Abstract
Context. Superclusters form from the largest enhancements in the primordial density perturbation field and extend for tens of Mpc, tracing the large-scale structure of the Universe. X-ray detections and systematic characterisations of superclusters and the properties of their galaxies have only been possible in the last few years. Aims. We characterise XLSSsC N01, a rich supercluster at z ~ 0.3 detected in the XXL Survey, composed of X-ray clusters of different virial masses and X-ray luminosities. As one of the first studies on this topic, we investigate the stellar populations of galaxies in different environments in the supercluster region. Methods. We study a magnitude-limited (r ≤ 20) and a mass-limited sample (log(M*∕M⊙) ≥ 10.8) of galaxies in the virialised region and in the outskirts of 11 XLSSsC N01 clusters, in high-density field regions, and in the low-density field. We compute the stellar population properties of galaxies using spectral energy distribution (SED) and spectral fitting techniques, and study the dependence of star formation rates (SFR), colours, and stellar ages on environment. Results. For r ≤ 20, the fraction of star-forming/blue galaxies, computed either from the specific-SFR (sSFR) or rest-frame colour, shows depletion within the cluster virial radii, where the number of galaxies with log (sSFR/ yr−1) > −12 and with (g − r)restframe < 0.6 is lower than in the field. For log(M*∕M⊙) ≥ 10.8, no trends with environment emerge, as massive galaxies are mostly already passive in all environments. No differences among low- and high-density field members and cluster members emerge in the sSFR-mass relation in the mass-complete regime. Finally, the luminosity-weighted age–mass relation of the passive populations within cluster virial radii show signatures of recent environmental quenching. Conclusions. The study of luminous and massive galaxies in this supercluster shows that while environment has a prominent role in determining the fractions of star-forming/blue galaxies, its effects on the star formation activity in star-forming galaxies are negligible.
Highlights
Galaxy stellar population properties, especially star formation history and colour, depend on the environment in which galaxies reside (Spitzer & Baade 1951; Oemler 1974; Davis & Geller 1976; Dressler 1980; Blanton et al 2005; Ball et al 2008)
To identify the threshold in colour that best separates the blue and red populations, we investigate the correlation between sSFR, (g − r)rest-frame colour and Mr, as shown in Fig. 5, for the subsample analysed by both LePhare and SINOPSIS
While the star formation rates (SFR) is a snapshot measuring the number of stars produced by the galaxy at the moment it is observed, the colour is sensitive to the past history of the galaxy itself, especially the recent history, being determined by its predominant stellar population
Summary
Especially star formation history and colour, depend on the environment in which galaxies reside (Spitzer & Baade 1951; Oemler 1974; Davis & Geller 1976; Dressler 1980; Blanton et al 2005; Ball et al 2008). Wilman et al 2005; Cucciati et al 2006; Cooper et al 2006, 2010; Coil et al 2008) At these redshifts, the inverse of the specific star formation rate (sSFR), i.e. the time for a galaxy to double its stellar mass, is higher in denser environments (Scoville et al 2013). This supercluster is the best candidate for environmental studies on galaxies since it is the richest in the XXL-N field (14 groups and clusters; hereafter clusters), and because it is located in a region of the sky with highly complete spectroscopic and photometric data This analysis is a first attempt to directly study the impact of large-scale environment, i.e. clusters which are considered all part of the same superstructure, on the star formation activity and stellar population properties of galaxies.
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