This thesis presents one of the first census of the properties of galaxies in X-ray selected groups and clusters at intermediate redshift, with the aim of assessing the role of envi- ronment on the galaxy stellar mass assembly, star formation activity and observed stellar population properties.
My project is framed in the XXL Survey (Pierre et al. 2016), the largest XMM-Newton programme approved to date, covering two extragalactic regions in the sky of 25 deg2 each one. Extended X-ray sources identified as groups and clusters are spectroscopically confirmed and their main properties are characterised either via direct measurements or by means of scaling relations. Among them, inferred X-ray luminosities and temperatures, virial masses and radii are of fundamental importance for the development of this thesis. The great advantage of XXL is that the XXL-North field (XXL-N) is fully covered by photometric and spectroscopic observations coming from the most recent extragalactic surveys of galaxies.
The availability of such a treasure trove of information motivates the development of my research on galaxy populations at 0.1≤z≤0.6 in XXL-N, exploring the most diverse environments ranging from the field, to groups, clusters and superclusters.
The first task of my work consists in the creation of a homogeneous spectrophotometric sample of galaxies, released in Guglielmo et al. (2017), suitable for scientific purposes. The catalogue contains spectroscopic redshifts, membership information on groups and clus- ters, spectroscopic completeness weights as a function of position in the sky and observed magnitude, stellar masses and absolute magnitudes computed by means of a spectral en- ergy distribution (SED) technique. The catalogue is fundamental for all XXL studies that aims at relating optical properties derived from galaxies with X-ray information and is widely used in the whole XXL collaboration.
The released spectrophotometric catalogue enables the first scientific achievement of this thesis regarding the study of the galaxy stellar mass function (GSMF). The goal of this analysis is to unveil whether the mass assembly of galaxies depends on global environment, i.e. field vs groups and clusters and, among groups and clusters, on X-ray luminosity, used as a proxy for the halo mass. I performed the analysis in four redshift bins in the range 0.1≤z≤0.6, finding overall that environment does not affect the GSMF, at least in the mass range probed. The result is further confirmed by the invariance of the mean mass of member galaxies on X-ray luminosity. I also looked into the evolution of the mass assembly from z=0.6 down to z=0.1, finding that the high mass end is already in place at the oldest epoch and does not evolve and detecting an increase in the low-mass galaxy population in the same redshift range. This study is one of the first systematic studies on the GSMF conducted for X-ray extended sources ranging from the group to the cluster environment, and is published in the second part of Guglielmo et al. (2017).
Having assessed the independence of the mass distributions on the global environment, I proceed investigating whether and to what extent the environment affects the star formation activity and the observed properties of the galaxy stellar populations.
I started this analysis from the richest supercluster identified in XXL-N, XLSSsC N01, located at redshift z∼0.3 and composed of 14 groups and clusters. This work has been submitted in Guglielmo et al. (2018a). With focus on the region surrounding XLSSsC N01, I divided galaxies in different environments, ranging from the virial regions of groups and clusters to the field, using a combination of global and local environment parametri- sations. The main results of this study are that, in the supercluster environment, while the star forming fractions and quenching efficiency strongly depend on environment, the SFR-mass relation does not. The star forming fraction progressively declines from the field to filaments to the virialised regions of groups and clusters, with an interesting en- hancement in the outer regions of the X-ray structures. Moreover, while the average luminosity weighted (LW)-age-mass relation is independent of the environment, a clear signature for recent star formation quenching is found in the stellar ages of passive galaxies in the virialised regions of X-ray structures.
Finally, I extend the analysis of this peculiar supercluster to the whole XXL-N field. This work will be enclosed in two articles in preparation (Guglielmo et al. 2018 b,c in prep.). Thanks to the higher statistics of the entire sample, I investigated the properties of galaxies and their evolution at 0.1≤z≤0.5 in different environments, with the goal of characterising the changing in the stellar population properties and the build up of the passive population via environmental quenching. Besides distinguishing among galaxies in the field, and in groups and clusters (virial regions and outskirts), I also focused on galaxies located in structures of different X-ray luminosity and in galaxies located within superclusters. Simultaneously, I also investigated the properties of galaxies located at different projected local densities (LD).
In particular, I characterised the fraction of star forming/blue galaxies and of the SFR- mass relation, as a function of both global and local environment. The fraction of star forming and blue galaxies is strictly related to the environment, having the lowest value in the virial regions of groups and clusters, and the highest in the field. In outer members, the same fraction is similar to that in the field at z≥0.2, and assumes intermediate values with respect to virial members and the field at 0.1≤z<0.2.
The SFR-mass relation is also environment dependent, and in particular the number of virial member galaxies having reduced SFR (galaxies in transition) nearly doubles that of field galaxies. Again, outer members show intermediate properties: the fraction of galaxies in transition is similar to the virial population at z>0.3, when it is found to be associated to the supercluster environment, and then reduces to values typical of field galaxies at 0.1≤z<0.3. The star forming and blue fractions also decrease with increasing LD at all redshifts. On the contrary, the fraction of galaxies in transition does not vary in the same LD range. These significant differences emerging among the global and local environments are intrinsically related to the different physical meaning of the two parametrisations, thus to the different physical mechanisms acting on galaxies when bound in the potential well of a dark matter halo (according to the global definition) or when exposed to interactions with other galaxies in over dense and highly populated regions (according to the local definition).
During the first stages of my PhD, I also completed the analysis of my master thesis, and I report the full text of the published paper in the Appendix of the thesis (Guglielmo et al. 2015). The results are closely related to the scientific questions tackled in my PhD project, addressed through a complementary approach that reconstructed the star formation history of low- redshift galaxies in clusters and in the field to study the dependence on global environment, stellar mass and observed morphology
