# Ultraviolet imaging observations of three jellyfish galaxies: Star formation
suppression in the centre and ongoing star formation in stripped tails

K. George1, B. M. Poggianti2, Neven Tomičić2,3,4, J. Postma5, P. Côté6, J.
Fritz7, S. K. Ghosh8, M. Gullieuszik2, J. B. Hutchings6, A. Moretti2, A.
Omizzolo9,2, M. Radovich2, P. Sreekumar10,11 A. Subramaniam11, S.N.
Tandon11,12, B. Vulcani2
1Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679,
Munich, Germany
2INAF-Astronomical Observatory of Padova, vicolo dell’Osservatorio 5 35122
Padova, Italy
3 Department of Physics and Astronomy, University of Florence, Via G. Sansone
1, 50019 Sesto Fiorentino, Florence, Italy
4INAF - Arcetri Astrophysical Observatory, Largo E. Fermi 5, 50127 Firenze,
Italy
5University of Calgary, Calgary, Alberta, Canada
6National Research Council of Canada, Herzberg Astronomy and Astrophysics
Research Centre, Victoria, Canada
7Instituto de Radioastronomia y Astrofisica, UNAM, Campus Morelia, A.P. 3-72,
C.P. 58089, Mexico
8Tata Institute of Fundamental Research, Mumbai, India
9Vatican Observatory, Vatican City, Vatican State
10ISRO HQ, Antariksh Bhavan, Bangalore 560094, India
11Indian Institute of Astrophysics, Koramangala II Block, Bangalore, India
12Inter-University Center for Astronomy and Astrophysics, Pune, India
<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

Spiral galaxies undergo strong ram-pressure effects when they fall into the
galaxy cluster potential. As a consequence, their gas is stripped to form
extended tails within which star formation can happen, giving them the typical
jellyfish appearance. The ultraviolet imaging observations of jellyfish
galaxies provide an opportunity to understand ongoing star formation in the
stripped tails. We report the ultraviolet observations of the jellyfish
galaxies JW39, JO60, JO194 and compare with observations in optical continuum
and $\mathrm{H}{\alpha}$. We detect knots of star formation in the disk and
tails of the galaxies and find that their UV and H$\alpha$ flux are well
correlated. The optical emission line ratio maps of these galaxies are used to
identify for every region the emission mechanism, due to either star
formation, LINER or a mix of the two phenomena. The star-forming regions in
the emission line maps match very well with the regions having significant UV
flux. The central regions of two galaxies (JW39, JO194) show a reduction in UV
flux which coincides with composite or LINER regions in the emission line
maps. The galaxies studied here demonstrate significant star formation in the
stripped tails, suppressed star formation in the central regions and present a
possible case of accelerated quenching happening in jellyfish galaxies.

###### keywords:

galaxies: clusters: intracluster medium, galaxies: star formation

††pubyear: 2018††pagerange: Ultraviolet imaging observations of three
jellyfish galaxies: Star formation suppression in the centre and ongoing star
formation in stripped tails–Ultraviolet imaging observations of three
jellyfish galaxies: Star formation suppression in the centre and ongoing star
formation in stripped tails

##  1 Introduction

Galaxies accrete hot gas from the intergalactic medium which cools and settles
in the disk as neutral and molecular hydrogen. Star formation is fueled by
this cold gas, which can be sustained by the continuous supply of gas. There
are conditions, either external or internal to the galaxy, that can alter the
gas supply or content and affect the molecular hydrogen required for star
formation. There are a few processes like stellar feedback, AGN feedback,
action of the stellar bar/bulge, which happen internally within the galaxy and
do not allow the gas to cool and thus sustain continuous star formation. The
external processes like major mergers, ram-pressure stripping and starvation
remove or cut down the supply of cold gas in galaxies. These processes have
different efficiencies and time scales, but the net effect is the gradual
shutdown of star formation within the galaxy, known as star formation
quenching. The time scales involved in star formation quenching can change
with the environment, galaxy stellar mass as well as the nature of different
processes that can act alone or in tandem (Man & Belli, 2018).

Ram pressure stripping is the process by which gas is removed from spiral
galaxies when they fall into galaxy clusters due to the action of the hot
plasma present in the immediate vicinity of the galaxy (Gunn & Gott, 1972).
Spiral galaxies in groups and clusters are observed to undergo this process
depending on the infalling galaxy mass and velocity with respect to the host
halo and the nature of the intracluster medium (ICM) (Hester, 2006). The
stripped gas can form tails of ionised gas extending to several kpc. Spiral
galaxies undergoing ram pressure stripping can resemble a jellyfish when
observed in ultraviolet (UV) and optical wavebands (Owen et al., 2006; Cortese
et al., 2007; Owers et al., 2012; Fumagalli et al., 2014; Ebeling et al.,
2014; Rawle et al., 2014; Poggianti et al., 2016; Bellhouse et al., 2017;
Gullieuszik et al., 2017; Boselli et al., 2018). The presence of ionised gas
in the tails of such "jellyfish galaxies" can be due to in situ star
formation, while other processes related to the interaction with the
surrounding hot ICM (e.g. thermal conduction, mixing etc) can also happen and
are not completely ruled out based on recent observations of selected galaxies
(Poggianti et al., 2019). Massive, hot and young stars (i.e., OBA spectral
types) emit the bulk of their radiation at ultraviolet wavelengths, which
makes UV imaging observations a direct probe to study ongoing star formation.
Hence, the ongoing star formation in the stripped tails of jellyfish galaxies
can be best studied from ultraviolet imaging observations. Previous UV
observations of jellyfish galaxies were done on galaxies residing in
relatively nearby Coma and Virgo clusters (Chung et al., 2009; Smith et al.,
2010; Hester et al., 2010; Fumagalli et al., 2011; Boissier et al., 2012;
Kenney et al., 2014; Boselli et al., 2018) but there is a dearth of UV
observations of jellyfish galaxies belonging to different clusters. It is not
clear whether the star formation along the tails and disk of jellyfish
galaxies depend on the host cluster properties. More observations in UV of
such galaxies in different clusters are therefore very much needed to
understand the details of ongoing star formation in jellyfish galaxies.

The goal of this study is to detect regions of ongoing star formation in the
stripped tail and the disk of three jellyfish galaxies observed in UV. One
edge-on and two face-on jellyfish galaxies are observed as part of GAs
Stripping Phenomena in galaxies with MUSE (GASP) survey. The main aim of GASP
survey is to investigate the gas removal process in galaxies using the ionized
gas observed with the MUSE integral-field spectrograph on VLT (Poggianti et
al., 2017). The ongoing star formation in the stripped gas can be directly
observed in UV and a better understanding of the progression of star formation
in jellyfish galaxies (both on tail and disk) is the motivation for observing
selected GASP galaxies in UV. The UV also helps to study any lack of star
formation in the disk of the galaxy and correlate that with energetic feedback
from active galactic nuclei (AGN) (see (George et al., 2019b)). The H$\alpha$
emission from the stripped tails of jellyfish galaxies could be caused by gas
ionised by OB stars with life times $\sim$ 10 Myr, whereas the UV emission is
coming from stellar photospheres with lifetimes $\sim$ 200 Myr (Kennicutt &
Evans, 2012). H$\alpha$ and UV emission thus traces different stellar
population ages and a comparison of regions in different wavelengths can give
clues to the time scales of star formation in jellyfish galaxies.

Previous studies of UV observations of galaxies from the GASP survey have been
published for two jellyfish galaxies, a nearly face-on and an edge-on, which
give a different view of star formation along the stripped tail George et al.
(2018); Poggianti et al. (2019). The observations presented here are from the
subsequent observations of three more galaxies. These galaxies are selected
based on the spectacular nature of strong ram-pressure stripping as evident
from optical B-band, H$\alpha$ observations and also on the viewing constrains
imposed by satellite. The three galaxies are assigned with Jclass=5 and 4
typical for extreme jellyfish galaxies (Poggianti et al., 2016). We have now
acquired UVIT observations of more GASP jellyfish galaxies with spectacular
stripping features, which will be presented in a followup paper (George et al.
in prep).

We describe below the UV observations of jellyfish galaxies JO60, JW39 and
JO194 along with the details of identification of UV knots along the stripped
tail and disk. We compare the $\mathrm{H}{\alpha}$ flux in the detected knots
with UV. We discuss the observations in section 2, and present the results in
section 3. We summarize the key findings from this study in section 4.
Throughout this paper we adopt a Chabrier initial mass function, concordance
$\Lambda$ CDM cosmology with $H_{0}=70\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$,
$\Omega_{\rm{M}}=0.3$, $\Omega_{\Lambda}=0.7$.

##  2 Observations, Data & Analysis

ID | RA | Dec. | Cluster | $\sigma_{cl}$ | logM$\star$/M$\odot$ | z | Jclass | BCGsep | BCGsep
---|---|---|---|---|---|---|---|---|---
| (J2000) | (J2000) |  | (km/s) |  |  |  | (Mpc) | (R200)
JO60 | 14:53:51.57 |  $+$18:39:06.4 | A1991 | 570 | 10.40 | 0.0622 | 5 | 0.633 | 0.48
JW39 | 13:04:07.71 |  $+$19:12:38.5 | A1668 | 654 | 11.21 | 0.0663 | 5 | 0.442 | 0.33
JO194 | 23:57:00.68 |  $-$34:40:50.1 | A4059 | 744 | 11.18 | 0.0420 | 4 | 0.286 | 0.17
Table 1:  Details of three galaxies used in this work. (1) GASP ID taken from
Poggianti et al. (2016); (2) and (3) equatorial coordinates of the galaxy
centre; (4) host galaxy cluster name; (5) velocity dispersion of the host
galaxy cluster from Gullieuszik et al. (2020); (6) logarithm of the galaxy
stellar mass (in solar masses from Vulcani et al. (2018)); (7) galaxy
redshift; (8) Jellyfish Class from Poggianti et al. (2016); (9) & (10)
separation between galaxy and BCG in Mpc and R200

The three galaxies presented in this paper have a spiral morphology and belong
to three different galaxy clusters. JO60 is seen almost edge-on and JW39 and
JO194 are seen face-on. All the three galaxies are undergoing extreme
stripping following the first infall to galaxy cluster according to their
position in the phase-space diagram showing the projected infalling velocity
normalized for the cluster velocity dispersion as a function of the projected
distance from the cluster center (Jaffé et al., 2018). Details of these
galaxies and the host galaxy clusters are given in Table 1 and a description
on the host cluster properties are given in Lourenco et al 2022 (submitted).
Lourenco et al. (2022) have carried out an in-depth analysis of several
indicators (both X-ray and optical) of the dynamical status of clusters. Here
we report some of their results for the clusters hosting the three jellyfish
galaxies studied here, but we refer to Lourenco et al. (2022) for details. The
host clusters of all three jellyfish galaxies show a bright cluster galaxy
(BCG) that matches the X-ray center, which is considered a sign of relaxation.
A1991 (host of JO60) is a relaxed cluster, with a concentrated and single
peaked X-ray emission. A1668 and A4059, instead, have a regular X-ray emission
but show small X-ray substructures. In this sense, A1668 and A4059 are similar
to the clusters hosting JO201 (A85) and JW100 (A2626), both with small X-ray
substructures. Based on the analysis of all WINGS/OMEGAWINGS optical
spectroscopic redshifts (Moretti et al., 2017), all three jellyfishes galaxies
analyzed in this paper belong to substructures (Biviano, Private
communication), as also JW100 ((Poggianti et al., 2019)) and JO201 ((Bellhouse
et al., 2017)). The projected distance of the three jellyfish galaxies from
the BCG are given in Table 1. The luminosity distance for JO60 is $\sim$ 261
Mpc and the angular scale of 1$\arcsec$ on the sky corresponds to 1.13 kpc at
the galaxy cluster rest frame. The luminosity distance for JW39 is $\sim$ 285
Mpc and the angular scale of 1$\arcsec$ on the sky corresponds to 1.22 kpc at
the galaxy cluster rest frame. The luminosity distance for JO194 is $\sim$ 213
Mpc and the angular scale of 1$\arcsec$ on the sky corresponds to 0.941 kpc at
the galaxy cluster rest frame.

The galaxy cluster fields hosting JO60, JW39, JO194 were observed at optical
wavelengths (B and V passbands) as part of the WINGS and OmegaWINGS surveys
(Fasano et al., 2006; Gullieuszik et al., 2015; Moretti et al., 2014, 2017).
The three jellyfish galaxies were observed with MUSE on the VLT under the
programme GASP using the spatially resolved integral field unit spectrograph
MUSE (Poggianti et al., 2017). We use the MUSE $\mathrm{H}{\beta}$ (4861.33
Å), [OIII] (5006.84 Å), [NII] (6583.45 Å), $\mathrm{H}{\alpha}$ (6562.82 Å)
and [SII] (6716.44 Å, 6730.81 Å) emission line flux maps of these galaxies in
this study.

The UV observations are from the ultra-violet imaging telescope (UVIT) onboard
the Indian multi wavelength astronomy satellite AstroSat (Agrawal, 2006). The
UVIT consists of twin telescopes, a FUV (130-180nm) telescope and a NUV
(200-300nm),VIS (320-550nm) telescope which operate with a dichroic beam
splitter. The telescopes are of 38cm diameter and generate circular images
over a 28′ diameter field simultaneously in all three channels (Kumar et al.,
2012). There are options for a set of narrow and broad band filters, out of
which we used the NUV N242W and FUV F148W/F154W filters for NUV and FUV
imaging observations for JO60. UVIT NUV channel stopped working since March,
2018 and we have only FUV imaging observations for JW39 and JO194 (Ghosh et
al., 2021). Table 2 gives details on the UVIT observations of the three
jellyfish galaxies.111PI: Koshy George, Proposal ID: G07${\\_}$002 JO60,
G08$\\_$002 JW39, A05$\\_$108 JO194.

The UVIT observations have an angular resolution of $\sim$
1$\aas@@fstack{\prime\prime}$2 for the NUV and $\sim$
1$\aas@@fstack{\prime\prime}$4 for the FUV channels (Subramaniam et al., 2016;
Tandon et al., 2017a). The NUV and FUV images are corrected for distortion
(Girish et al., 2017), flat field and satellite drift using the software
CCDLAB (Postma & Leahy, 2017). The images from multiple orbits are coadded to
create the master image. The astrometric calibration is performed using the
astrometry.net package where solutions are performed using USNO-B catalog
(Lang et al., 2010). The photometric calibration is done using the zero point
values generated for photometric calibration stars as described in Tandon et
al. (2017b) and updated in Tandon et al. (2020). The UV magnitudes are in AB
system.

The $\mathrm{H}{\alpha}$ emission line flux map of the three galaxies obtained
from MUSE observations, which we mention as $\mathrm{H}{\alpha}$ image for
convenience, is used in this study to compare with UV observations. The
1$\arcmin$ $\times$ 1$\arcmin$ field of view requires at least two MUSE
pointings to cover the disk and the tail of the galaxies studied here. We note
that H$\alpha$ imaging from VLT/MUSE has a plate scale of 0.2 arcsec pixel-1
with angular resolution $\sim$ 1$\arcsec$ whereas the UVIT plate scale is 0.4
arcsec pixel-1 with angular resolution of $\sim$ 1.2-1.4$\arcsec$ at the
position of the galaxy.

Galaxy | Channel | Filter |  $\lambda_{mean}$(Å) |  $\delta$$\lambda$(Å) | Int.time(s)
---|---|---|---|---|---
JO60 |  |  |  |  |
| FUV | F148W | 1481 | 500 | 12900
| NUV | N242W | 2418 | 785 | 12516
JW39 |  |  |  |  |
| FUV | F154W | 1541 | 380 | 17989
JO194 |  |  |  |  |
| FUV | F148W | 1481 | 500 | 37504
Table 2:  Log of UVIT observations

##  3 Results

###  3.1 UV and H$\alpha$ imaging

The young stars in the stripped tail of jellyfish galaxies can have different
ages depending on the time since the start of the stripping and subsequent gas
condensation to stars. The UV and H$\alpha$ imaging of jellyfish galaxies with
flux coming from different stellar spectral types can then show different
morphology. In addition, the H$\alpha$ flux can have a contribution from other
ionisation mechanisms like shocks, AGN, ionisation from older stellar
generations etc. Below we compare the UV and H$\alpha$ images of the three
jellyfish galaxies in our sample. We note that UV traces 100-200 Myr old star
formation in FUV and NUV passband imaging used here. We present here a
detailed analysis of the UV imaging data for JO60, JW39 and JO194.

####  3.1.1 JO60

Figure 1: The NUV and optical colour composite image of the region around
JO60. The NUV image is from observations with UVIT and the optical imaging
from the OmegaWINGS survey (Gullieuszik et al., 2015). The image is made by
combining NUV (coloured blue) and optical $B,V$ filter band pass images
(coloured red and green). 1$\arcmin$ $\times$ 1$\arcmin$ field of view of
VLT/MUSE pointings are marked with a white dashed line box. The direction
towards the bright cluster galaxy is shown with a white arrow. The image
measures 4$\arcmin$ $\times$ 4$\arcmin$ ($\sim$ 271 kpc $\times$ 271 kpc).

The UVIT observations of Abell 1991 cover a diameter of $\sim$ 1.9 Mpc at the
galaxy cluster rest frame. The UV and optical colour composite image of the
region centred on JO60 is shown in Figure 1. The galaxy is seen almost edge-on
with an unwinding pattern of tail at two sides, very similar to the scenario
proposed in Bellhouse et al. (2021) but not very clear due to the edge-on view
of the galaxy. We note a pattern of knots that is visible towards the south.
It is not clear whether these knots are part of the system as we lack MUSE
coverage for this region. We note that the UV emission along the tails is more
diffuse in nature with a lower number of knots comparing to the UV image of
the other jellyfish galaxy JO201 studied under GASP (See Fig 2 of George et
al. (2018) where the UV emission is seen more in the form of knots along with
diffuse emission close to the disk of the galaxy). We point out that JO60 is
seen nearly edge-on and the projection effects should also be taken into
account while looking for UV features like knots. Perhaps the knots are
present in the orthogonal direction to the line of sight and hence they are
not visible when viewed edge-on. However there are few UV knots that could be
star forming regions confined to the southern part of the tail of JO60. There
is a bright star just on the west of the southern side of the galaxy which is
visible in optical as well as NUV (but not in FUV) imaging data. The galaxy
seen at the west is not having redshift information to make any association
with JO60. The flux calibrated FUV, NUV and H$\alpha$ imaging data of JO60 are
shown in Figure 2 and 3. We overlay the contour that defines the galaxy main
body (in green) and the H$\alpha$ contours in dashed white colour. The galaxy
main body is determined based on galaxy boundary definition of isophotes from
the MUSE spectral continuum map as described in section 3.1 of Gullieuszik et
al. (2020). The prominent features that are present in the disk and tail are
detected in all three images. There are more UV bright knots in the lower tail
compared to the tail in the upper part of the galaxy.

(a)  (b)

Figure 2: The $\mathrm{H}{\alpha}$ flux contours in white colour is overlaid
over the FUV (a) and $\mathrm{H}{\alpha}$ image of JO60 (b). The green line
define the galaxy main body as described in Gullieuszik et al. (2020).  Figure
3: The $\mathrm{H}{\alpha}$ flux contours in white colour is overlaid over the
NUV image of JO60. The green line define the galaxy main body.

####  3.1.2 JW39

Figure 4: The FUV and optical colour composite image of the region around
JW39. This colour composite image was made by combining the FUV image from
UVIT with optical B,V imaging from the WINGS survey (Fasano et al., 2006).
1$\arcmin$ $\times$ 1$\arcmin$ field of view of VLT/MUSE pointings are marked
with a white dashed line box. The direction towards the bright cluster galaxy
is shown with a white arrow. The image measures 4$\arcmin$ $\times$4$\arcmin$
($\sim$ 293 kpc $\times$ 293kpc).

The UVIT observations of the Abell 1668 galaxy cluster correspond to a
diameter of $\sim$ 2 Mpc at the cluster rest frame. The UV and optical colour
composite image of the region centred on JW39 is shown in Figure 4. The UV
emission along the tails of the galaxy is prominent. The flux calibrated FUV
and H$\alpha$ imaging data of JW39 are shown in Figure 5. The central region
of the galaxy is dominated in the optical by the presence of a bulge.

The stripping process forms tails that appear as an unwinding of the arms of
the galaxy as reported in Bellhouse et al. (2021). There are regions on the UV
image, particularly at the lower side of the stripped tail with low levels of
UV flux contained in H$\alpha$ contour. H$\alpha$ and UV otherwise match very
well on the disk of the galaxy. We note that there is a cavity around the
central region source seen strikingly in UV and partly also in H$\alpha$. The
galaxy shows enhanced UV and H$\alpha$ emission towards the upper side of the
disk (marked with a white arrow in Figure 5a) at the region that most probably
is impacting on the ICM during the galaxy infall. The uppermost side of the
disk is however already devoid of both UV and H$\alpha$ emission, thus it has
been quenched already at least $\sim$ 100-200 Myr ago.

(a)  (b)

Figure 5: The $\mathrm{H}{\alpha}$ flux contours in white colour is overlaid
over the FUV image of JW39 (a) and the $\mathrm{H}{\alpha}$ image of JW39 (b).
The green line define the galaxy main body as described in Gullieuszik et al.
(2020). The white colour arrow points to the enhanced UV emission region and
therefore possible interaction site of galaxy disk with the ICM.

####  3.1.3 JO194

Figure 6: The FUV and optical colour composite image of the region around
JO194. This colour composite image was made by combining the FUV image from
UVIT with optical B,V imaging from the OmegaWINGS survey (Gullieuszik et al.,
2015). 1$\arcmin$ $\times$ 1$\arcmin$ field of view of VLT/MUSE pointings are
marked with a white dashed line box. The direction towards the bright cluster
galaxy is shown with a white arrow. The image measures 4$\arcmin$ $\times$
4$\arcmin$ ($\sim$ 226 kpc $\times$ 226kpc). The vertical stripe is due to a
gap in optical B and V imaging.

The UVIT observations of the Abell 4059 galaxy cluster correspond to a
diameter of $\sim$ 1.58 Mpc at the galaxy cluster rest frame. The UV and
optical colour composite image of the region centred on JO194 is shown in
Figure 6. The galaxy is seen face on with the stripped tails appearing as an
unwinding of the spiral arms (Bellhouse et al., 2021). The UV emission along
the tails is showing knots of star formation along with a diffuse component.
There are regions along the arms in the colour composite image in Figure 6
with no much UV emission but having detectable emission in B,V images. The
unwinding arm in the lower side of galaxy is seen as having the optical (B,V)
emission slightly displaced from the UV. There is a redder bar situated within
the disk seen in optical images. The flux calibrated FUV and H$\alpha$ imaging
data of JO194 is shown in Figure 7. There are regions in the tails,
particularly at the lower tail of the galaxy, that are quite bright in
H$\alpha$ but not in UV. The spatial offset between UV and H$\alpha$ emission
is consistent with the star formation occurring in spiral arms that are
progressively opening, as reported for JO194 using spectrophometric modeling
in Bellhouse et al. (2021). The galaxy shows enhanced UV and H$\alpha$
emission on east side of the disk (marked with a white arrow in Figure 7a) at
the region that most probably interacts first with the ICM during the galaxy
infall. This region also shows an offset between the galaxy main body (defined
from optical continuum emission) and UV emission suggesting an ongoing
quenching propagating along the disk due to ram-pressure stripping over the
last 100-200 Myr.

(a)  (b)

Figure 7: The $\mathrm{H}{\alpha}$ flux contours in white colour is overlaid
over the FUV image of JO194 (a) and the $\mathrm{H}{\alpha}$ image of JO194
(b). The green line define the galaxy main body as described in Gullieuszik et
al. (2020). The white colour arrow points to the enhanced UV emission region
and therefore possible interaction site of galaxy disk with the ICM.

###  3.2 Segmentation maps from UV and H$\alpha$ imaging

(a)  (b)  (c)

Figure 8: The segmentation map for galaxies JO60, JW39 and JO194 created from
ProFound run over FUV image. The colour scheme used to map the segments goes
from red for faint segments, to blue and to green for the brightest segments.

We used ProFound (Robotham et al., 2018) to identify UV segments from the FUV
images of the three galaxies. ProFound is a source finding package that
generates dilated segmentation map of pixels belonging to an individual source
and performs photometry that fully measures the associated flux from the
region. This is a different technique comparing to the traditional approach of
using circular or elliptical aperture based photometry. The isophote of the
source with irregular shapes called segments are dilated iteratively and
converge to measure the background subtracted total flux. The iterative
dilation ensures de-blending where segments are not allowed to grow into each
other. For the segments thus identified we obtain the central RA, Dec, total
flux and the total number of pixels within which the flux is measured. We
first run ProFound on the FUV image of three galaxies creating a segmentation
map of the galaxy and identifying various segments on the disk and stripped
tails of the galaxy. The positional information of segments from the
segmentation map created from FUV image is then used to extract the H$\alpha$
flux in the same region from MUSE data. We note that due to slightly coarser
spatial resolution of UVIT images, we decided to use UVIT based segmentation
map to measure the total UV and H$\alpha$ fluxes, rather than resampling the
Halpha image to the UVIT one. This make sure that the morphological
information of the segments are retained and same region position and area is
used on the UV and H$\alpha$ images of the galaxy to measure the flux. The
segmentation maps created for the three galaxies are shown in Fig 8. The
colour scheme used to map the segments change from red for faint segments, to
blue and to green for the brightest segments. The bonafide segments with
redshifts belonging to three galaxies are used in further analysis. The
segments that are background or foreground objects with different redshifts
are isolated based on the redshift information from the MUSE data cube and not
used in our analysis. We identify the segments falling within the galaxy main
body (shown in green line in Fig 2, Fig 5, and Fig 7) as contributing from the
galaxy disk and the rest from the tails.

The ProFound run on the JO60 FUV image detects 77 segments and by careful
analysis overlaying the segments on the images, we could find 67 segments in
common between FUV and H$\alpha$. There are 34 segments that are identified
from the disk of the galaxy and the remaining 33 belong to the stripped tail.
The ProFound run on the JW39 FUV image identifies 115 segments and we could
find 87 segments in common between FUV and H$\alpha$. There are 42 segments
that are identified from the disk of the galaxy and the remaining 45 belong to
the stripped tail of the galaxy. The ProFound run on the JO194 FUV image
identifies 59 segments out of which we find 54 segments in common between FUV
and H$\alpha$. There are 32 segments that are identified from the disk of the
galaxy and the remaining 22 belong to the stripped tail. Table 3 gives details
on the detected segments in common between UV and H$\alpha$ images for the
three jellyfish galaxies.

Galaxy | Disk | Tail | Total
---|---|---|---
JO60 | 34 | 33 | 67
JW39 | 42 | 45 | 87
JO194 | 32 | 22 | 54
Table 3:  Number of segments used in the study detected from UV images of
three galaxies.

###  3.3 Comparing UV and H$\alpha$ flux

We now compare the UV and H$\alpha$ flux from segments in the three galaxies.
The NUV, FUV flux values are plotted against the H$\alpha$ flux value of JO60
galaxy segments in Fig 9 with the blue colour assigned to FUV, green to NUV.
The segments on the disk are marked with open diamonds and the segments on the
tails are marked with filled diamonds. The FUV flux values are plotted against
the H$\alpha$ flux value in Fig 10 for JW39 and in Fig 11 for JO194. There is
a good correspondence between the UV and H$\alpha$ flux values for these
segments on the disk and the tail of all three galaxies. The segments on the
disk are having a larger area in general (see Fig 8a, Fig 8b and Fig 8c) with
larger UV and H$\alpha$ flux values as expected.

Figure 9: The FUV (in blue) and NUV flux (in green) of the 78 knots detected
for JO60 is compared to the $\mathrm{H}{\alpha}$ flux. The 23 knots detected
within the disk of the galaxy are shown by the open diamonds, while the filled
diamonds are tail knots.  Figure 10: The FUV flux of the 87 knots detected for
JW39 is compared to the $\mathrm{H}{\alpha}$ flux. The 42 knots detected
within the disk of the galaxy are shown by the open diamonds, while the filled
diamonds are tail knots. The red points are segments from the disk that cover
regions with LINER or composite emission as discussed in the text. Figure 11:
The FUV flux of the 54 knots detected for JO194 is compared to the
$\mathrm{H}{\alpha}$ flux. The 32 knots detected from the disk of the galaxy
are shown as open diamonds, while the filled diamonds are tail knots. The red
points are segments from the disk that cover regions with LINER or composite
emission as discussed in the text.

###  3.4 Emission line maps & UV imaging

Emission line diagnostic ratio diagrams (Baldwin et al., 1981) (BPT) created
from MUSE data can be used to distinguish different gas ionization process
operating in different regions of the galaxy. We now check whether the regions
with UV flux on the disk and tails of the galaxy are contained in star-forming
regions in the emission line maps. The BPT maps of JO194, JW39 and JO60 are
discussed in detail in Poggianti et al. (2017); Peluso et al. (2022);
Poggianti et al. (2019); Radovich et al. (2019) which we used here for our
analysis. The $\mathrm{H}{\alpha}$, [SII], [OIII] and [NII] emission line flux
maps are used to create the line diagnostic diagrams and distinguish regions
dominated by star formation, composite (AGN+SF), LINER and AGN. The LINER
emission region can in-fact be LINER or LIER. LIER corresponds to non-SF
emission coming from the ionisation due to older stellar population or shocks,
and spatially corresponds well with diffuse ionised gas (Tomičić et al.,
2021a, b). LINER corresponds to emission due to the central black hole
feedback. We show the regions corresponding to LINER (green), AGN (red),
Composite (orange) and star formation (blue) along with the NUV or FUV image
of the galaxies. JW39 and JO194 host a low-luminosity AGN at the centre,
detected as LINER and X-ray emission.

(a)  (b)  (c)

Figure 12: Emission line diagnostic map of galaxy JO60, JW39 and JO194 with
regions covered due to emission from LINER, composite (AGN+SF) and star
formation. FUV detected segments outline is overlaid in grey.

Figure 12a presents the emission line regions for JO60, which is dominated by
star formation along the disk and tail of the galaxy. There is a small
presence of composite emission at the centre and edges of the disk. The FUV
image is showing the presence of UV flux coinciding with the star formation
region in the emission line map. We note that the FUV, NUV and H$\alpha$
fluxes from the detected segments of this almost edge-on galaxy are having a
good correlation as shown in Fig 9. We also analysed the NUV image of the
galaxy and confirm that NUV and FUV shows identical features. Combining the
UV, H$\alpha$ and emission line region map thus gives a picture of ongoing
star formation happening in the tail and disk of JO60.

Figure 12b presents the emission line regions for JW39, which is dominated by
star formation along the rim of the disk and in the tail of the galaxy. There
is composite and LINER emission in the central region of the galaxy.
Interestingly, in this region, there is no significant UV flux other than the
point source at the centre, which could be the UV emission from the LINER
region. The integrated FUV flux density (Flux/Area) drops by a factor $\sim$ 3
with respect to rest of galaxy disk in an elliptical central region that has a
major axis $\sim$ 23 kpc and minor axis $\sim$ 17 kpc. Along the tail there
are a few spaxels with composite emission. The FUV image is showing the
presence of UV flux coinciding with the star-forming regions in the emission
line map.

Figure 12c presents the emission line regions for JO194. The emission line map
shows regions on the disk and the tail with composite and star formation.
There is a region close to the centre dominated by LINER emission. The FUV
image is showing the presence of UV flux coinciding with the star formation
regions in the emission line map. This is consistent both on the disk and the
tails in the sense there is a good correspondence between UV flux and star
forming region, which is not the case with the composite region. There is not
much UV flux at the central region of the galaxy, and in fact there is a
"suppressed UV emission" seen which coincides with the composite+LINER region.
There is a point source at the centre seen in FUV image and this coincides
with the LINER region in the emission line map. The integrated FUV flux
density drops by a factor $\sim$ 4 with respect to rest of galaxy disk in an
elliptical central disk region that has a major axis $\sim$ 19 kpc and minor
axis $\sim$ 14 kpc.

The emission line maps of JW39 and JO194 reveal regions with emission due to
composite and LINER. This is seen mostly at the centre for JW39, while for
JO194, an almost face-on galaxy, the LINER emission is central and the
composite emission is found both at the centre and extending partly along the
stripped tails. We checked whether segments with significant contribution from
composite and LINER emission deviate from the general UV-$\mathrm{H}{\alpha}$
correlation for star-forming regions. The red points in Fig 10 and Fig 11
correspond to such segments (7 for JW39 and 5 for JO194) from the disk of the
galaxy. For both galaxies the segment containing the LINER emission is the
offset point with a high H$\alpha$ flux with respect to FUV. The segments with
significant contribution from composite region are generally showing a similar
trend comparing to segments detected from star-forming regions in the emission
line diagnostic region maps. The segments with contribution from
composite/LINER regions are having slightly lower UV flux values for JW39
compared to JO194.

##  4 Discussion

The detected UV flux from the stripped tail and disk of the three galaxies
studied here is from the UV integrated spectral energy distribution of hot
young OBA spectral type stars on the main sequence. The brightest of these
stars will remain on the main sequence for $\sim$ 1-200 Myr which sets an
upper limit on the timescale of the star formation visibility as seen in UV.
The star formation in the stripped tails of these jellyfish galaxies happens
in situ and is not formed in the disk of the galaxy prior to stripping (in
fact stars in the disk are not affected by ram pressure effects). Thus the UV
is directly probing very recent star formation in the tentacles and disk of
these jellyfish galaxies. The $\mathrm{H}{\alpha}$ emission is due to the
recombination of hydrogen gas ionized by OB spectral type stars with a main
sequence life of $<$ 10 Myr (Kennicutt, 1998; Kennicutt & Evans, 2012). Hence
$\mathrm{H}{\alpha}$ and UV are sensitive to different star formation
timescales. It is therefore important to understand the features seen in both
UV and $\mathrm{H}{\alpha}$ imaging data to make a clear picture of ongoing
star formation along the galaxy disk and the tentacles. Stars formed more than
10 Myr ago will not contribute to the $\mathrm{H}{\alpha}$ emission as the
O-type stars will have left the main sequence. It is also important to note
that the $\mathrm{H}{\alpha}$ emission originates from the ionised gas. There
can be mechanisms other than photoionization by young/massive stars that can
ionise the gas, whereas the UV emission is from stellar photospheres. UV is
therefore a direct probe to understand star formation in jellyfish galaxies.
The good correlation between UV and H$\alpha$ segmentation for the three
galaxies presented here demonstrates that there is significant ongoing star
formation happening in these galaxies. This star formation can be enhanced in
the disk and the stripped tails, following the infall of galaxies into the
cluster. The location of most intense UV flux on the disk corresponds to the
region of interaction of the galaxy with the ICM. There is a displacement
between the galaxy main body traced in optical continuum and the UV emission
on the disk where the galaxy likely interacts with the ICM, hinting at the
start of quenching process due to ram-pressure stripping on the disk.

The scatter in the FUV and H$\alpha$ flux values in Figure 9, Figure 10 and
Figure 11 can in principle be explained by the contribution from other
emission mechanisms, but we note that here we didn’t perform any extinction
correction for both UV and $\rm H\alpha$. We aim to perform extinction
correction for a pixel-pixel SFR computation from UV and $\rm H\alpha$, which
will be reported in a follow up paper (Tomicic et al. in prep). The segments
on the disk with contribution from composite emission are not showing any
significant offset as described in previous section for JW39 and JO194, while
segments that cover the LINER emission show an offset.

The UV imaging of the three galaxies presented here shows loose spiral arms
which show signatures of arm unwinding, as in the optical. This effect was
studied in detail from optical and H$\alpha$ imaging of 11 GASP jellyfish
galaxies including JW39 and JO194 by Bellhouse et al. (2021). The gas is first
stripped from the outer regions of the spiral arm, which then collapses and
forms new stars at higher orbits as seen in UV imaging. This increases the
pitch angle from the disk and gives the visual appearance of unwinding of
spiral arms. We note that the unwinding phenomenon might be rather common in
jellyfish galaxies and refer to Vulcani et al. (2022), where a catalog of
unwinding galaxies is presented and their properties are characterized.

The ram-pressure can also drive the gas into the central regions of the galaxy
feeding the black hole forming a active galactic nuclei (AGN). This can
initiate energetic feedback in the form of outflows, jets into the cold
molecular gas needed for sustaining star formation. AGN driven ionized
outflows have been observed in GASP jellyfish galaxies (Radovich et al.,
2019). JW39 and JO194 hosts a LINER emitting source in the central region
which could be the effect of a black hole fed by gas inflow towards the centre
of the galaxy due to ram-pressure stripping (Poggianti et al., 2017; Peluso et
al., 2022). The outflows and energetic feedback from AGN can inhibit star
formation. This is demonstrated in observations and simulations for galaxies
in general (Sanders et al., 1988; Springel et al., 2005; Di Matteo et al.,
2005; Schawinski et al., 2007; Somerville et al., 2008; Hopkins et al., 2008;
Schawinski et al., 2010; Wang et al., 2010; Liu et al., 2015; Cheung et al.,
2016; Bing et al., 2019) and for jellyfish galaxies in particular (George et
al., 2019b; Ricarte et al., 2020). The emission line diagnostics based on the
BPT diagram show an emission dominated by LINER and composite-like emission in
the central region of JO194 and JW39. Interestingly this region matches with
that of reduced FUV flux. This might indicate that the region with reduced FUV
flux corresponds to suppressed star formation due to the effect of LINER
dominated ionisation in the same region with a possible connection to AGN
feedback. The composite regions in JO194 are too extended to be all likely
connected with the AGN. As the LINER emission is concerned, as discussed in
Radovich et al. (2019), the BPT ratios in JO194 could be explained either by
an AGN or by shocks, though the X-ray luminosities measured by Chandra could
indicate that an AGN may be indeed present, but possibly obscured. In the case
of JO194 there is also a stellar bar present as in the case for JO201. Stellar
bars can redistribute gas and quench star formation along the length of the
bar (Khoperskov et al., 2018; George et al., 2019a, 2020) which is further
complicated by the presence of AGN. Ram-pressure stripping is an efficient way
of removing gas necessary for star formation in the disk of the galaxy. This
process also compresses the gas in the disk and triggers starbursts which use
the remaining gas in a fast mode. The AGN feedback mechanism operating in the
centre of these galaxies undergoing gas stripping will enhance the conditions
necessary for star formation quenching. The ram-pressure stripping itself is
considered an efficient mechanism to quench star formation for galaxies in
groups and clusters. The evidences of suppressed star formation at the centre
therefore provides an interesting scenario of accelerated quenching. This can
expedite the migration of the galaxy position from the star forming main
sequence to the non-star forming passive population. The presence of UV
suppression in jellyfish galaxies seems ubiquitous in all face-on galaxies
(JO201, JW39, JO194) studied. This will be further explored in detail with the
ongoing UV imaging of a larger sample of GASP galaxies.

The stripped tails of jellyfish galaxies are not always dominated by star
formation. There exists observations of jellyfish galaxies with long H$\alpha$
tails with no significant UV flux (Boselli et al., 2016; Boissier et al.,
2012; Yagi et al., 2007, 2010, 2017; Jáchym et al., 2017; Laudari et al.,
2022). This implies a different star formation efficiency in the stripped
tails and in those cases the H$\alpha$ emission can be produced by other
mechanisms for these galaxies. The three galaxies presented here have
significant star formation along the tail with a good correspondence between
the UV and H$\alpha$ imaging.

The UV study of jellyfish galaxies in the local universe can give insights
into scenarios of ongoing and suppressed star formation in galaxies undergoing
morphological transformation in different environments. The current sample of
such galaxies is limited. The ongoing surveys like GASP with a multi-
wavelength data set on jellyfish galaxies with different inclinations can
provide more insights into the different phases of ram-pressure stripping, the
associated star formation and quenching. This can give important constrains to
future surveys of jellyfish galaxies at different redshift in the era of space
telescopes like Euclid, JWST and the ground telescopes like E-ELT.

##  5 Summary

We have studied the ongoing star formation in the GASP jellyfish galaxies
JO60, JW39, JO194 using the ultraviolet imaging observations from UVIT. We
note that very few UV observations of jellyfish galaxies exist in the
literature (Chung et al., 2009; Smith et al., 2010; Hester et al., 2010;
Fumagalli et al., 2011; Boissier et al., 2012; Kenney et al., 2014; Boselli et
al., 2018; Mahajan et al., 2022). These are for galaxies in nearby Coma and
Virgo galaxy clusters. It is therefore important to study the UV properties of
a statistical sample of such galaxies from different clusters. We are
observing spectacular cases of GASP jellyfish galaxies in UV, beginning with
JO201 and JW100 that were published in George et al. (2018); Poggianti et al.
(2019). Here we present UV imaging of three additional GASP galaxies selected
based on spectacular nature of tails as evident from optical B-band and
$\mathrm{H}{\alpha}$. We compared the FUV/NUV imaging data with the
$\mathrm{H}{\alpha}$ imaging data of galaxies and the following inferences are
made.

  * •

The main body and the stripped tails for all three galaxies show strong UV
emission with the emitting regions in UV and $\mathrm{H}{\alpha}$ showing a
very good correlation. This suggests that the major source responsible for the
emission are star-forming regions that contain also stars younger than 10 Myr.

  * •

There could be other components responsible for the $\mathrm{H}{\alpha}$
emission as revealed from the emission line maps. The UV emission shows a good
correspondence with star-forming and composite regions in the emission line
map but not with LINER regions.

  * •

Indeed, there are indications of reduced UV flux in the central regions of
JO194 and JW39 galaxy disks, which coincide with composite and LINER regions
in the optical emission line maps, while JO60 is too edge-on to inspect its
central regions. This suggests a suppression of the star formation in the
central regions of the disk of JO194 and JW39. This should be taken as
indication of outside-in and inside-out quenching happening in tandem in these
galaxies. Larger samples of jellyfish galaxies with deep UV imaging will be
able to ascertain how common it is to observe a reduced UV (and SF) in the
inner half of galactic disks (George et al. in prep.).

## Acknowledgements

This publication uses the data from the AstroSat mission of the Indian Space
Research Organisation (ISRO), archived at the Indian Space Science Data Centre
(ISSDC). UVIT project is a result of collaboration between IIA, Bengaluru,
IUCAA, Pune, TIFR, Mumbai, several centres of ISRO, and CSA. Based on
observations collected by the European Organisation for Astronomical Research
in the Southern Hemisphere under ESO program 196.B-0578 (MUSE). This project
has received funding from the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation program (grant agreement
No. 833824, GASP project). We acknowledge financial contribution from the
contract ASI-INAF n.2017-14-H.0, from the grant PRIN MIUR 2017
n.20173ML3WW_001 (PI Cimatti) and from the INAF main-stream funding programme
(PI Vulcani). This research made use of Astropy, a community-developed core
Python package for Astronomy (Astropy Collaboration et al., 2018).

## Data Availability

The Astrosat UVIT imaging data underlying this article are available in ISSDC
Astrobrowse archive
(https://astrobrowse.issdc.gov.in/astro_archive/archive/Home.jsp), and can be
accessed with proposal ID: G07_002, G08_002, A05_108. The data can also be
shared directly on request to the corresponding author.

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