021). Further, the H
i morphologies of the four H i detections in Hydra, the orientation of their H
i and H$\alpha$ tails–in two cases perpendicular to the direction to the
cluster center–and their velocities relative to the cluster center tell a more
complex story than the simple radial accretion of H i-rich objects which has
been revealed to date in other clusters (e.g. Virgo, Chung et al. 2009; Coma,
Solanes et al. 2001; Molnar et al. 2021; and Antlia, Hess et al. 2015).

In the following subsections we discuss in detail the H i and optical
morphologies that are the result of ram pressure stripping as well as the
inferred direction of motion with respect to the cluster, and we compare these
to simulations. We also quantify the amount of stripped material that is
detected and the star formation that is occurring in the stripped material.
Table 6 lists the measured and derived properties of the H i detected galaxies
and Table 5 lists the total star formation in the galaxies and estimated star
formation in the stripped material.

###  3.1 Ram pressure stripping in action

The interstellar medium of galaxies can be stripped by the hot intracluster
medium if the pressure of moving through the medium is greater than the
gravitational restoring force of the galaxy (Gunn & Gott 1972; see also Köppen
et al. 2018):

$\rho_{ICM}v_{ICM}^{2}>2\pi G\Sigma_{*}\Sigma_{g}$ (1)

where $\rho_{ICM}$ is the density of the ICM, $v_{ICM}$ is the wind velocity
of the galaxy relative to the ICM, $G$ is the gravitational constant,
$\Sigma_{*}$ is the stellar mass surface density, $\Sigma_{g}$ is the gas
surface density. To be more precise $2\pi G\Sigma_{*}$ should be generalized
to represent the gravitational potential of the galaxy, including the
contribution of dark matter halo, however the above formulation allows for
easy comparison with measurable baryonic quantities.

In particular, the ram pressure condition from Gunn & Gott (1972) is for a
galaxy experiencing a wind face-on (wind angle of $0^{\circ}$). NGC 3312, NGC
3314a, and NGC 3314b all appear to be inclined to some degree with respect to
the ICM wind. A number of authors have attempted to generalize the formula and
found through simulations that there is no apparent correlation between
inclination angle with respect to the ICM wind and total mass loss, except for
the largest angle values ($>60^{\circ}$ Vollmer et al. 2001b; Roediger &
Brüggen 2006; Jáchym et al. 2009). Roediger & Brüggen (2006) also point out
that the tails are not always aligned with the direction of motion: their
orientation can also be dependent on the column density at which they are
measured and the observed inclination angle of the galaxy. What happens to the
stripped material likely depends on a combination of factors so that some
fraction of the gas may be heated and lost to the ICM; some fraction may be in
gravitationally bound clumps which can be stabilized or collapse to form
stars; and some gas may not escape the galaxy at all, but fall back onto the
disk (Vollmer et al., 2001b). In NGC 3312, NGC 3314a, and NGC 3314b we see
evidence for all three of these scenarios occurring.

####  3.1.1 H i morphology

Comparison with simulations can help infer a more accurate estimate of the
direction of motion. In the top right of Figures 2 and 3 we show the projected
direction of motion that is implied for the galaxies by (1) where the steepest
H i contours encounter the highest stellar mass surface density of the disk
(red dashed line) and (2) the angle of the H i tails with respect to the major
axis of the disk (orange dot-dashed line). In general, these agree within a
projected $\sim 30^{\circ}$ of each other. We also include the vector between
the galaxy center and the center of Hydra (NGC 3311; purple arrow).
Strikingly, for NGC 3312 and NGC 3314a, this vector is nearly perpendicular to
the implied direction of motion, in contrast to the simple picture of
infalling galaxies on radial orbits. Below, we compare the H i morphologies to
simulations of ram pressure at different wind angles from Roediger & Brüggen
(2006) (see also figures from simulations in Vollmer et al. 2001b, Roediger et
al. 2006, Roediger & Brüggen 2008).

  * •

_NGC 3312_ appears to be experiencing a relatively low wind angle (nearly
face-on). The central H i disk appears largely unimpacted by the ram pressure,
with the inner disk having a relatively flat H i distribution at $10^{21}$
cm-2, while the galaxy outskirts are swept back (Figure 3). The H i tail off
the southern edge of the galaxy is longer than the northern edge. Compared to
the top panels of Figure 5 in Roediger & Brüggen (2006) (see also Roediger &
Brüggen 2008), this would suggest that the wind angle is $\sim$30∘ from face-
on. This is also consistent with the sharpest dust lanes appearing on the
north side of the galaxy. The simulations also suggest that NGC 3312 has only
been under the influence of ram pressure for a few 100 Myrs, however, the
galaxy is also massive: its uncorrected H i line width spans almost 600 km
s-1. Thus it may able to hold onto gas at its center despite pressure from the
ICM.

  * •

_NGC 3314a_ appears to have a large wind angle (close to 90∘) which is seen
nearly face-on in the plane of the sky, akin to the central column of Figure 6
in Roediger & Brüggen (2006). Nearly the entire disk appears to be impacted by
the ram pressure. Compared to NGC 3312 and despite the confusing background
galaxy, this is almost certainly a relatively low mass spiral.

  * •

_NGC 3314b_ also appears to have a large wind angle, but is viewed close to
edge-on in the plane of the sky, akin to the left column of Figure 6 in
Roediger & Brüggen (2006) (or middle panel of Figure 2 in Jáchym et al. 2009).
However, we propose it is observed at much later times or is experiencing
higher ram pressure than displayed in the hydrodynamic simulation: the galaxy
has essentially no H i tail, although the H i contours are compressed on the
western side of the galaxy, and the H i disk appears to flare slightly on the
east side of the galaxy.

  * •

_LEDA 753342_ : based on the H i, this galaxy is either just beginning to
experience ram pressure, or does not appear to at all (Figure 4). The galaxy
is optically faint, and relatively narrow in H i and thus expected to be very
low mass. Its optical appearance, which could be a result of tidal
interactions, has yet to be explained as there are no obvious interacting
neighbors. The H i is symmetrically distributed while the optical has a ring-
or arrow-like (pointing down) morphology.

####  3.1.2 H i velocity maps

The intensity-weighted velocity (moment 1) maps provide information on the gas
motions along the line of sight. Since stripped gas most likely originated in
the rotating thin or thick disk of the galaxies, motion which deviates from
rotation can tell us about how the galaxy is moving along the line of sight,
or how drag from the ICM may be impacting the stripped gas (Haan & Braun,
2014).

  * •

_NGC 3312_ : the H i disk appears to follow normal galaxy rotation, and the
velocity of stripped gas is clearly imprinted with this rotation: the northern
tail is redshifted compared to the systemic velocity of the galaxy and the
southern tail is blue-shifted compared to the systemic velocity. However, if
we compare the velocity of gas in the tails relative to the velocities in the
disk from which the gas appears to originate (draw a straight line along the
length of the tail to where they intersect the main disk), the gas in both
tails is at higher velocities relative to the disk. If the stripped gas is
experiencing drag from interaction with the ICM, this would imply NGC 3312 is
moving towards us in its orbit.

  * •

_NGC 3314a_ : this velocity field is complex and the galaxy is viewed close to
face-on. We propose that the major axis is aligned just east of north: the gas
on the top and leading edge of the galaxy is generally redshifted with respect
to the systemic velocity, while the gas coincident with the southwest part of
the stellar disk is blue-shifted. The two H i tails differ in their velocity
structure. The western tail increases in redshift with distance from the disk
and contains the highest redshifted gas. The eastern tail is less blue-shifted
than the most blue-shifted gas in the disk. We suggest that the gas in the
tails originated in different parts of the galaxy and, if they are
experiencing drag, the velocities are also consistent with the galaxy moving
towards us with respect to the cluster. Another possibility may be that the
complex tail kinematics indicate gas is falling back onto the galaxy after it
has been stripped (J. Kenney, private communication), although simulations
suggest this may take a few 100 Myr before a steady state with backflow can be
reached (Roediger et al., 2015; Tonnesen, 2019).

  * •

_NGC 3314b_ : the H i kinematics appear generally consistent with a rotating
disk viewed close to edge-on. However, on the northeast side of the galaxy,
there is blue-shifted gas that appears to have been swept back by ram pressure
because the velocity contours are bent counter-clockwise. This would be
consistent with an ICM wind felt from the northeast as suggested by the H i
contours of the total intensity map (Figure 2).

Despite the above discussion, we caution that the MeerKAT data were observed
at low velocity resolution, $\sim 44$ km s-1, compared to the typical H i
velocity dispersion in late-type galaxy disks of $\sim$10 km s-1. Detailed
modeling of the ram pressure in these galaxies would benefit from future high
resolution imaging with MeerKAT in 32K mode.

Figure 5: Results from the spatially resolved SED fitting for NGC 3312. Left
panel: Stellar mass surface density; Middle panel:: Observed g-i color index;
Right panel: Inferred internal dust reddening E(B-V). Not shown are the
stellar population age map, showing consistently old ages throughout the
entire face of the galaxy, with the exception of several very young regions
coinciding with sites of active star formation where the youngest stellar
population outshines the underlying, older generations.

####  3.1.3 Star formation and stellar properties

In deep optical data, both NGC 3312 and NGC 3314a show obvious signs of
ongoing ram-pressure stripping, in the form of clumpy filaments extending
outwards towards the southwest from the main body of the galaxy (Figures 3 and
6).

Besides the filaments, NGC 3312 presents morphologically as a largely
undisturbed spiral galaxy in the optical with one peculiarity: the north-
eastern edge (i.e. the side opposite the filaments) is noticeably bluer than
the south-western side, with only weak spiral features visible in any optical
or infrared bands, and no apparent dust lanes (see the color-map in the middle
panel of Figure 5). The trailing, south-western side shows more typical
features with discernible dust lanes and spiral arms. At the boundary between
the blue and dust-free outer north-eastern parts and the more dusty central
parts, as well as near the front of the spiral arms towards the north, we
observe a number of bright star formation sites luminous in both narrow-band
H$\alpha$ and GALEX near- and far-UV (Martin et al., 2005; Morrissey et al.,
2007).

To determine the underlying cause for the bluer colors in the NW part of the
galaxy – both a younger stellar population as well as reduced dust content
would be plausible explanations – we performed a spatially resolved spectral
energy distribution (SED) fitting. After extracting the relevant areas from
the full mosaic data, we binned the data 2x2 to a pixel-size of 0.66 arcsec to
improve signal-to-noise and extracted five-band SEDs for each pixel. For
comparison we generated a synthetic stellar population model using GALEV
(Kotulla et al., 2009), assuming a star formation history with constant star
formation rate (SFR), solar metallicity, and a Salpeter (1955) initial mass
function. Foreground dust reddening was based on Schlafly & Finkbeiner (2011)
and taken from NED to convert the observed reddening into band-pass specific
extinctions using the empirical calibrations from Yuan et al. (2013). Free
parameters during the fit were the galaxy age, stellar mass, and dust
extinction (assuming a Calzetti et al. 2000 extinction law). This yielded, for
each pixel, a corresponding stellar population age, stellar mass, and dust
content.

The results of this modeling are shown in Figure 5; not shown are the stellar
population age, which were nearly constant near the maximum allowed age of
13.6 Gyr. The only exceptions were several very young regions that coincide
with the location of sites of intense star formation activity mentioned above.
As expected, the stellar mass distribution is largely smooth and centrally
concentrated. One key finding is the distribution of dust extinction; the
bluer regions on the NE side of the galaxy have significantly lower dust
contents than the central and SW parts of the galaxy. In the context of ram-
pressure stripping this suggests that the interaction with the ICM on the
wind-facing side of the galaxy either removed or destroyed most, if not all
dust in this area, exposing the intrinsic, unobscured stellar populations
Crowl et al. (2005); Abramson et al. (2011). This lack of dusty, cold gas and
the associated lack of star formation activity then also explains the absence
of obvious spiral structure in this region. Finally, we note that the stellar
mass surface density shows no signs of tidal interactions on either the north
or south side of NGC 3312, in agreement with deep imaging in both the optical
and mid-IR from Spitzer.

###  3.2 Quantifying the stripped material

NGC 3312 and NGC 3314a are mostly likely still early in their interaction with
the cluster ICM and so while gas has been removed, we suppose that most of the
stripped material is still in the form of H i in the tail, or has collapsed to
form stars. In the following subsections we quantify the amount of stripped
material in both H i and ongoing star formation to make an order of magnitude
quantification of the fate of the stripped H i.

####  3.2.1 Stripped H i

In order to estimate the amount of H i that has been stripped from its
original location in the galaxy disk, we use a combination of the H i contours
and binned stellar mass surface density from _Spitzer_ IRAC images (Figure 9).
We assume that the majority of the gas seen in projection outside the stellar
disk is stripped. Therefore, we exclude H i that is coincident with the
stellar disk above a stellar mass surface density of 0.25 M⊙ kpc-2, and H i
that appears still connected to the stellar disk and with column densities
greater than $2.4\times 10^{20}$ cm-2. This is the same contour level at which
we will use to estimate the impact of ram pressure at the leading edge of the
galaxy disks in Section 4. In particular, the H i criteria allows us to
account for gas that is still in the face-on disk of NGC 3314a but, being
dominated by young stars and dust, the stellar component is not bright in the
infrared.

For NGC 3312, we estimate $5.0\times 10^{8}$ M⊙ or $8\pm 1$% of the measured
gas mas is in the stripped component. The H i on the southwest edge of the
galaxy, which also features star forming streams, extends to about 30 kpc from
the disk, as measured from perpendicular to the kinematic major axis.

For NGC 3314a, we estimate $1.1\times 10^{9}$ M⊙ or $38\pm 4$% of the measured
gas mass. The southernmost part of the H i tail extends to about 40 kpc from
the optical galaxy center.

For NGC 3314b, the H i disk is truncated with respect to optical disk, and in
fact it would seem that any H i tail that previously existed has been
destroyed through its ongoing interaction with the cluster–whether by heating
from the ICM or multiple encounters with other galaxies.

Figure 6: Color-composite images combining observations in GALEX near-UV (blue
channel), DECam r-band (green channel), and continuum subtracted H$\alpha$
(red channel) of NGC 3312 (top panel) and NGC 3314 (bottom panel). Each panel
also shows the regions used to estimate the current star formation rate in the
stripped material. The tail region of NGC 3312 is contaminated by several
foreground stars; these have been excluded (see crossed-out patches for masked
areas) before integrating the total H$\alpha$ luminosity of the region.

####  3.2.2 Star formation in stripped material

To estimate star formation rates in the stripped material we visually defined
regions encompassing most to all of the detectable emission outside the main
body of the galaxy in either UV or continuum-subtracted H$\alpha$. The regions
are shown as contours in Figure 6, overlaid on a color-composite made from
imaging in NUV, optical, and narrowband H$\alpha$. We only included regions
that likely belong to the tails based on location, shape, and brightness,
omitting more distant regions of H$\alpha$emission where this association
would have been less certain. (Defining regions algorithmically, for example
based on a specific surface brightness limit, would have resulted in
unphysically complex and fragmented regions, or encompassed large chunks of
empty sky that only adds noise with little to no incremental signal.) In the
case of NGC 3312 the star forming material is more diffuse, so the selected
region is larger to include as much of the low-surface brightness emission as
possible; several foreground stars, shown as crossed-out boxes, are excluded
from the region. In NGC 3314a we observe a more clumpy distribution and thus
could select significant emission in a number of brighter clumps. We note that
for both galaxies there is likely additional diffuse UV and H$\alpha$ emission
outside these regions, so our quoted luminosities and derived star formation
rates for these regions only present lower limits to the true values. However,
given the comparably larger uncertainties in discriminating between tail and
disk star formation, we believe they represent the best possible
approximations given the data at hand.

To integrate the observed fluxes across all bandpasses – continuum-subtracted
H$\alpha$, NUV, and FUV – we add up the flux in all enclosed pixels, excluding
the areas of the foreground stars in NGC 3312. To account for background
contamination and to estimate uncertainties, we placed the identical aperture
on a large number of random positions throughout the image. The median
brightness across these random apertures, after excluding outliers, was then
subtracted as background from the integrated fluxes in our science apertures.
The scatter of these random apertures is taken to represent the inherent
measurement uncertainties. All intrinsic measurements were then converted
first to physical fluxes and subsequently to luminosities, corrected for dust
extinction (see description for the optical SED fitting above). They are
listed in Table 5. Overall we find a reasonable agreement between results from
the different bandpasses for the outlying areas; measurements in the Near-UV
are most sensitive to dust-corrections, and a small overestimation of the
foreground extinction can account for the larger SFRs derived from the NUV.
Integrated values for NGC 3312 and NGC 3314 as a whole agree less well,
largely due to the unknown correction for dust internal to each galaxy. As
such the quoted values for the total FUV, NUV and H$\alpha$ represent lower
limits to the actual star formation rate.

Table 2: Star formation rates for the stripped material and total star
formation rates over the entire galaxy.

Bandpass | SFR in tails [M⊙ yr-1] | Total SFR [M⊙ yr-1]
---|---|---
NGC 3312 | NGC 3314a | NGC 3312 | NGC 3314a | LEDA 753342
FUV | $0.62\pm 0.24$ | $0.53\pm 0.05$ | $1.45\pm 0.35$ | $1.33\pm 0.08$ | $0.152\pm 0.010$
NUV | $0.72\pm 0.52$ | $0.89\pm 0.11$ | $2.60\pm 0.82$ | $2.29\pm 0.16$ | $0.245\pm 0.019$
H$\alpha$ | $0.77\pm 0.81$ | $0.38\pm 0.08$ | $3.99\pm 1.00$ | $1.70\pm 0.11$ | $0.145\pm 0.012$
WISE 12$\mu$m | – | – | $4.03\pm 1.40$ | $3.85\pm 1.33$ | $0.054\pm 0.021$
555See Figure 6 for regions and text for explanation. Disk SFRs can be
estimated from the total, minus stripped. Note that these estimates do not
account for intrinsic dust extinction within the galaxy and as such present
lower limits to the actual star formation rate (and thus apparent trends with
bandpass largely reflect the unknown effect of internal dust obscuration).

Table 3: Galaxy properties and ram pressure parameters

Galaxy | $M_{*}$ | $M_{HI}$ | $v_{sys}$ | $\sigma_{v}$ | $\Sigma_{*}$ | $\Sigma_{HI}$ | $\rho_{ICM}$ | $d_{cluster}$ | Velocity
---|---|---|---|---|---|---|---|---|---
Name |  $\log$(M⊙) |  $\log$(M⊙) |  km s-1 |  km s-1 | M⊙ kpc-2 | cm-2 | cm-3 | kpc | Substructure
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10)
NGC 3312* | $11.07\pm 0.11$ | 9.83 | 2851 | 690 | $8.5\times 10^{7}$ | $2.4\times 10^{20}$ | $2.5\times 10^{-4}$ | 550 | Foreground
NGC 3314a | $10.58\pm 0.12$ | 9.43 | 2849 | 690 | $7.5\times 10^{7}$ | $2.4\times 10^{20}$ | $1.1\times 10^{-4}$ | 800 | Foreground
NGC 3314b | 8.85 | 4657 | 440 | $2.5\times 10^{8}$ | $2.4\times 10^{20}$ | $9.1\times 10^{-4}$ | 300 | Bkgrd/clstr core
LEDA 753342 | $8.79\pm 0.16$ | 8.62 | 2743 | – | – | – | – | – | Foreground

666Column (1): galaxy name; (2) stellar mass derived from WISE; (3) measured H
i mass; we assume a typical measurement error of $\sim$15%; see also Appendix
A; (4) systemic velocity in the barycentric frame measured from the H i data
by SoFiA; (5) assumed velocity dispersion for the subgroup along the line-of-
sight to which the galaxies belong; (6) estimated stellar mass surface density
in the disk where ram pressure is occurring; (7) estimated H i column density
at which gas is being stripped; (8) derived ICM density; (9) implied distance
to the cluster center based on the Hayakawa et al. (2006) Beta model; (10)
group classification along the line-of-sight. Quantities derived assuming all
galaxies are at the distance of the Hydra Cluster, $D=58.6$ Mpc. Note that the
stellar masses of NGC 3314a/b cannot be disentangled, and so are reported
together.

##  4 Discussion

Multiple authors have suggested that NGC 3312, NGC 3314a, and LEDA 753342 may
belong to either foreground cluster substructure, or a foreground interacting
galaxy group which is seen in projection along the line of sight (Kurtz et
al., 1985; Fitchett & Merritt, 1988; McMahon et al., 1992; Valluri et al., in
preparation). As a foundation for our discussion, Figure 7 shows the velocity
distribution of galaxies with known redshift within in the Hydra Cluster from
the 6dF Galaxy Survey Data Release 3 (6dFGS; Jones et al. 2009). We indicate
the velocities of our four H i detections, as well as the giant cD elliptical
at the core, NGC 3311. We also show the velocity distribution of galaxies
within $40^{\prime}$ of the cluster center. Fitchett & Merritt (1988)
presented a similar plot to demonstrate evidence for the presence substructure
along the line-of-sight. Specifically, they suggested 2-3 substructures in the
velocity distribution at $\leq 3100$ km s-1 (foreground), $>3100$ km s-1
(cluster center), and peaking at $\sim 4400$ km s-1 (background). While our
presentation of the 6dFGS data contains 45% more galaxies within
$40^{\prime\prime}$ than the original Fitchett & Merritt (1988) histogram, the
suggestion of a velocity component centered around 2850 km s-1 is still
evident – as well as a relatively flat distribution at higher velocities.

While NGC 3312, NGC 3314a, and LEDA 753342 lie within the apparent velocity
dispersion of the Hydra Cluster, a pure redshift distance would place them at
46 Mpc as compared to the 58.6 Mpc cluster distance. Given the strong ram
pressure experienced by NGC 3314a and NGC 3312, and the lack of an X-ray
substructure at that location which could be attributed to the foreground
group (implying a hot local IGM responsible for the stripping), we find the 12
Mpc distance from Hydra implied by the objects’ redshift to be at odds with
the typical 1.4 Mpc virial radius of the cluster within which galaxies
experience strong ram pressure. Our multi-wavelength data, and in particular
the H i morphology, allows us to put new quantitative constraints on the
position and orbits of the galaxies that were not possible before, allowing us
to locate the position of potential cluster substructure.

Figure 7: Hydra Cluster galaxy velocities from 6dFGS (Jones et al., 2009).
Blue histogram includes all galaxies within $\sim 3^{\circ}$ ($\sim 2r_{vir}$)
of the cluster center. Orange histogram includes galaxies within $40^{\prime}$
of the cluster center. NGC 3311 is the nominal cluster center. NGC 3312, NGC
3314a, and LEDA 753342 appear to belong to substructure at lower systemic
velocity along the line of sight. NGC 3314b is substantially redshifted with
respect to the cluster center.

###  4.1 Constraining real galaxy distances to the cluster center

The strength of the ram pressure force felt by a galaxy is dependent in part
on the density of the medium through which it moves. The dominant X-ray
emission mechanism in clusters is thermal bremsstrahlung and is generally
modeled as an isothermal gas in hydrostatic equilibrium. From this Beta model
one can calculate the ICM density as a function of cluster radius (Cavaliere &
Fusco-Femiano, 1976, 1978; Sarazin, 1988):

$\rho(r)=\rho_{0}\left[1+\left(\frac{r}{r_{c}}\right)^{2}\right]^{3\beta/2}$
(2)

where $\rho_{0}$ is the central density of the cluster; $r$ is distance from
the cluster center; $r_{c}$ is the core radius of the cluster ICM; and $\beta$
is the power law index.

McMahon et al. (1992) modeled the Einstein IPC X-ray surface brightness using
a geometric deprojection to estimate the ICM density of $2.6\times 10^{-3}$
cm-3 at the projected location of NGC 3312. Using this value and the fact that
their lowest H i contour, at $N_{HI}=6\times 10^{20}$ cm-2, did not clearly
show evidence for ram pressure stripping, they argued that the galaxy must be
in the foreground of the main Hydra Cluster core. Recently Wang et al. (2021)
also showed that at the projected position in the cluster, NGC 3312 should be
stripped over the entire disk.

Figure 8: Beta models of the Hydra Cluster intracluster medium versus
estimates of ram pressure experienced by each galaxy. The lines provide the
best estimate for each galaxy. The shaded areas show an estimated uncertainty
for the contributions of dark matter, the wind angle, velocity through the
ICM, and errors in stellar mass surface density. These uncertainties have been
added in quadrature and are displayed as the vertical bars to the right of the
plot. See text for details.

We now re-examine this calculation in the context of our deeper H i data. We
combine the ram pressure stripping condition (Equation 1) with a Beta model of
the ICM (Equation 2) based on the diffuse X-ray emission to estimate the ICM
density as a function of radius and constrain the location of each galaxy
within a shell around the cluster (Figure 8).

We consider two different Beta models based on different X-ray observations.
In the first instance, we adopt the same central density as McMahon et al.
(1992) of $\rho_{0}=5.0\times 10^{-3}$ cm-3 and assume a value of $\beta=0.67$
and $r_{500}=823$ kpc estimated from ROSAT observations (Eckert et al. 2011;
$r_{500}=0.19r_{c}$). In the second instance, we use the same model adopted by
Reynolds et al. (2021b) to model ram pressure stripping in ESO 501-G075 at the
outskirts of Hydra. In this case, $\rho_{0}=1.1\times 10^{-2}$ cm-3,
$\beta=0.69$, and $r_{c}=90.9$ kpc based on XMM-Newton data from Hayakawa et
al. (2006). The difference between these two ICM models can be seen in Figure
8. The two models predict roughly the same values for ICM density at the
projected distances of NGC 3312 and NGC 3314a/b, however they vary by roughly
20% at larger radii.

The ram pressure condition in Equation 1 can then be rearranged to directly
compare the observed H i and optical galaxy properties with the modeled ICM
density:

$\rho_{ICM}=2\pi G\Sigma_{*}\Sigma_{g}/v^{2}_{ICM}.$ (3)

Our assumed values for each term are presented in Table 6. We estimate the
velocity of the galaxy with respect to the ICM from the velocity dispersion of
the cluster, $v_{ICM}=\sqrt{3}\sigma_{v}$, where $\sigma_{v}=440$ km s-1 for
the Hydra Cluster core (McMahon et al., 1992), or $\sigma_{v}=690$ km s-1
which includes all galaxies along the line of sight (Lima-Dias et al., 2021).
We note that for each galaxy, these $v_{ICM}$ values are consistent with the
line-of-sight difference in redshift between the galaxy and NGC 3311.

For the gas surface density, $\Sigma_{g}$, we take the second lowest H i
contour in Figures 2 and 3. This column density is consistent with where the
contours on the leading edge of the galaxies are steeply rising and where, on
the tailing edge, the H i morphology just shows signs of gas dragged out the
disks by ram pressure.

Finally, we estimate the stellar mass surface density, by taking binned values
of $\Sigma_{*}$, and identifying where our chosen H i contour intrudes on the
stellar disk (Figure 9). This is obviously imperfect for NGC 3314a/b. Based on
the examination, of Figure 3 and Figure 9, we believe that the $\Sigma_{*}$ at
which the steepest H i contours for NGC 3314a are observed lie roughly above
the inclined disk of NGC 3314b, and therefore assume they are mostly
contributed to by the (fainter) face-on NGC 3314a spiral. However, NGC 3314b
could be contributing a third of the stellar mass at that radius which we
include in our error budget in below. The $\Sigma_{*}$ at which the steepest H
i contours for NGC 3314b are more clearly observed to lie outside the
brightest part of the NGC 3314a and are estimated to be dominated by the
relatively edge-on NGC 3314b disk. By using Spitzer IRAC imaging, we assume
that the calculation of $\Sigma_{*}$ is not greatly impacted by dust
attenuation (typical dust extinctions in spiral galaxies have been found to be
$A_{V}\sim 0.6$ mag (Masters et al., 2010), corresponding to a $A_{\rm
IRAC}\sim 0.03$ mag using the scaling factors from Yuan et al., 2013), and
therefore our estimates are, at worst, an over-estimate due to the
superposition of the galaxies.

Figure 8 shows the Beta model versus the restoring force within NGC 3312, NGC
3314a, and NGC 3314b at the point where gas is estimated to now being
stripped. Where the Beta model and the estimated ram pressure density
intersect represents a best estimate of the galaxies’ current distance from
the cluster. In fact, there is significant uncertainty, which we attempt to
capture in the plot. At least four factors contribute to change the ram
pressure estimate in different ways. First, gas and stars are not the only
component of galaxies, and dark matter may also exhibit some gravitational
restoring force. This will be more important for low mass, low surface
brightness galaxies (NGC 3314a) than for high mass and/or high surface
brightness galaxies (NGC 3312, NGC 3314b). More mass in the disk will increase
the necessary ICM density, suggesting the galaxies are closer to the cluster
core. We include 50%, 30%, and 30% uncertainty for NGC 3314a to NGC 3314b, and
NGC 3312, respectively, in general agreement with the broad distribution of
the baryonic-to-dark matter mass fraction estimated by Martinsson et al.
(2013, see their Figure 14) for disk-like galaxies.

Second, galaxies experiencing an edge-on wind may require a greater force to
remove the gas, also effectively increasing the required ICM density in this
plot. This may be important in NGC 3314a and NGC 3314b, which are experiencing
a more edge-on ICM wind (likely ¿60∘). The impact of inclination is highlighy
uncertain. We choose to estimate a 30% uncertainty, consistent with the case
of medium ram pressure in Roediger & Brüggen 2006.

Third, our $v_{ICM}$ estimates may still be underestimates since, in all
cases, they are approximately equal to the $\delta v$ between the galaxy and
NGC 3311, allowing no budget for velocity in the plane of the sky. If the
galaxies are moving faster than we estimate, they require a lower ICM density
to remove the gas, implying the galaxies are further from the cluster center.
We include a factor of $\sqrt{(}2)$, equivalent to a $\sim$40% uncertainty for
all galaxies, to account for additional motion in the plane of the sky.

Finally, in the case of NGC 3314a and NGC 3314b we may be overestimating the
stellar mass surface density due to their overlap (and if for example dark
matter is unimportant in these disks). With less mass in their disks, they
require less ICM density to be impacted at the same level by ram pressure. We
include a 33% (as stated above) and 20% uncertainty in the overestimate of the
$\Sigma_{*}$ for these two galaxies, respectively.

In Figure 8 we show the contribution of these factors by plotting them in
shaded bands around our ram pressure “best estimate”. Finally, we add them in
quadrature to give a sense of the full error budget. Of course, these are not
formal errors, but are meant to be indicative of the unknowns. The
uncertainties may change the cluster-centric distance estimates by $\pm 15$%
for NGC 3312; $\pm 30$% for NGC 3314a; and ${}^{+60\%}_{-30\%}$ for NGC 3314b,
assuming the Beta model described by Hayakawa et al. (2006), or more if the
Beta model is shallower (McMahon et al., 1992; Eckert et al., 2011).

Nonetheless, our current best estimate is that NGC 3314b sits at $\sim$300 kpc
from the cluster center. NGC 3314a must be in the foreground, and we estimate
it is $\sim$800 kpc from the cluster center. NGC 3312 is estimated to be at
$\sim$550 kpc from the cluster center; the question is whether it is in the
foreground or background. NGC 3312 and NGC 3314a share essentially the same
systemic velocity, are experiencing an ICM wind from similar directions, and
appear to belong to the same velocity substructure group along the line of
sight (Figure 7). We propose that the favored scenario is that NGC 3312, NGC
3314a, and a handful of other galaxies belong to coherently moving
substructure in the foreground of the cluster, moving towards us. As discussed
later this implies they have passed the cluster pericenter and survived with
significant amounts of H i.

###  4.2 Timescales for stripping

To estimate a lower limit to the timescale for the stripping duration we can
use the maximum extent of the stellar material and the velocity of the galaxy
relative to the cluster ICM. Consistent with our earlier assumptions of
$v_{ICM}=\sqrt{3}\sigma_{v}$, we estimate a relative velocity of $\sim$1100 km
s-1 for NGC 3312 and NGC 3314a through the cluster. In a conservative scenario
we can expect that the stripped gas is instantly slowed down to ICM rest
velocity as it is stripped out of the galaxy (in reality, as shown by
simulations, at least some of the gas follows the main body in a slipstream
and thus at a reduced relative velocity; Roediger et al. 2015). Based on our
deep narrow-band imaging, we find faint SFR activity in the tail of NGC 3312
out to a relative distance of 36 kpc (2.1 arcmin; assuming distance of 58.6
Mpc) and potentially out to 66 kpc. For NGC 3314a, we find similar extents,
with detections out to distances of 37 kpc and maybe as far 59 kpc. Note that
these distances, derived from optical data, extend beyond the outermost
contours in the H i data where the low column densities are likely due to gas
being dispersed and/or being used up in star formation. With these distances
and relative velocities we derive a conservative estimate to the duration of
the stripping event of 32-59 Myr for NGC 3312 and 33-53 Myr for NGC 3314a.

Note that these timescales are lower limits to the real duration; older star
formation at larger distances are no longer detectable, and we have made no
corrections due to projection angles, or gas not instantly coming to a full
stop with respect to the ICM, which could easily add factors of several to the
estimate. In addition, we could be overestimating $v_{ICM}$ which would also
imply a longer timescale over which the material had been stripped (see also
our discussion of tangential orbits in Section 4.3). Estimates for ram
pressure timescales from simulations are of order a few 100 Myr (Schulz &
Struck, 2001; Roediger & Hensler, 2005), or longer when a varying ICM,
radiative cooling, or magnetic fields are taken into account (Tonnesen, 2019).

Comparing these timescales with the current SFR obtained earlier (see Table
5), we estimate a total stellar mass of the order of $1-48\times 10^{7}$ M⊙ in
the stripped material in each of the two galaxies. This is significantly lower
than the total mass estimate derived from H i, which is of the order of a few
times $10^{8}$ M⊙. However, if we account for typical star formation
efficiencies of the order of a less than 1 to a few percent (Evans et al.,
2009; Murray, 2011) both the estimates for total H i mass, current SFR, and
total new stellar mass agree with each other. Our estimate for the amount of
stripped mass is also consistent with recent simulations of galaxies under
similar $v_{ICM}$, $\rho_{ICM}$ conditions (Lee et al., 2020).

###  4.3 Final thoughts: galaxies on outfall, tangential orbits, past
interactions, formation of UDGs

With H i and H$\alpha$ tails nearly perpendicular to the cluster-centric
direction, and highly blue-shifted velocities with respect to the cluster
systemic velocity, NGC 3312 and NGC 3314a must be past their first pericentric
passage of the cluster and effectively on outfall. This implies they have also
already experienced their strongest maximum pressure and it will only get
weaker, until they hit apocenter of their orbits and infall again.

The general wisdom on how gas loss occurs in clusters is mixed. On the one
hand, simulations or analytical arguments have been made that infalling
galaxies are mostly stripped of gas by the time they reach the center of the
cluster (e.g. Brüggen & De Lucia 2008). In this context, how NGC 3312 and NGC
3314a could have passed pericenter and still be so gas-rich is not clear.
Whether NGC 3314b is still on first infall or outfall, its H i content has
also survived to reach significantly close to the cluster center. On the other
hand, more recent analysis suggests that galaxies can survive pericentric
passage with some of their H i reservoir intact if they are massive enough
($M_{*}\gtrsim 10^{9.5}$M⊙; Cortese et al. 2021). Vollmer (2009) describe a
stripping model in Virgo in which galaxies can still have an H i reservoir 200
Myr after experiencing peak ram pressure. It may also be that the bulk motion
of the galaxy group substructure IGM through the cluster can protect the
galaxies from the harshest effects of RPS, resulting in higher H i masses
after their pericenter passage. If LEDA 753342 sits in the leeside of the
infalling group, that may explain why it does not show signs of RPS.

In the absence of detailed modeling (see Conclusions), we also consider
whether NGC 3312 and/or NGC 3314a could be on more tangential orbits with
respect to the cluster to explain the orientation of their H i tails. Other
instances of large H i tails suggesting tangential orbits have been seen: for
example, NGC 4388 crossing in front of M86 in Virgo (Oosterloo & van Gorkom,
2005). In the case of tangential orbits, most of the galaxies’ velocity would
be in the plane of the sky. The estimated circular velocity would be
$v_{circ}=\sqrt{(}GM_{cluster}/r)=795-840$ km s-1, where $M_{cluster}\equiv
M_{500}\sim 0.9\times 10^{14}$ is the mass of the cluster within the
approximate orbit of the galaxies ($r_{500}\sim 700$ kpc; Zhao et al. 2013;
Piffaretti et al. 2011), and $r=550-800$ kpc is the cluster-centric distance
(Figure 8). This velocity is consistent with that estimated in Section 4.1, so
that the possibility of the ram pressure morphologies in NGC 3312 and NGC
3314a being due to tangential motion is within our uncertainties. We note that
jellyfish galaxies are estimated to have large impact parameters ($>400$ kpc;
McPartland et al. 2016), consistent with more tangential orbits, and
consistent with the cluster-centric distances we have derived in Section 4.1.

Tidal interactions could also have an impact on the gas disks in clusters.
McMahon et al. (1992) suggested that NGC 3312 and NGC 3314a may have
interacted in the past as a result of their proximity in projection. At our
assumed distance, the two galaxies have a projected separation of 128 kpc. In
galaxies which are suspected to be experiencing tidal interactions, this may
stir-up the disk and loosen the gas, making it easier for them to be stripped
by ram pressure (Vollmer, 2003; Chung et al., 2007; Sorgho et al., 2017). If
their gas is more loosely bound, they may also be further way from the cluster
center than has been inferred from purely the ram pressure criterion. We do
not detect any signs from ongoing and/or recent interactions in the stellar
bodies of these galaxies, based on both visual inspection and isophote
analysis, at least down to our surface brightness limit of $\sim 27$ mag
arcsec-2 in the $r$-band or $26.5$ mag arcsec-2 in $g$, in agreement with
findings by Iodice et al. 2021. We especially caution that what appears to be
tidal debrid near the northern edge of NGC 3312 is an artifact of its
disturbed spiral structure; a more close analysis of its isophotes reveals a
smooth outer profile with no offset from the center of its inner disk, making
tidal interactions with either NGC 3314 or any of its nearby (at least in
projection) companions very unlikely. Furthermore, we also do not detect any
tidal disturbances in its H imorphology. Nevertheless, it has also been shown
in simulations that the bridge between interacting galaxies in a cluster
environment can be destroyed by the ICM wind, while the ram pressure stripped
tails remain (Kapferer et al., 2008). Thus, we cannot completely rule out the
possibility of multiple environmental effects including tidal interactions.

We also consider the option that NGC 3312 and NGC 3314a belong to a large
foreground group unassociated with Hydra, and that the ram pressure they are
experiencing is not due to the cluster ICM but to a hot intragroup medium of
the halo to which the galaxies would then belong. This could explain their
significantly lower recessional velocity relative to the cluster systemic
velocity if the redshift is dominated by Hubble flow rather than movement
through the cluster potential well. Evidence to support this possibility
includes the Tully-Fisher distance for NGC 3312 which suggests it should be at
53-55 Mpc (Theureau et al., 2007; Tully et al., 2016)777These authors use a
h=0.57 and h=0.75, respectively. We have scaled the distance to h=0.7., 3-5
Mpc in front of Hydra. This possibility would imply the presence of an
intragroup medium at the level of few $\times 10^{-4}$ cm-3 that is undetected
in X-rays (Freeland & Wilcots, 2011). It would make NGC 3312 and NGC 3314a the
first example of massive galaxies exhibiting ram pressure in a group halo that
is not also detected in X-rays. However, to our knowledge, the only non-dwarf,
ram-pressure-stripped candidates identified so far have only been detected in
groups with X-rays counterparts (Freeland et al., 2010; Murugeshan et al.,
2021).

Finally, we return to the recent consideration of Iodice et al. (2021) as to
whether or not UDG 32 could have formed as a result of ram pressure stripping
in NGC 3314a. Unfortunately our data does not offer any confirmation of this
scenario. UDG 32 is not detected in H$\alpha$ within our narrowband
observations, and it is not detected anywhere in our H i cube which spans
recessional velocities of roughly 500-8100 km s-1. Most of the stellar
material we detect is compact and bright in H$\alpha$. UDG 32 by comparison is
more diffuse and somewhat redder. Nonetheless, Iodice et al. (2021) point out
that it still has similar colors to regions in the stripped tail, and since
UDG 32 is beyond the extent of the H i detected to date, we cannot rule out
that it is made of stars formed out of stripped material for which the H i has
dropped below currently detected levels. Deeper 32K H i observations with
MeerKAT that are currently under investigation may shed greater light on this
issue.

##  5 Conclusions

In this paper we have analyzed the H i morphologies and star forming
properties of NGC 3312, NGC 3314a, NGC 3314b and LEDA 753342, with a
particular emphasis on the stripped material. We have presented the deepest
and highest resolution H i imaging to date combined with very deep broad and
narrow-band optical images of the core of the Hydra cluster to show
conclusively that NGC 3312 and NGC 3314a are experiencing ram pressure
stripping, which is removing H i and resulting in star formation in the
stripped material of order 0.5 M⊙ yr-1. These galaxies have distinct H i and
H$\alpha$ tails extending a projected 30-40 kpc and 40-60 kpc from the disk.
We also resolve NGC 3314b for the first time and show that it is in an
advanced state of RPS, with no evidence for an H i (or star forming) tail.
Finally, LEDA 753342, an unusual ring galaxy, may be just starting to feel the
impact of the cluster environment.

The amount of material in the ram pressure stripped tails is dominated by the
H i component, which makes up 8 to 35% ($0.5-1\times 10^{9}$ M⊙) of the total
detected gas mass in NGC 3312 and NGC 3314a, respectively. The estimated
stellar material in the tails, from order of magnitude arguments, is of order
$0.1-5\times 10^{8}$ M⊙ by comparison. We argue that this is consistent with
estimated star formation efficiencies, as well as with a number of
simulations.

We use the exceptional multi-wavelength, spectroscopic, and kinematic data,
with the serendipitous alignment of NGC 3314a and NGC 3314b to estimate the
real distances from the cluster core of all galaxies obviously experiencing
ram pressure. In particular, the favored scenario is that NGC 3312 and NGC
3314a are moving towards us – based on the drag seen in the H i tails – as
part of a foreground substructure that has past its pericenter with a
significant amount of the gas still intact. Given the strength of the ram
pressure felt by NGC 3312 and NGC 3314a, the similar velocity of LEDA 753342
and its spatial proximity is likely a coincidence, rather than the three
galaxies belonging to the same cluster substructure or being at the same
distance. However, future X-ray observations, combined with simulations, are
needed to say for certain whether bulk motions of substructure within the
cluster could protect LEDA 753342 from the harshest effects of RPS. Finally,
NGC 3314b is likely on a highly radial orbit and is in a significantly more
advanced ram pressure state. Despite the relatively strong constraints we can
place on the location of these galaxies, there are number of uncertainties in
our calculations. We propose that future studies which combine higher spectral
resolution observations of the galaxies already obtained with MeerKAT, and
detailed hydrodynamic simulations may provide greater insight into the galaxy
orbits and the physical conditions they are experiencing within the cluster.

The Hydra Cluster has been studied before in H i with the Very Large Array
McMahon et al. (1992), and most recently with ASKAP as part of WALLABY Early
Science (Wang et al., 2021; Reynolds et al., 2021a). The results presented
here show the game-changing capability of the MeerKAT telescope to H i studies
in the nearby Universe, even at relatively low spectral resolution.

###### Acknowledgements.

We thank the anonymous referee whose comments helped improve the quality of
this paper. KMH would like to thank Enrichetta Iodice, Paolo Serra, and
William Keel for useful discussion, and Amidou Sorgho for the GBT beam
comparison. We also thank Eric Peng and Thomas Puzia for their generous
permission to use their N662 narrowband filter for our DECam observations. RK
is also thankful to Kathy Vivas and the staff of CTIO for their help and
support while preparing and executing the DECam observations, and for the LSST
helpdesk staff for assistance while adapting the LSST pipeline for use with
DECam data. KMH acknowledges financial support from the State Agency for
Research of the Spanish Ministry of Science, Innovation and Universities
through the “Center of Excellence Severo Ochoa” awarded to the Instituto de
Astrofísica de Andalucía (SEV-2017-0709) from the coordination of the
participation in SKA-SPAIN, funded by the Ministry of Science and innovation
(MICIN) and grant RTI2018-096228-B-C31 (MCIU/AEI/FEDER,UE). RK gratefully
acknowledges partial funding support from the National Aeronautics and Space
Administration under project 80NSSC18K1498, and from the National Science
Foundation under grants No 1852136 and 2150222. HC is supported by Key
Research Project of Zhejiang Lab (No. 2021PE0AC03); the South African
Department of Science and Innovation and the National Research Foundation
through SARChI’s South African SKA Fellowship within the SARAO Research Chair
held by RC Kraan-Korteweg. JSG thanks the University of Wisconsin College of
Letters and Science for partial support of this work. The MeerKAT telescope is
operated by the South African Radio Astronomy Observatory, which is a facility
of the National Research Foundation, an agency of the Department of Science
and Innovation. Part of the data published here have been reduced using the
CARACal pipeline, partially supported by ERC Starting grant number 679629
”FORNAX”, MAECI Grant Number ZA18GR02, DST-NRF Grant Number 113121 as part of
the ISARP Joint Research Scheme, and BMBF project 05A17PC2 for D-MeerKAT, and
partially supported by the South African Research Chairs Initiative of the
Department of Science and Technology and National Research Foundation.
Information about CARACal can be obtained online under the URL:
https://caracal.readthedocs.io/en/latest/. This work made use of the CARTA
(Cube Analysis and Rendering Tool for Astronomy) software (DOI
10.5281/zenodo.3377984 – https://cartavis.github.io). We acknowledge the use
of the Ilifu cloud computing facility – www.ilifu.ac.za, a partnership between
the University of Cape Town, the University of the Western Cape, the
University of Stellenbosch, Sol Plaatje University, the Cape Peninsula
University of Technology and the South African Radio Astronomy Observatory.
The Ilifu facility is supported by contributions from the Inter-University
Institute for Data Intensive Astronomy (IDIA – a partnership between the
University of Cape Town, the University of Pretoria, the University of the
Western Cape and the South African Radio astronomy Observatory), the
Computational Biology division at UCT and the Data Intensive Research
Initiative of South Africa (DIRISA).

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##  Appendix A HI mass check

Anecdotally, a number of observers have come across mismatched continuum or H
i fluxes from MeerKAT as compared to other instruments. A rigorous discussion
of our H i fluxes will be presented in a future paper on the full Hydra
Cluster mosaic. Here we provide a simple comparison between our H i fluxes and
those found in the literature.

First, we scale the H i detected by McMahon et al. (1992) with the VLA from
their assumed distance of 45 Mpc to 58.6 Mpc. After this, we find that we
report 50% more gas in NGC 3312, 18% more gas in NGC 3314a, and 12% less gas
in LEDA 753342. This is a wide range of values, so the difference cannot be a
simple a flux offset between the reduced data. The VLA data includes the most
compact array configurations, whose shortest baselines cover the largest
scales of these galaxies so we also do not expect much gas to be ‘resolved
out’. We expect some of the difference ($\sim$10 to 20%) between our H i mass
measurements of NGC 3312 and NGC 3314a simply comes from our deeper
observations detecting H i in the ram pressure stripped tails that were not
seen before. Considering LEDA 753342, the VLA observations have a resolution
of order the size of the galaxy, therefore is it possible the galaxy is
sitting on a positive noise peak which would bias the mass to a higher value.
On the other hand, LEDA 753342 is well resolved by the MeerKAT observations
and thus will not be susceptible to the same bias in such a strong way.

We are also able to compare with single-dish Green Bank Telescope (GBT)
observations of NGC 3312 and NGC 3314a reported in Courtois et al. (2009).
Unfortunately the line profile of NGC 3314a (NED) seems to be impacted by
either RFI or confusion with an absorbing source in the GBT beam which will
result in underestimate its H i mass. Nonetheless, without any other
considerations or flux corrections for the shape of the GBT beam, we only
report 14% more gas in MeerKAT as compared to the GBT observations, which
would suggest these observations may actually be in good agreement. For NGC
3312, the GBT spectrum looks free of confusing sources. In this case, we
primary beam correct our MeerKAT image with the GBT beam and find that we
still report 25% more gas.

If the flux calibration of our observations are high, the column densities at
which we evaluate the ram pressure criterium are also high. This would mean
that the galaxies may be more distant from the cluster than is implied in
Figure 8. The absolute H i mass measurement does not effect our estimate for
the relative fraction of H i in the stripped tails of NGC 3312 and NGC 3314a.

##  Appendix B Stellar mass surface density

Here we present the _Spitzer_ IRAC images converted to units of stellar mass
surface density. We use these maps for two purposes: (1) to estimate the
$\Sigma_{*}$ at which ram pressure is most impacting the disk, and (2) to
estimate where the H i is considered as “stripped” (see Section 3.2).

Figure 9: Spitzer IRAC images with H i contours of NGC 3312 (top), NGC 3314a
(center), and NGC 3314b (bottom) in units of stellar mass surface density. The
right image is the binned version of the left image (see Section 2.3). The
gray lines indicate the direction of ram pressure as suggested by either how
far the lowest H i contours penetrate into the stellar disk (dashed), or by
the approximate orientation of the H i tail (dot-dashed). See Section 3.1,
Figures 2, 3 for more details.