Recently filaments, in particular the warm hot intergalactic medium (WHIM) in filaments, have been the pivot of several studies. Using emission in the soft X-ray bands, such studies estimate the WHIM temperature in filaments to be ∼3–8 keV (Eckert et al. 2015; Akamatsu et al. 2017; Parekh et al. 2017; Tanimura et al. 2017). Furthermore, the cosmic microwave background map from the Planck together with the Canada–France–Hawaii Telescope Lensing Survey, as well as the Two-Micron All-Sky Redshift Survey of galaxies suggest that at least half of the missing baryons in the Universe may reside as WHIM in large-scale filaments tracing the dark matter distribution (Van Waerbeke, Hinshaw & Murray 2014; Génova-Santos et al. 2015). Therefore, undeniably it is crucial to characterize the large-scale structure (LSS) of the Universe and comprehend the impact of the cosmic-web on properties of galaxies. The Coma supercluster is one of the richest large-scale structures (Chincarini & Rood 1976) in the nearby Universe comprising two clusters of galaxies, connected by a web of large-scale filaments around $30\,h_{70}^{-1}$ Mpc long (e.g. Fontanelli 1984). The two clusters, Coma (Abell 1656) and Abell 1367, along with the filaments of galaxies dispersed with several small galaxy groups span ∼500 deg2 on the sky (Mahajan et al. 2010). The large-scale filaments in the Coma supercluster have not just been observed by means of the galaxy distribution in the optical wavebands (Gregory & Thompson 1978; Mahajan et al. 2010), but also diffuse emission in the radio continuum (Kim et al. 1989). Studies of clusters and groups at $z$ ∼ 0 have evidently shown that outskirts of groups and clusters (Zabludoff & Mulchaey 1998; Wang, Owen & Ledlow 2004; Rines et al. 2005; Cortese et al. 2007; Tran et al. 2009; Gavazzi et al. 2010; Smith et al. 2010; Sun et al. 2010; Coppin et al. 2011; Mahajan, Raychaudhury & Pimbblet 2012; Verdugo et al. 2012; Mahajan 2013) and filaments of galaxies (Porter & Raychaudhury 2007; Boué et al. 2008; Fadda et al. 2008; Porter et al. 2008; Edwards et al. 2010a; Biviano et al. 2011) are favourable sites for galaxy transformations. Based on a study using optical data from the Sloan Digital Sky Survey (SDSS) data release (DR) 7, Mahajan et al. (2010) found that the star formation (SF)–density relation in the Coma supercluster for the giant galaxies is much weaker than their dwarf counterparts. However, the fraction of star-forming galaxies for both declines to ∼0 at the centre of the clusters (also see Mahajan, Haines & Raychaudhury 2011). Cybulski et al. (2014) further study the star formation in the Coma supercluster by combining a complementary optical data set from SDSS DR 9, with IR data from the Wide-Field Infrared Survey Explorer (Wright et al. 2010) and UV data from the Galaxy Evolution Explorer (GALEX; Martin et al. 2005). Cybulski et al. (2014) corroborated the results of Mahajan et al. (2010, 2011) by probing both obscured and unobscured star formation down to ∼0.02 M⊙ yr−1, in order to quantify the effect of different types of large-scale environments: groups, clusters, filaments, and voids, on quenching SF in galaxies. In the absence of dust in star-forming galaxies, the UV emission is a good tracer of massive (>10 M⊙) star formation. On the other hand, optical emission lines such as H α probe instantaneous star formation over a time-scale of ≲20 Myr (Kennicutt 1998). Assuming that the UV luminosity is not overwhelmed by contribution from the old stellar populations due to the UV upturn such as in massive early-type galaxies (O’Connell 1999), the UV luminosity measures star formation over a time-scale of ∼100 Myr (Kennicutt 1998). Hence, the star formation rate (SFR) estimated from optical emission lines delineates the continuous SF in a galaxy, while the SFR determined from the UV is representative of its recent SF activity. But even though GALEXand its predecessor UV imagers have been used to investigate individual galaxies within clusters and groups (e.g. Hicks & Mushotzky 2005), or galaxy populations therein (e.g. Donas et al. 1990; Donas, Milliard & Laget 1995; Cornett et al. 1998; Boselli et al. 2005b), limited work has been done to analyse the UV properties of galaxies in the large-scale cosmic-web. Since the Coma supercluster is one of the most well studied regions in the nearby Universe, many other authors (e.g. Bernstein et al. 1995; Mobasher et al. 2003; Hammer et al. 2012; Smith et al. 2010; Smith, Lucey & Carter 2012b) have made use of optical and UV data to study the Coma and Abell 1367 clusters and their surroundings. With the advent of large redshift surveys, several studies (Gavazzi et al. 2010; Mahajan et al. 2010, 2011; Gavazzi et al. 2013; Cybulski et al. 2014 ) have also used multiwavelength data at optical, UV, and 21 cm continuum to study the properties of galaxies in the entire supercluster region. In this paper, we make use of similar data sets: UV data derived from GALEX and optical spectroscopic and photometric data from the SDSS for the entire Coma supercluster to further explore the impact of environment on the properties of galaxies. Conventionally, the ‘environment’ of galaxies is quantified as the projected density of galaxies in a fixed 2D or 3D region of the sky (e.g. Dressler 1980). Muldrew et al. (2012) combined 20 published methods of defining environment into two methods: (i) method that uses nearest-neighbours to probe the underlying galaxy density and (ii) fixed aperture methods. Muldrew et al. (2012) found that while the former are better suited for quantifying internal density within massive haloes, the latter fixed-aperture methods are better for probing the large-scale environment. Therefore, in order to characterize the large-scale cosmic-web, a combination of these methodologies is required to quantify the environment on different scales. In this work, we implement their result by making use of two different algorithms to define the large-scale filaments, and high density nodes of the cosmic-web characterized as clusters and groups. We also present a catalogue of all spectroscopically confirmed galaxy members of the Coma supercluster detected in the UV. This paper is organized as follows: in the next section we describe our data sets, followed by definition of environment in Section 3. In Section 4, we analyse the broad-band colours of galaxies as a function of environment, while in Section 5 we study the impact of the large-scale filaments on the properties of galaxies. Finally, we discuss our results in the context of the existing literature in Section 6 and summarize our results in Section 7. Throughout this paper, we use concordance Λ cold dark matter cosmological model with H0 = 70  km s−1 Mpc−1, $\Omega _\Lambda =0.7$, and Ωm = 0.3 to calculate distances and magnitudes. We note that at the redshift of the Coma cluster ($z$ = 0.023) our results are independent of the cosmological model used.
Citation: Porter & Raychaudhury 2007
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