# Exploring filament galaxies using AstroSat/ UVIT

Divya Pandey  Aryabhatta Research Institute of Observational Sciences, Manora
Peak, Nainital 263 002, India  Department of Physics and Astronomy, National
Institute of Technology, Rourkela, Odisha 769 008, India  Kanak Saha  Inter-
University Centre for Astronomy & Astrophysics, Postbag 4, Ganeshkhind, Pune
411 007, India  Ananta C. Pradhan  Department of Physics and Astronomy,
National Institute of Technology, Rourkela, Odisha 769 008, India

###### Abstract

We present results from our deep Far-ultraviolet (FUV) survey using
AstroSat/UVIT of a filamentary structure at $z$ $\sim$ $0.072$. A total of
four filaments comprising 58 galaxies were probed in our study. We detect 18
filament galaxies in our FUV observation. All filament galaxies are further
classified based on their photometric color, nuclear activity, and morphology.
The filaments contain galaxies with mixed stellar population types and
structures. We do not detect galaxies in our UVIT survey up to a distance of
0.4 Mpc from the filament axis, implying a dearth of recent star formation in
the inner region of filaments. We witness an increase in FUV specific star
formation rate (sSFR) of filament galaxies with increasing distance from the
filament spine (Dfil). On the contrary, no such gradient in FUV sSFR with
cluster-centric distance is observed in the case of cluster galaxies. The high
stellar mass filament galaxies (M⋆ $\sim$ 1011 M⊙) were more star-forming than
cluster galaxies in a fixed mass range. The FUV morphology of some filament
galaxies detected in the filament outskirts (Dfil $\sim$ 0.9 Mpc) is
comparable to or slightly extended than their optical counterpart. The mass
assembly of galaxies probed by estimating $(FUV-r)$ color gradients show that
more centrally star-forming galaxies reside closer to the filament axis
regardless of stellar mass. Our results prove that the likelihood of merger
interaction and gas starvation increases on approaching the filament spine. We
report a definitive and in-homogeneous impact of filaments on the galaxies
residing inside them.

Large-scale structure of the universe(902) — Ultraviolet astronomy(1736) —
Galaxy evolution(594) — Galaxy environments(2029) — Star formation(1569)

††facilities: AstroSat/UVIT, GALEX, SDSS††software: CCDLAB (Postma & Leahy,
2021), Source Extractor (Bertin & Arnouts, 1996), StatMorph (Rodriguez-Gomez
et al., 2019)

##  1 Introduction

Filaments are the most visually dominant component of the cosmic web. Around
50% of galaxies or baryonic mass in the Universe resides within filaments
while the structure occupies only $\sim$5% of the cosmic volume (Cautun et
al., 2014). Filaments inhabit the spaces between massive clusters and inside
voids (Tempel et al., 2014). Filaments are not just an assembly of galaxies;
they provide a dynamical pathway in channeling the galaxies to massive
clusters. Apart from the cool baryonic matter in galaxies, filaments are shown
to be the carrier of warm-hot intergalactic matter (WHIM; Galárraga-Espinosa
et al., 2021). The filamentary pattern in the cosmic web is well-illustrated
both in simulations (Colberg et al., 2005; Cautun et al., 2014) as well as
observations (e.g., Durret et al., 2003; Porter & Raychaudhury, 2005). The
detection of filaments within the cosmic web is an intricate task due to their
low environmental density. There are various algorithms available for
extracting filaments in the cosmic web, such as Bisous (Tempel et al., 2014),
DisPerSE (Sousbie et al., 2011), NEXUS+ (Cautun et al., 2013, 2014), etc.

Several large-scale phenomena, such as cosmic web enhancement and detachment,
are reported to occur inside filaments. A few filament galaxies are shown to
host ionized gas clouds in their circumgalactic region due to cosmic web
enhancement (Vulcani et al., 2019). On the other hand, it is theoretically
indicated that galaxies experience gas starvation caused by cosmic web
detachment inside filaments due to the non-linear interaction of the cosmic
web (Aragon Calvo et al., 2019). These physical mechanisms affect the ongoing
star formation activity of the constituent galaxies. Filaments also affect
their resident galaxies dynamically due to non-uniform large-scale matter
distribution (Lee & Pen, 2000; Tempel & Libeskind, 2013). Therefore, it is
evident that the filaments significantly impact the evolution of the galaxies
inside them.

There are a number of observation- (e.g., Kuutma et al., 2017; Mahajan et al.,
2018; Lee et al., 2021) and simulation-based (e.g., Singh et al., 2020)
studies performed to probe the structural and physical properties of filament
galaxies. Most of these analyses are based on optical imaging and
spectroscopic datasets. The local environment density around filament galaxies
and the distance of galaxies from the filament spine are found to impact their
growth (Bonjean et al., 2020). Studies focusing on undermining the properties
of galaxies as a function of their distances from the spine of filaments found
that the passive galaxy fraction increases closer to the filament axis (Kuutma
et al., 2017; Laigle et al., 2018; Singh et al., 2020; Bonjean et al., 2020).
The authors report a morphological transformation of filament galaxies from
late- to early-type, followed by a suppression in ongoing star formation when
approaching the filament spine from the void environment. An increased
likelihood of finding bright and red galaxies at a similar large-scale
luminosity density is observed in filaments than outside them due to a higher
number of elliptical galaxy populations (Poudel et al., 2017). Although
filaments are found to host red and passively evolving galaxies, they are
comparatively less massive and bluer than their counterpart in the clusters
(Mahajan et al., 2018). Meanwhile, filaments are reported to act as a
stimulator for star formation activities in galaxies, e.g., Darvish et al.
(2014) uncovers the possibility of enhanced gas accretion in filament galaxies
through inter-filamentary channels. Filaments provide an active environment to
trap cold gas, assist its cooling, and enhance star formation activity (Liao &
Gao, 2019). The discussion above reveals an inhomogeneity in the star
formation properties among filament galaxies.

The ultraviolet (UV) emission broadly divided into two wavelength ranges, far-
UV (FUV) and near-UV (NUV), from galaxies is an ideal tracer of recent star
formation in galaxies (Gil de Paz et al., 2007). A few investigations in the
past have probed the properties of filament galaxies with the help of UV and
UV-optical colors (e.g., Mahajan et al., 2018; Lee et al., 2021) using
observations from Galaxy Evolution Explorer (GALEX, Morrissey et al., 2007).
The authors examine the gradient in UV colors of filament galaxies when
approaching the filament spine. Another study inspected the changes in GALEX-
based NUV star formation rates (SFRs) and stellar mass of spiral galaxies
along filaments (Alpaslan et al., 2016). These analyses hinted towards a
definitive transformation in UV properties along the filament. However, none
of the studies have focused on deep FUV properties of filament galaxies as
GALEX All-sky Imaging (AIS) survey, which covers a maximum portion of the sky
and has a typical exposure time of 100 s with a resolution of 5′′ (Morrissey
et al., 2007). The FUV emission provides a clear separation between recently
star-forming and passive galaxies. The average timescale of FUV emission is
roughly 0.1 Gyr, even shorter than the NUV emission timescale of about 1 Gyr
(Gil de Paz et al., 2007). Therefore, deep and spatially-resolved FUV
observation of filaments could help us trace the ongoing recent star formation
along the filaments.

This work is based on a proposed deep FUV survey of filamentary structure
conducted by AstroSat/ Ultraviolet Imaging Telescope (UVIT, Tandon et al.,
2017). The rectangular box shown in Figure 1 encloses the targeted filamentary
structure. An enhanced spatial resolution ($\approx$ three times better than
GALEX) and greater sensitivity of UVIT would help us identify recently star-
forming galaxies in the filaments, which may be undetected in previously done
shallow UV imaging surveys. We have conducted a similar deep UVIT survey of
the Bootes void (red circle in Figure 1; Kirshner et al., 1981). The
observations led to the detection of a few void galaxy candidates, which were
undetected in GALEX observation. We reported a distinct blueward shift in
color of void galaxies compared to a sample of nearby galaxy (Pandey et al.,
2021). Voids and clusters are the rarest and densest environments,
respectively. At the same time, filaments are thought to represent the
intermediate environment density. A detailed understanding of star formation
activity across environments will be possible when enough studies explore
these environments in FUV. Our FUV observation of the filaments would be
utilized to investigate the gradient in UV emission across its width. We plan
to compare the results with galaxies in different environments.

Physical phenomena such as galaxy mergers, gas starvation and accretion, etc.,
occur inside filaments. These mechanisms, along with the star formation
history of a galaxy, can lead to different stellar mass assembly mechanisms
(Lin et al., 2019). We would compare the FUV morphology of filament galaxies
with their optical to identify mergers or gas accretion signatures. Moreover,
we plan to utilize multi-wavelength observations to probe the mass assembly of
filament galaxies to highlight the dominant physical mechanisms responsible
for galaxy evolution in filaments. This study would provide insights into the
role of cosmic filaments in shaping the star formation activity of galaxy
populations.

This paper is organized as follows: Section 2 describes the observational data
used in the work and UVIT data reduction techniques followed, and Section 3
explains our methodology applied in extracting filament galaxies from UVIT
observations. We derive the photometric and chemical properties of filament
galaxies in Section 4. Various aspects of FUV emission of the sample, such as
morphology and FUV SFRs are discussed in Section 5. In Section 6, we examine
the mass assembly of filament galaxies. Our findings are discussed in Section
7. A standard $\Lambda$CDM cosmology with ${\Omega}_{M}$ = 0.3,
${\Omega}_{\Lambda}$ = 0.7, and H0 = 70 km s-1 Mpc-1 is assumed in this paper.

Figure 1: Redshift cone diagram of galaxies in the line-of-sight of UVIT
observation. Each black dot in the figure represents a galaxy taken from SDSS
DR 16. Blue rectangular box surrounds the filamentary structure observed in
our UVIT survey. The box is zoomed in the inset plot. The galaxy distribution
in light-green color highlights one of the filaments (Filament ID 1029 in
Tempel et al. (2014)) studied in this work. The set of magenta and green lines
encloses the UVIT FOVs observed in the present study. The red circle
represents the boundary of the Bootes void while the pair of blue line
encloses the FOV observed by us to examine the properties of void galaxies
(Pandey et al., 2021).

##  2 Data

###  2.1 Observations

We primarily use proposed AstroSat/UVIT data observed in BaF2 filter centered
at ${\rm\lambda_{e}=1541}$ Å (Observation ID: A09_127T01_9000003816,
A09_127T03_9000003820; PI: Divya Pandey). The total field-of-view (FOV) of the
UVIT channel is 28′ in diameter. The pixel scale of the reduced image $=$
0.417′′, whereas the zero-point of the filter is $\sim$ 17.78 mag (Tandon et
al., 2020). Under the proposal, we observe a filamentary structure at z $\sim$
0.073. The observed structure is located close to one of the largest voids in
the northern hemisphere, the Bootes void (See Figure 1). We proposed to
observe the structure in two FOVs with an exposure time of 7 kilo seconds
each. The central coordinates of the two pointing are: $\alpha$ = 217.15∘,
$\delta$ = 46.12∘ (FOV2) and $\alpha$ = 215.85∘, $\delta$ = 46.1∘ (FOV1).
Figure 1 shows the direction of UVIT pointings on the cosmic web.

Apart from the UV survey, we use archival imaging and spectroscopic data from
Sloan Digital Sky Survey Data Release 16 ($SDSS$ DR 16, Ahumada et al., 2020)
and Galaxy Evolution Explorer (GALEX, Morrissey et al., 2007) in this work.

###  2.2 UVIT data reduction and analysis

We obtain the raw Level 1 version of the observations from AstroSat data
archive111https://webapps.issdc.gov.in/astro_archive/archive/Home.jsp. The
level 1 data is fed into CCDLAB (Postma & Leahy, 2021). CCDLAB is an automated
pipeline used for reducing the L1 version of UVIT data into science-ready
images. The data reduction includes drift correction, flat-field correction
and is further corrected for additional distortion and fixed noise pattern.
The corrected images from different orbits are stacked together through the
centroid alignment process to obtain a deep, science-ready image. CCDLAB
removes frames containing large numbers of photon counts due to cosmic ray
shower, leading to a reduction in the total exposure time of the image. The
effective exposure time (texp) of the final images was 5.7 ks and 6.3 ks. The
world coordinate system (WCS) was assigned to the final images using
Astroquery (Ginsburg et al., 2019) appended with CCDLAB. Astroquery uses the
GAIA (Gaia Collaboration et al., 2016) catalog to assign coordinates to each
pixel in the final image. The mean error in the astrometry was $\approx$
0.′′28\. Along with the science-ready images, we get corresponding exposure
maps as an output from the CCDLAB.

We run source extractor (Bertin & Arnouts, 1996) for detecting sources
observed in the two sets of FUV images. The background is detected in
automated mode. The detection and analysis threshold is fixed at 1.5$\sigma$
for source extraction and photometry. We consider the minimum number of pixels
required to classify an intensity distribution as a source (DETECT_MINAREA) to
be 9. The 3$\sigma$ limiting magnitudes for our observation in FOV 1 and FOV 2
are 24.79 mag and 25.07 mag, respectively. We detected a total of 3574 sources
combining both the FOVs. We perform Petrosian photometry to measure the
integrated magnitude of the detected sources. We expect to detect irregular
and disky galaxies in our UVIT FOVs wherein the Petrosian apertures may
recover nearly all flux from an exponential light distribution (Blanton et
al., 2001).

##  3 Identifying filament galaxies

Only a fraction of the detected sources in our the FUV observation belong to
the filaments. Hence, the identification of filament galaxies is imperative
part of our study. Multiple catalogs employ various algorithms to find
filaments and associated galaxies in large-scale structures (Tempel et al.,
2014; Carrón Duque et al., 2022; Alpaslan et al., 2014). We use a catalog
given by Tempel et al. (2014, hereafter Tempel catalog) in our study. The
authors use the Bious model to detect filaments in an extensive database of
galaxies observed with SDSS. The catalog provides a detailed list of filaments
and constituent filament galaxies. The information regarding the distance from
the nearest filament spine (Dfil), comoving distance, integrated magnitudes,
and optical luminosities for each filament galaxy are included in the catalog.

Figure 2: Sky map showing the distribution of galaxies in the line of sight of
our observation. Different symbols for each filament are used to indicate the
number of galaxies present in each of the four filaments. Green and magenta
circles enclose the UVIT FOV. The galaxies depicted by blue markers were
detected in the FUV survey whereas The red colored markers within FOVs
represent filament galaxies which were undetected.

We matched our UVIT source catalog with the Tempel catalog within a radius of
$\sim$2.′′5 to identify filament galaxies in our FUV observations. The
detected galaxies belong to four different filaments (Filament ID: 1029, 4188,
9987, and 9374 in the Tempel catalog) of different lengths. The four filaments
together contain 58 galaxies, of which 18 were detected in the UVIT sample. We
restrict our filament sample up to Dfil = 1 Mpc, as previous reports on
filament galaxies point out that the filament considerably impacts the
properties of galaxies situated within 1 Mpc distance from their axis (Tempel
& Tamm, 2015; Tempel et al., 2015). A sky map depicting the distribution of
galaxies in each filament is shown in Figure 2. The inset figure highlights
the UVIT FOVs, UVIT targeted and detected galaxies. The red-colored markers
within FOVs represent filament galaxies undetected in our survey, whereas the
galaxies depicted by blue markers were detected in the FUV survey. Prior to
UVIT, a shallow FUV survey (texp $\sim$ 100 s) of the observed FOVs was
conducted by GALEX. We find that six of the 18 filament galaxies detected by
UVIT remained missing in GALEX AIS catalog. In other words, these filament
galaxies are newly detected in our UVIT survey. Table 1 consists of
information on each filament, their constituent galaxies, and a comparison
between filament galaxies detected by UVIT and GALEX AIS observation.

Table 1: Details of the filament observed in our UVIT observation.

Fil. ID | Length | Total Galaxies | FUVUVIT | FUVGALEX
---|---|---|---|---
(1) | (2) | (3) | (4) | (5)
9987 | 3 Mpc | 7 | 5 | 5
9374 | 7.55 Mpc | 19 | 6 | 5
1029 | 13.69 Mpc | 24 | 6 | 2
4188 | 3 Mpc | 9 | 1 | 0

Col (1): Filament identity from Tempel et al. (2014), Col (2): Length of each
filament, Col (3): Total number of galaxies present in each filament within 1
Mpc from the filament spine, Col (4): Number of galaxies detected in UVIT FUV
and Col (5): Number of galaxies present in GALEX FUV catalog .

We extract Petrosian magnitudes corresponding to all SDSS broadband filters
and spectroscopic information for all 58 galaxies from SDSS DR 16. The
magnitudes used in this work are corrected for Galactic extinction using
Galactic dust extinction maps provided by Schlegel et al. (1998).

##  4 Properties of filament galaxies

We study the star-forming, nuclear, and structural properties of all 58
galaxies in the four filaments. As shown in Figure 2, only a portion of these
filaments were observed with UVIT. Therefore, we create two subsets among the
total galaxies observed with UVIT (within 1 Mpc from the nearest filament
axis): the FUV detected and FUV undetected galaxies. The histogram shown in
the upper left panel of Figure 3 highlights the cross-section distribution of
galaxies inside filaments from the filament axis for the entire sample and
both subsets. The filaments seem to be more dense in the inner region.
Interestingly, we detect no galaxy in our FUV observation up to Dfil $\approx$
0.4 Mpc.

Figure 3: (a) Distribution of distance from the filament spine (Dfil) for all
filament galaxies (black) with Dfil $\leq$ 1 Mpc, galaxies detected (blue) and
undetected (red) in our UVIT FUV survey. (b) g$-$r vs. Mr color-magnitude
diagram for filament galaxies. The contour is composed of sample of local
galaxies observed in SDSS DR 16. (c) BPT diagram used to separate HII region
galaxies from composite and seyfert galaxies. The solid and dashed polynomial
are described in Kewley et al. (2001) and Kauffmann et al. (2003a),
respectively. Each grey dot represents a galaxy present in Kauffmann et al.
(2003b) spectroscopic catalog. (d) Gini coefficient vs. M20 coefficient for
all filament galaxies. The markers have the same meaning as (a).

###  4.1 Color-magnitude diagram

We use optical g$-$r vs. Mr color-magnitude diagram (CMD) to classify all 58
filament galaxies into the blue cloud, red sequence, and green valley galaxies
(Blanton et al., 2005; Salim, 2014). The blue cloud contains star-forming
galaxies, mostly disk-dominated spirals, and irregulars. The red sequence
comprises star formation quenched galaxies. Lenticular and elliptical galaxies
mostly dominate this region. The intermediate region between the two modes of
distribution is known as the green valley, where galaxies with intermediate
star formation histories lie. The parallel lines dividing the CMD are given by
Blanton et al. (2005). The background galaxies in the sample are taken from
SDSS DR 16 within $z$ $<$ 0.1.

As seen in the upper right panel of Figure 3, our filament sample is spread
across the blue cloud and green valley region. We detect a comparable
population of star-forming galaxies and green valley galaxies in our FUV
survey. On the contrary, we only detected star-forming and blue galaxies in
our previous study of the Bootes void (Pandey et al., 2021). We highlight the
dominance of green valley galaxies in filaments which contain a mix of stellar
population. Such galaxies are in transition phase where the star formation is
declining but not entirely ceased. Galaxy interactions and mergers, feedback
from active-galactic nuclei (AGN) are few physical mechanisms that can drive a
galaxy to this state (Salim, 2014). The galaxies present in filaments studied
by us are brighter than Mr $\sim$ $-$20 mag. However, previous reports have
confirmed the presence of faint dwarf galaxies in filament environment as well
(Lee et al., 2021).

###  4.2 BPT diagram

The BPT (Baldwin, Phillips, and Terlevich, Baldwin et al., 1981) diagram is
ubiquitously used to classify and understand the ionization sources of
galaxies based on their emission line ratios. It provides insights into the
dominant mechanisms that power the emission lines observed in the spectra of
galaxies. We use the line flux ratio of [O III]/Hβ vs. [N II]/ Hα. The diagram
is divided into the three regions containing purely star-forming or ‘HII
region’ galaxies, composite galaxies, and galaxies with AGN based on
prescription given by Kewley et al. (2001) and Kauffmann et al. (2003a).

The emission line fluxes used in the diagram were corrected for internal
extinction prior to use. The reddening $E(B-V)$ value was determined using Hα
and Hβ emission lines following the Balmar decrement method given by
Osterbrock (1989). It can be seen in the bottom left panel of Figure 3 that
filaments contain a mix of populations in terms of nuclear activity. All
purely star-forming galaxies in the FOVs were detected in FUV observation. We
also detect a few galaxies with Active Galactic Nuclei (AGN) in our UV survey.
The AGN-host filament galaxies are star-marked in Table 2.

###  4.3 Morphological classification using non-parametric methods

We employ a non-parametric approach to determine the morphological
classification of filament galaxies studied in this work. This method requires
us to compute the Gini and M20 coefficients. The Gini coefficient ($G$) is a
statistical parameter based on the Lorentz curve (Lotz et al., 2004). The
parameter is computed with the following equation:

$G=\frac{1}{\bar{X}n(n-1)}\sum^{n}_{i}(2i-n-1)X_{i}$ (1)

here, Xi represents all pixel value , $\bar{X}$ is the average of all Xi, $n$
is total number of pixels. All pixel values are arranged in an increasing
order before calculating the $G$. For a homogeneous system, $G$ converges to
0, whereas $G$ = 1 when the entire light of a galaxy is concentrated in a
pixel (Glasser, 1962).

The parameter $M20$ corresponds to the second-order moment of light for the
brightest region in the galaxy (Lotz et al., 2004). The parameter is
calculated by summing over the brightest pixel values ($f_{i}$) until the sum
corresponds to 20% of the total galaxy flux. This value is normalized by Mtot
corresponding to the second-order moment of the total light in a galaxy. These
quantities are defined as:

$M_{tot}=\sum_{i}^{n}M_{i}=\sum
f_{i}\big{[}(x_{i}-x_{c})^{2}+(y_{i}-y_{c})^{2}\big{]}$ (2)

$M_{20}=\log\Big{(}\frac{\sum_{i}^{n}M_{i}}{M_{tot}}\Big{)},{\rm where}\
\sum_{i}f_{i}<0.2f_{tot}$ (3)

here, xc, yc corresponds to galaxy central coordinates.

We use statmorph, a python-based code provided by Rodriguez-Gomez et al.
(2019) to calculate the above-defined statistical parameters for $SDSS$
$r$-band images of all filament galaxies. The code creates internal
segmentation maps for the galaxies by using a threshold value for source
detection and convolution filter size, both given by the user. The maps are
used for calculating $G$ and $M20$. The galaxies whose fitting was suspected
were not included in the analysis.

The bottom right panel of Figure 3 shows the distribution for all filament
galaxies. We refer to Lotz et al. (2008) for demarcating the galaxy population
into spirals/ irregular, elliptical/ lenticular, and merger candidates on $G$
vs. $M20$ plane. The filament galaxies from our study are equally distributed
in the late- and early-type galaxy regions. Only two clear cases of galaxy
merger have been identified in the sample. These galaxies appear to have
elliptical morphology. We primarily detect disky-spiral galaxies in our UVIT
FUV survey. Filaments contain mixed galaxy populations compared to void
galaxies, which are primarily blue and disky in morphology (Kreckel et al.,
2012; Pandey et al., 2021).

Table 2: Detailed information on all 18 galaxies detected in UVIT FUV survey.

ID | RA | Dec | z | Dfil | FUV | FUV SFR | M⋆ | Fil ID
---|---|---|---|---|---|---|---|---
| [deg] | [deg] |  | [Mpc] | [mag] | [M⊙ yr-1] | [M⊙] |
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9)
1 | 215.894 | 46.176 | 0.072 | 0.90 | 19.784 $\pm$ 0.042 | 2.372 $\pm$ 0.100 | 10.14 | 9987
2 | 217.090 | 45.966 | 0.074 | 0.61 | 21.247 $\pm$ 0.088 | 0.855 $\pm$ 0.076 | 10.65 | 4188
3 | 216.988 | 46.202 | 0.073 | 0.53 | 19.660 $\pm$ 0.042 | 1.993 $\pm$ 0.085 | 10.110 | 1029
4 | 215.788 | 46.289 | 0.073 | 0.45 | 20.347 $\pm$ 0.056 | 1.919 $\pm$ 0.108 | 10.22 | 9987
5∗ | 215.819 | 46.146 | 0.073 | 0.98 | 20.594 $\pm$ 0.074 | 8.309 $\pm$ 0.612 | 11.20 | 9987
6 | 217.159 | 46.123 | 0.074 | 0.46 | 21.710 $\pm$ 0.102 | 0.880 $\pm$ 0.089 | 10.30 | 1029
7 | 215.817 | 46.028 | 0.073 | 0.81 | 21.440 $\pm$ 0.091 | 0.652 $\pm$ 0.059 | 10.02 | 9374
8 | 215.670 | 46.075 | 0.073 | 0.78 | 20.911 $\pm$ 0.080 | 1.017 $\pm$ 0.082 | 10.26 | 9374
9 | 215.772 | 46.149 | 0.074 | 1.00 | 19.649 $\pm$ 0.043 | 8.119 $\pm$ 0.350 | 10.78 | 9987
10 | 215.903 | 46.111 | 0.072 | 0.85 | 19.580 $\pm$ 0.041 | 4.851 $\pm$ 0.199 | 11.00 | 9374
11 | 217.302 | 46.240 | 0.075 | 0.85 | 20.410 $\pm$ 0.078 | 0.832 $\pm$ 0.065 | 9.91 | 1029
12 | 215.947 | 46.058 | 0.072 | 0.88 | 21.175 $\pm$ 0.082 | 1.006 $\pm$ 0.083 | 10.11 | 9374
13 | 217.152 | 46.316 | 0.075 | 0.88 | 19.206 $\pm$ 0.037 | 3.415 $\pm$ 0.126 | 10.58 | 1029
14 | 217.145 | 46.307 | 0.074 | 0.78 | 23.200 $\pm$ 0.256 | 0.084 $\pm$ 0.022 | 10.28 | 1029
15 | 215.838 | 46.247 | 0.072 | 0.74 | 19.629 $\pm$ 0.046 | 2.975 $\pm$ 0.138 | 10.50 | 9987
16∗ | 215.941 | 45.931 | 0.072 | 0.74 | 18.890 $\pm$ 0.033 | 2.977 $\pm$ 0.100 | 11.34 | 9374
17∗ | 217.108 | 45.969 | 0.074 | 0.43 | 22.755 $\pm$ 0.175 | 0.0023 $\pm$ 0.0004 | 11.41 | 1029
18∗ | 215.848 | 45.905 | 0.072 | 0.67 | 24.893 $\pm$ 0.466 | 0.008 $\pm$ 0.004 | 10.30 | 9374

Col. (1) Identity number of each galaxy; (2), (3), and (4) represent the
coordinates and redshift; (5) distance of each galaxy from the nearest
filament spine; (6) FUV magnitudes (uncorrected for internal and galactic
extinction); (7) intrinsic FUV SFRs; (8) stellar mass; (9) Serial number of
the parent filament taken from Tempel et al. (2014). The star-marked galaxies
in the table host an AGN.

##  5 FUV emission in filament galaxies

Figure 4: FUV sSFR vs. M⋆ for UVIT detected and undetected filament galaxies.
The downward triangular markers indicate the upper limit of FUV sSFR. The
color bar represents the Dfil for each galaxy. Background galaxies shown with
grey markers are taken from Salim et al. (2016).

The FUV SFRs would serve as an excellent tool to identify the most recently
star-forming galaxy in our sample. As highlighted in the previous section,
apart from UVIT-identified galaxies, we find a set of filament galaxies that
do not emit in FUV above a threshold of 1.5$\sigma$. These galaxies are termed
as FUV undetected. We perform fixed aperture photometry with an aperture
radius of $\approx$ 1.5′′ on the FUV images at the centroid positions of the
undetected galaxies. The magnitudes, thus obtained, were aperture corrected up
to the saturation radius of a point source. The average aperture corrected
magnitudes were approximately 1 - 1.5 mag higher than the FUV 3$\sigma$
limiting magnitude. These magnitudes are used in the subsequent analysis in
this work.

We calculated internal extinction corrected FUV SFRs for all (UVIT detected
and undetected) filament galaxies using following relation taken from
Kennicutt (1998):

$\rm SFR=1.4\times L_{FUV}(erg\,s^{-1}\,Hz^{-1})$ (4)

Where LFUVis the intrinsic FUV luminosity. We determine the value of internal
extinction by the method discussed earlier in section 4.2. The ratio between
stellar-to-gas attenuation is assumed to be 0.44. The median FUV SFRs of UVIT
detected filament galaxies is $\approx$ 1.0 M⊙ yr-1, similar to the Milky Way
galaxy. The stellar mass of filament galaxies were procured from GALEX-SDSS-
WISE Legacy Catalog (GSWLC, Salim et al., 2016). Table 2 contains the FUV
magnitudes, SFRs, stellar mass, and Dfil for all filament galaxies detected in
UVIT. Figure 4 shows FUV specific star formation rates (sSFR $=\,\rm
SFR/M_{\star}$) vs. M⋆ distribution for all filament galaxies targeted in our
UVIT survey. We use aperture-corrected FUV magnitudes for calculating FUV SFRs
in the case of UVIT undetected galaxies. The color bar indicates the Dfil for
each galaxy.

As seen in the figure, we find two distinct populations of galaxies in the
filaments - star-forming and quiescent. A majority of the UVIT undetected
galaxies lie closer to the filament spine. The non-detection of galaxies near
the spine could be associated with the extra-galactic gas supply cutoff deep
inside filaments suggested by Kuutma et al. (2017), which may lead to a dearth
of recent star formation in the region. We find a weak positive trend between
FUV sSFR and Dfil. A similar trend of decreasing optical color index is
reported in the literature (Kuutma et al., 2017; Singh et al., 2020). The
relation is extensively discussed in the following section.

The massive filament galaxies (M⋆ $\approx$ 1011 M⊙) detected in our FUV
survey are mostly star-forming. Some of these galaxies also host an AGN.
However, it is well-established that several stellar mass quenching processes,
viz. gravitation quenching (Genzel et al., 2014), AGN feedback (Croton et al.,
2006), active in massive galaxies lead to the cessation of the ongoing star
formation. Intriguingly, a few studies on filaments establish that the inter-
filamentary gas can replenish the HI content of filament galaxies (Kereš et
al., 2005). Kleiner et al. (2017) reported the evidence of cold-mode gas
accretion in several galaxies. This effect is more prominently observed in
high-mass galaxies due to their deep gravitational potential well. As detected
in our study, the accumulated cold gas can further ignite the star formation
activities in the massive filament galaxies. However, the likelihood of gas
accretion in the sampled filament galaxies needs further investigation.

###  5.1 Filament vs. clusters galaxies

Figure 5: Variation of FUV sSFR in filament/ cluster galaxies as a function of
the distance of galaxies from the filament spine/ cluster center. The FUV
detected, and undetected filament galaxies are shown with circle and downward
triangle symbols. The downward arrow used for FUV undetected filament galaxies
shows the upper limit of FUV sSFR calculated using aperture-corrected FUV
magnitude. Star markers represent the cluster galaxies. The best linear fit
over the cluster and filament galaxy (detected/ detected and undetected)
distributions are represented with grey dash-dotted and solid/ dashed lines,
respectively. The color bar represents the stellar mass of galaxies.

One of the key objectives of this work is to investigate whether the star-
forming properties of filament galaxies are different from those existing in
other large-scale environments. Most of the galaxies detected in our FUV
survey lie on the massive side of the stellar mass range. We do not include
void galaxies in this analysis as voids are mostly dominated with low-stellar
mass galaxies (Kreckel et al., 2012; Pandey et al., 2021). Galaxy clusters are
another LSS known to host the most massive galaxies in the Universe. We
compare two aspects of the FUV emission from cluster and filament galaxies.
Firstly, we are interested in studying the gradient in FUV sSFRs of galaxies
inside filaments and clusters as a function of their distances from the
centers of the respective structures. Moreover, we are motivated to compare
the FUV emission in massive filament and cluster galaxies.

We use the RedMaPPer catalog (Rykoff et al., 2016) for identifying cluster
galaxies in the nearby Universe (z $<$ 0.1). We select galaxies with cluster
membership probability greater than 50%. The FUV magnitudes of cluster members
were obtained from the GALEX AIS catalog. The galaxies with FUV magnitude
error less than 0.3 mag are included in the sample. Using the FUV magnitudes,
we calculate internal dust extinction FUV SFR using Equation 4. The value of
internal reddening in the galaxies is deduced using the Balmar decrement ratio
method (see Section 4.2). The stellar masses corresponding to the galaxies
were taken from GSWLC. Forth applying all the cuts, we obtain a small sample
of 33 cluster galaxies between a mass range of 1010.4 M⊙ to 1011.8 M⊙.

Figure 5 depicts the dependence of FUV sSFR on the distance of filament spine
and cluster center for filament and cluster galaxies, respectively. We include
FUV undetected galaxies in the analysis for a coherent interpretation. It can
be clearly seen that the recent star formation for filament galaxies decreases
on approaching the filament spine. On the other hand, cluster galaxies show
little-to-no dependence between FUV sSFRs and distance from the cluster
center. Previously done studies pointed out that for nearby galaxies (z $\sim$
0), the SFR and sSFR do not show any significant variation with clustercentric
distance (Laganá & Ulmer, 2017). Even after including FUV undetected filament
galaxies in the study, the correlation between sSFR and Dfil remains
unaltered. The results highlight the difference in evolution galaxies
depending on the large-scale environment.

Figure 5 qualitatively shows that the average FUV sSFR of filament galaxies
detected in FUV observation is greater than the cluster galaxies. We perform
the Kolmogorov–Smirnov (KS) test and the Anderson-Darling (AD) test on the
distribution of FUV sSFR for cluster and filament galaxies in a fixed stellar
mass bin. We only selected galaxies having stellar mass greater than 1010.5
M⊙. The p-values corresponding to both tests were much less than 0.05, whereas
KS-statistic (= 0.73) and AD-statistic (= 7.2) were high on comparing the FUV
sSFR of UVIT detected filament and cluster galaxies. We combine the aperture-
corrected FUV sSFR of undetected filament galaxies with the detected galaxies
and perform the same statistical tests. The p-value increases to 0.02, less
than the significance level of 0.05. Hence, we conclude that the FUV sSFR of
massive filament and cluster galaxies belong to different parent populations.
The statistical tests imply that the star formation process is more rapid in
filaments than clusters. The observed results could arise from a slower gas
depletion time scale in the former environment than the latter one, as pointed
out by Castignani et al. (2022). Alternatively, the cold mode gas accretion
from inter-filamentary channels or gas-rich mergers could rejuvenate the star
formation in massive galaxies.

###  5.2 FUV morphology of filament galaxies

Figure 6: Grey-scale FUV image stamps of all 18 galaxies detected in the FUV
survey are shown. The optical morphology in SDSS r-band is superimposed on the
image stamps. The three levels of the blue contours represent 10$\sigma$,
5$\sigma$, and 1.5$\sigma$ threshold level above the background noise. The
surface brightness of the outermost contour is 24.14 mag arcsec-2. The
identity number, stellar mass, and distance of galaxies from the nearest
filament are mentioned on each image from top to bottom. We mark the centroids
of FUV and r-band observation with cyan and red crosses, respectively.

The FUV morphology of galaxies is an efficient probe to identify the recently
star-forming regions in them. We utilize our well-resolved UVIT observation to
investigate the FUV morphology of filament galaxies and compare them with
their optical counterparts. Figure 6 illustrates FUV images of all UVIT-
detected filament galaxies. The blue contours on the figure show the SDSS-r
band profile of galaxies. The three contour levels in blue represent
10$\sigma$, 5$\sigma$, and 1.5$\sigma$ level, where $\sigma$ represents the
mean standard deviation in the SDSS r-band observation. The surface brightness
of the outermost contour is 24.14 mag arcsec-2. The galaxies are serially
assigned a number ($ID$) according to Table 2.

We show FUV and optical centroids of the galaxies using cyan and red crosses,
respectively. The young stellar population inside galaxies represented by FUV
emission mostly aligns with the main stellar population traced by optical
emission. On the contrary, a similar analysis done for cluster galaxies by
Mahajan et al. (2022) highlights that the FUV morphology for a sample of
cluster galaxies was skewed relative to their optical emission primarily due
to ram-pressure stripping (Gunn & Gott, 1972) rampantly observed in the
cluster environment. The result establishes that filament galaxies may not
witness extreme environment disturbances, which are prevalent in clusters.
However, the FUV emission appears clumpy in the filament galaxies and shows
considerable size variation. For a few galaxies, the emission is asymmetric
within the galaxy, e.g., $ID\,9,ID\,15$. Moreover, we observe FUV emission in
some galaxies beyond the outermost optical contour, e.g.,
$ID\,1,ID\,5,ID\,11,ID\,15$. These observations hint at the possibility of
recent gas accretion episodes. A thorough comparison between deep optical and
UV observation of the filament galaxies is required for confirming the
existence of extended-UV disks (Thilker et al., 2007).

As discussed in the previous section, massive galaxies in filament exhibit a
faster rate of recent star formation than those in the clusters. The FUV disk
sizes of most of the massive filament galaxies (M⋆ $>$ 1010.5 M⊙) in the
sample are slightly extended or comparable to their optical emission. These
galaxies are mostly detected in the outer edge of filaments (D${}_{\rm
fil}\,\sim 0.9$ Mpc). Previous studies establish that high stellar mass
galaxies primarily reside closer to the filament spine (Chen et al., 2017). In
our survey, massive galaxies near the filament axis were undetected or
detected with weak UV emission (see $ID\,17$ in Figure 6; also see upper left
panel of Figure 3). Some moderate stellar mass galaxies ($\lesssim$ 1010.5 M⊙)
also show extended UV emission in our sample (e.g., $ID$ 11). Previous
discussion in this work hints towards a gradient in the level of recent star
formation as we progress from the filament axis to its outer edges. An earlier
study analyzing the HI gas content of galaxies in the filament environment
reported that at fixed stellar mass and color, the galaxies closer to the
filament spine are increasingly HI deficient (Odekon et al., 2018). This
finding corroborates our result wherein the galaxies closer to the spine do
not show a signature of recent star formation.

##  6 Impact on the mass assembly

The results presented in the previous sections establishes that the filaments
provide an in-homogeneous environment for galaxies to evolve. We detect a
negative gradient in FUV sSFRs on approaching the filament spine. Moreover, we
find hints of gas accretion on the filament galaxies by comparing their UV and
optical morphology. The observations suggest existence of physical mechanisms,
such as galaxy interaction, gas starvation, inside filaments. The study of the
stellar mass assembly of filament galaxies would highlight the mechanisms
dominantly influencing their evolution. Multiple physical in-situ and
externally driven processes may lead to stellar mass accumulation in galaxies
over the cosmic time. The in-situ process includes star formation, gas
recycling, and gas accretion from the inter-galactic medium (Fall &
Efstathiou, 1980; Carroll & Ostlie, 2017) whereas merger-interaction,
accretion of satellite galaxies, gas stripping (Toomre et al., 1977) are a few
externally driven processes that steer stellar mass growth. There are two
prominent mass assembly mechanisms are thought to be in action in galaxies,
i.e., ‘inside-out’ and ‘outside-in’ modes in the literature (Sánchez-Blázquez
et al., 2007; Pérez et al., 2013; Pan et al., 2015). In the case of inside-out
mass assembly, the central regions of a galaxy form stars earlier and more
rapidly than the outer regions. Outside-in growth, on the other hand,
describes a pattern where star formation starts in the outer regions of a
galaxy and then progresses inward toward the central region. The secularly
evolving galaxies mainly grow via an inside-out mechanism whereas galaxies
influenced due to environment show outside-in stellar mass build-up.

Broadband colors of galaxies are often used to determine stellar population
age of galaxies in case of unavailability of spectral information. In this
section, we use UV-optical ($FUV-r$) color index to indicate stellar
population age. The ($FUV-r$) color is used actively to segregate star-forming
and passive galaxies (Hammer et al., 2012). Both SDSS and UVIT have similar
resolution and pixel scale ($\sim$ 0.′′4). We perform photometry on a central
aperture of r = 1.5′′ as it is roughly comparable to the point-source function
full-width half maxima (PSF FWHM) of the two surveys. The radius is equivalent
to $\approx$ 2.25 kpc at $z$ between 0.07 and 0.08. We consider the Petrosian
magnitude of the sources as a measure of their total magnitude. The color
gradient, $\Delta(FUV-r)$, is computed using the following equation:

$\Delta(FUV-r)=(FUV-r)_{r=1.5^{\prime\prime}}-(FUV-r)_{\rm total}$ (5)

We divide our galaxy sample into three categories based on the value of
$\Delta(FUV-r)$. The color gradient is positive ($>2\sigma$) for red-cored
galaxies whereas $\Delta(FUV-r)\,<\,2\sigma$ for blue-cored galaxies. The
galaxies with $|\Delta(FUV-r)|\,<\,2\sigma$ are considered flat-cored. The
average error ($\sigma$) associated with (FUV$-$r) color is 0.12 mag.

Figure 7: Left panel: Ratio of FUV SFR (aperture size (r) = 1.5" and Petrosian
aperture) vs. $\Delta(FUV-r)$. Right panel: Distribution of $\Delta(FUV-r)$
and Dfil. The color bar shown in the image represents stellar mass of
individual filament galaxies. The horizontal lines enclose the region
representing flat-color gradient galaxies.

We calculate the ratio of FUV SFR within a circular aperture of radius
$\sim$1.′′5 to total FUV SFR. The left panel of Figure 7 shows the central-to-
total SFR ratio vs. $\Delta(FUV-r)$ distribution for filament galaxies. Here,
galaxies belonging to the blue cloud in the upper right panel of Figure 3 are
termed star-forming, whereas the remaining sample detected in UVIT is grouped
as quenched. The quenched galaxies in the sample have the most negative color
gradients, similar to blue-cored galaxies. The star formation in these
galaxies is restricted to the inner region. On the other hand, the red-cored
filament galaxies have comparable FUV SFR throughout their extent.

The variation of $\Delta(FUV-r)$ and Dfil is shown in the right panel of
Figure 7. The sample has a nearly equal number of red- and blue-cored filament
galaxies. Galaxies with M⋆ $<$ 1010.5 M⊙ mostly have flat color gradients with
few exceptions showing a blue-core. Nearly all galaxies closer to the filament
spine, i.e., Dfil $\lesssim$ 0.7 Mpc, are blue-cored regardless of their
stellar masses. The result implies that these galaxies are likelier to witness
galaxy nurture activities. The galaxies situated radially outwards in the
filament are red-cored or, in other words, follow ‘inside-out’ mass assembly.
We compare the strength of the monotonic relation between $\Delta(FUV-r)$ and
Dfil against $\Delta(FUV-r)$ and M⋆ by computing their Spearman correlation
coefficient ($\rho_{s}$) for both pairs. The quantity Dfil proxy for the
environment ($\rho_{s}$ = 0.377 with a p-value of 0.123) has a stronger
positive correlation compared to the stellar mass ($\rho_{s}$ = 0.226 with a
p-value of 0.367). We conclude that the environment has a more substantial
influence on the mass assembly of filament galaxies. However, it is essential
to note that both correlations are positive and relatively weak.

##  7 Discussion and Conclusions

This study aims at understanding the star formation properties of galaxies
inside filaments. We conduct a deep FUV survey to identify galaxies undergoing
recent star formation and to subsequently investigate their FUV sSFRs, FUV
morphology, and mass-assembly mechanism. The results are compared with cluster
galaxies in a fixed stellar mass range. Although the filament comprises both
early- and late-type galaxies, we mostly detect star-forming and disky
galaxies in our FUV survey. Our UV survey does not detect galaxies closer than
0.4 Mpc from the spine. Previous studies have pointed out that the fraction of
elliptical galaxies increases as the distance towards the filament axis
decreases (Kuutma et al., 2017). The massive or red galaxies are more likely
to exist near the spine of the filament (Chen et al., 2017). The non-detection
of such galaxies in our deep UV survey confirms their dearth of star formation
deep inside filaments. The mixed galaxy population type detected in filament
could result from the pre-processing of galaxies inside filaments (Castignani
et al., 2022).

Moreover, we find a trend between the recent star formation timescale in
galaxies with Dfil. The galaxies at the filament outskirts are more star-
forming than those in the inner region. The result corroborates the finding of
Kuutma et al. (2017), where the spiral galaxy fraction increases with
increasing distance from the spine of the filament. On comparing the observed
trend with the cluster environment, the sSFR of filament galaxies increases as
a function of distance to the filament axis. In contrast, cluster galaxies
show no such effects. A previous study on cluster galaxies shows very slight
variation wherein the sSFR of galaxies increases slightly when approaching
cluster center (Laganá & Ulmer, 2017). According to a comparative study done
by Mahajan et al. (2018) on filament, void, and cluster galaxies observes a
similar result by analyzing their Hα equivalent width and colors. Ricciardelli
et al. (2014) found a decrease in the SFR as distance from the void center
increases up to a certain mean distance while no effect could be established
for sSFRs of galaxies. The discussion above illustrates the difference in the
evolution of galaxies in various LSSs. A comprehensive study combining more UV
observations of LSSs would be aimed.

In this study, we observe that galaxies with M⋆ than 1011 M⊙ placed in the
outer region of filaments have equivalent or slightly extended FUV disk size
compared to their optical counterpart. The result may be an outcome of cold-
mode gas accretion on the galaxies through filaments. However, some of these
galaxies host AGN or are surrounded by close neighboring galaxies, which may
have led to the observed result. Therefore, our study could neither confirm
nor deny the cold mode accretion process reported by Kleiner et al. (2017).
Agreeably, we detected a limited number of filament galaxies detected in our
UVIT survey. Hence, we aim to perform a similar study with an increased sample
to enhance our understanding of filament galaxies. Deep multi-band
observations could be utilized to undermine the prevalence cold-mode gas
accretion in filaments.

We study the mass assembly of filament galaxies based on $(FUV-r)$ color
gradient. The galaxies closer to the filament spine mainly evolve via outside-
in growth mass assembly. In other words, galaxies host blue cores closer to
the filament axis. Similar results were reported by Lee et al. (2021), where
the distribution of Hα equivalent width as a function of vertical distance
from the filament axis was studied. The author showed that the high stellar
mass galaxies closer to the filament spine are actively star-forming in their
central region. A considerable cause of the central star formation could be
the increased likelihood of merger interactions.

Our work establishes that filament could serve as a hotbed for several
activities impacting the growth of galaxies residing inside filaments., e.g.,
galaxy interactions, gas starvation, and gas accretion. These processes
attribute to a range of galaxy properties in color, SFR, and morphology. The
results from our study are summarized below:
i) We detect galaxies with Dfil $>$ 0.4 Mpc in our deep UV survey.
ii) The study establishes that the filament environment substantially affects
the evolution of galaxies inside them. We find early- and late-type galaxies
residing inside the filament. Most of the galaxies detected by in our FUV
survey are star-forming and late-type.
iii) There is a positive gradient in FUV sSFR as the distance from the
filament axis increases.
iv) We find that massive galaxies in the filament are undergoing recent star
formation on a rapid timescale compared to galaxies in clusters.
v) The UV morphology for a few galaxies residing in the outer edge of
filaments is extended more than their optical profile.
vi) ‘Outside-in’ mass-assembly mode is more prevalent in the inner region of
the filaments, i.e., galaxies inside filaments are centrally star-forming. The
result could be due to an increased rate of galaxy merger interaction in the
inner region.

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). DP and ACP would like to acknowledge Inter University Centre for
Astronomy and Astrophysics (IUCAA), Pune, India for providing facilities to
carry out this work.

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