Galaxy populations in clusters and proto-clusters

2012/2013The aim of my Thesis is to explore the physical properties of the galaxy population in clusters and proto-clusters. A large number of physical processes plays an important role in the formation and evolution of galaxies: cooling, that allows the condensation of gas in the centre of dark matter haloes; star formation, that converts cold gas in stars; feedback from Active Galactic Nuclei (AGN), that prevents the gas in the central regions of haloes from "over-cooling"; feedback from Supernovae, which liberates energy in the surrounding, mixing the gas and enriching it with heavy metals. Galaxy clusters are special environments in which additional important processes take place, and play an important role in the evolution of the cluster galaxy population. Galaxy merging, harassments, tidal interactions, ram pressure stripping and strangulation are all processes acting in dense environments such as clusters of galaxies. I will take advantage of a {\it state of the art}-semi-analytic model of galaxy formation and of a set of 27 high-resolution dark matter only simulations: the semi-analytic model is based on physically motivated and observationally constrained prescriptions for the physical processes listed above and makes use of merger-trees extracted from the simulations to generate mock catalogues of galaxies. First, I make use of this set of simulations to carry out a statistical study of dark matter substructures. In the framework of modern theories of galaxy formation, dark matter substructures can be considered as the birth-sites of luminous galaxies. Therefore, the analysis of subhaloes, and in particular of their mass and spatial distributions, merger and mass accretion histories, provides important information about the expected properties of galaxies in the framework of hierarchical galaxy formation models. I have studied the amount and distribution of dark matter substructures within dark matter haloes, focusing mainly on the measured properties of subhaloes as a function of the mass and physical properties of their parent haloes, and redshift. I show that the fraction of halo mass in substructures increases with increasing mass, reaching $10 \%$ for haloes with mass of the order of 10^{15} \,M_{\odot} \hm. The scatter in the relation is driven by halo concentration, with less concentrated haloes having larger fractions of mass in substructures. Most of this mass is locateted in the external regions of the parent haloes, in relatively few, but massive subhaloes, thus giving rise to a mass segregation which appears to be stronger at increasing redshift. Tidal stripping is found to be the process responsible for that. In fact, haloes that are more massive at the time of accretion, and that are supposed to host more luminous galaxies, are brought closer to the centre on shorter time-scales by dynamical friction, and therefore suffer of a more significant stripping. The results confirm that the main properties of galaxies, such as luminosity or stellar mass, are related to the mass of subhalos at infall, as found in previous studies.. The main results discussed in this part of the Thesis have been published in Contini et al. (2012), MNRAS.420.2978C. In a second part, I describe the implementation of physical processes responsible for the generation of the Intra-Cluster Light (ICL) in the available semi-analytic model, that, in its original form, does not account for them. The inclusion of these physical processes is, thus, an important improvement of the model. I take advantage of this upgrade of the model to investigate the origin of the ICL and to understand how the main properties of galaxies change with respect to a model that does not include these additional prescriptions. I find the fraction of ICL in groups and clusters predicted by the model to range between $10 \%$ and $40 \%$, with a large scatter and no halo mass dependence. Large part of the scatter on cluster scales is due to a range of dynamical histories, while on smaller scales it is mainly driven by individual accretion events and stripping of relatively massive satellites, with mass of the order of 10^{10.5} \, M_{\odot} \hm, found to be the major contributors to the ICL. The ICL forms very late, below $z \sim 1$ and a non negligible fraction (between $5 \%$ and $25 \%$) has been accreted during the hierarchical growth of haloes. Moreover, the ICL is made of stars which cover a relatively large range of metallicity, with the bulk of them being sub-solar, in agreement with recent observational data. The main results of this analysis have been submitted to MNRAS (Contini et al. 2013). In the last part of the thesis, the updated model is used to investigate the properties of the galaxy population in proto-cluster regions. The work is still in progress. I am testing the predictions of the semi-analytic model and comparing them with observations in terms of properties such as galaxy colours, star formation and stellar mass. A preliminary analysis of one very massive proto-cluster region shows that the galaxy population gets red and tend to cluster around the most massive galaxy as time goes by. There are, in literature, only a few attempts to probe such peculiar regions of the Universe from a theoretical point of view. The novelty of this work lies in the connection between massive clusters observed in the local Universe and the proto-cluster regions from which they have formed. I will try to define what a proto-cluster region is, and how it looks like, by studying the main properties of progenitors it contains. Specifically, I will investigate the spatial and velocity distributions of galaxies in simulated proto-clusters, looking at the red and blue galaxy distributions in these regions, as well as at BCG and satellite properties as a function of redshift. The main results of this work will be the subject of a paper in preparation.XXV Ciclo198

The Connection between the Intracluster Light and its Host Halo: Formation Time and Contribution from Different Channels

We extend the analysis presented in \cite{contini2023a} to higher redshifts, up to $z=2$, by focusing on the relation between the intracluster light (ICL) fraction and the halo mass, its dependence with redshift, role played by the halo concentration and formation time, in a large sample of simulated galaxy groups/clusters with $13\lesssim \log M_{halo} \lesssim 15$. Moreover, a key focus is to isolate the relative contributions provided by the main channels for the ICL formation to the total amount. The ICL fraction at higher redshift is weakly dependent on halo mass, and comparable with that at the present time, in agreement with recent observations. Stellar stripping, mergers and pre-processing are the major responsible channels of the ICL formation, with stellar stripping that accounts for $\sim 90\%$ of the total ICL, regardless of halo mass and redshift. Pre-processing is an important process for clusters to accrete already formed ICL. The diffuse component forms very early, $z\sim 0.6$, and its formation depends on both concentration and formation time of the halo, with more concentrated and earlier formed haloes that assemble their ICL earlier than later formed ones. The efficiency of this process is independent of halo mass, but increases with decreasing redshift, which implies that stellar stripping becomes more important with time as the concentration increases. This highlights the link between the ICL and the dynamical state of a halo: groups/clusters that have a higher fraction of diffuse light are more concentrated, relaxed and in an advanced stage of growth.Comment: Corrected for several bugs and typos. Clean no

The definition of environment and its relation to the quenching of galaxies at z=1-2 in a hierarchical Universe

A well calibrated method to describe the environment of galaxies at all redshifts is essential for the study of structure formation. Such a calibration should include well understood correlations with halo mass, and the possibility to identify galaxies which dominate their potential well (centrals), and their satellites. Focusing on z = 1 and 2 we propose a method of environmental calibration which can be applied to the next generation of low to medium resolution spectroscopic surveys. Using an up-to-date semi-analytic model of galaxy formation, we measure the local density of galaxies in fixed apertures on different scales. There is a clear correlation of density with halo mass for satellite galaxies, while a significant population of low mass centrals is found at high densities in the neighbourhood of massive haloes. In this case the density simply traces the mass of the most massive halo within the aperture. To identify central and satellite galaxies, we apply an observationally motivated stellar mass rank method which is both highly pure and complete, especially in the more massive haloes where such a division is most meaningful. Finally we examine a test case for the recovery of environmental trends: the passive fraction of galaxies and its dependence on stellar and halo mass for centrals and satellites. With careful calibration, observationally defined quantities do a good job of recovering known trends in the model. This result stands even with reduced redshift accuracy, provided the sample is deep enough to preserve a wide dynamic range of density.Comment: 19 pages, 12 figures, accepted for publication in MNRA

YZiCS: Unveiling Quenching History of Cluster Galaxies Using Phase-space Analysis

We used the time since infall (TSI) of galaxies, obtained from the Yonsei Zoom-in Cluster Simulation, and the star formation rate (SFR) from the Sloan Digital Sky Survey (SDSS) Data Release 10 to study how quickly star formation of disk galaxies is quenched in cluster environments. We first confirm that both simulated and observed galaxies are consistently distributed in phase space. We then hypothesize that the TSI and SFR are causally connected; thus, both the TSI and SFR of galaxies at each position of phase space can be associated through abundance matching. Using a flexible model, we derive the star formation history (SFH) of cluster galaxies that best reproduces the relationship between the TSI and SFR at $z\sim 0.08$. According to this SFH, we find that the galaxies with $M_{*} > 10^{9.5} M_{\odot}$ generally follow the so-called "delayed-then-rapid" quenching pattern. Our main results are as following: (i) Part of the quenching takes place outside clusters through mass quenching and pre-processing. The e-folding timescale of this "$ex\text{-}situ$ quenching phase" is roughly 3 Gyr with a strong inverse mass dependence. (ii) The pace of quenching is maintained roughly for 2 Gyr ("delay time") during the first crossing time into the cluster. During the delay time, quenching remains gentle probably because gas loss happens primarily on hot and neutral gases. (iii) Quenching becomes more dramatic (e-folding timescale of roughly 1 Gyr) after delay time, probably because ram pressure stripping is strongest near the cluster center. Counter-intuitively, more massive galaxies show shorter quenching timescales mainly because they enter their clusters with lower gas fractions due to $ex\text{-}situ$ quenching.Comment: 24 pages, 11 figures, 1 table, accepted to ApJ

YZiCS: Unveiling the Quenching History of Cluster Galaxies Using Phase-space Analysis

We used the time since infall (TSI) of galaxies, obtained from the Yonsei Zoom-in Cluster Simulation, and the star formation rate (SFR) from the Sloan Digital Sky Survey Data Release 10 to study how quickly the star formation of disk galaxies is quenched in cluster environments. We first confirm that both simulated and observed galaxies are consistently distributed in phase space. We then hypothesize that the TSI and SFR are causally connected; thus, both the TSI and SFR of galaxies at each position of phase space can be associated through abundance matching. Using a flexible model, we derive the star formation history (SFH) of cluster galaxies that best reproduces the relationship between the TSI and SFR at z ~ 0.08. According to this SFH, we find that galaxies with M * > 109.5 M ⊙ generally follow the so-called "delayed-then-rapid" quenching pattern. Our main results are as follows: (i) part of the quenching takes place outside clusters through mass quenching and preprocessing. The e-folding timescale of this "ex situ quenching phase" is roughly 3 Gyr with a strong inverse mass dependence. (ii) The pace of quenching is maintained roughly for 2 Gyr ("delay time") during the first crossing time into the cluster. During the delay time, quenching remains gentle, probably because gas loss happens primarily on hot and neutral gases. (iii) Quenching becomes more dramatic (e-folding timescale of roughly 1 Gyr) after delay time, probably because ram pressure stripping is strongest near the cluster center. Counterintuitively, more massive galaxies show shorter quenching timescales mainly because they enter their clusters with lower gas fractions due to ex situ quenching