The HectoMAP Cluster Survey: Spectroscopically Identified Clusters and their Brightest Cluster Galaxies (BCGs)

We apply a friends-of-friends (FoF) algorithm to identify galaxy clusters and we use the catalog to explore the evolutionary synergy between brightest cluster galaxies (BCGs) and their host clusters. We base the cluster catalog on the dense HectoMAP redshift survey (2000 redshifts deg−2). The HectoMAP FoF catalog includes 346 clusters with 10 or more spectroscopic members within the range 0.05 < z < 0.55 and with a median z = 0.29. We list these clusters and their members. We also include central velocity dispersions (σ *,BCG) for the FoF cluster BCGs, a distinctive feature of the HectoMAP FoF catalog. HectoMAP clusters with higher galaxy number density (80 systems) are all genuine clusters with a strong concentration and a prominent BCG in Subaru/Hyper Suprime-Cam images. The phase-space diagrams show the expected elongation along the line of sight. Lower-density systems include some low reliability systems. We establish a connection between BCGs and their host clusters by demonstrating that σ *,BCG /σ cl decreases as a function of cluster velocity dispersion (σ cl), in contrast, numerical simulations predict a constant σ *,BCG/σ cl. Sets of clusters at two different redshifts show that BCG evolution in massive systems is slow over the redshift range z < 0.4. The data strongly suggest that minor mergers may play an important role in BCG evolution in clusters with σ cl ≳ 300 km s−1. For lower mass systems (σ cl < 300 km s−1), major mergers may play a significant role. The coordinated evolution of BCGs and their host clusters provides an interesting test of simulations in high-density regions of the universe.

STAR FORMATION HISTORIES OF BRIGHTEST CLUSTER GALAXIES WITH INTERMEDIATE CENTRAL AGES

This work is devoted to the study of the star formation histories (SFHs) of the brightest cluster galaxies (BCGs) with intermediate central ages (from 5 to 10Gyr), to confirm if BCGs with these ages represent different accretion histories or simply a stochastic effect. The sample is composed of 6 BCGs with intermediate central ages and 3 BCGs with old central ages (> 12Gyr) as comparison galaxies. The galaxies were observed with the integrated field spectrograph VIMOS installed in the Very Large Telescope (VLT). The SFHs were obtained with the full spectrum fitting technique using the star population code STARLIGHT. The BCGs of intermediate central age analyzed formed almost 100% of their stars at z > 2 and their SFHs are similar to the SFHs of BCGs of old central ages and elliptical galaxies of similar mass (MDyn > 1011 Mʘ); therefore, these BCGs do not represent different SFHs.

Velocity dispersion of brightest cluster galaxies in cosmological simulations

ABSTRACT Using the DIANOGA hydrodynamical zoom-in simulation set of galaxy clusters, we analyse the dynamics traced by stars belonging to the brightest cluster galaxies (BCGs) and their surrounding diffuse component, forming the intracluster light (ICL), and compare it to the dynamics traced by dark matter and galaxies identified in the simulations. We compute scaling relations between the BCG and cluster velocity dispersions and their corresponding masses (i.e. $M_\mathrm{BCG}^{\star }$–$\sigma _\mathrm{BCG}^{\star }$, M200–σ200, $M_\mathrm{BCG}^{\star }$–M200, and $\sigma _\mathrm{BCG}^{\star }$–σ200), we find in general a good agreement with observational results. Our simulations also predict $\sigma _\mathrm{BCG}^{\star }$–σ200 relation to not change significantly up to redshift z = 1, in line with a relatively slow accretion of the BCG stellar mass at late times. We analyse the main features of the velocity dispersion profiles, as traced by stars, dark matter, and galaxies. As a result, we discuss that observed stellar velocity dispersion profiles in the inner cluster regions are in excellent agreement with simulations. We also report that the slopes of the BCG velocity dispersion profile from simulations agree with what is measured in observations, confirming the existence of a robust correlation between the stellar velocity dispersion slope and the cluster velocity dispersion (thus, cluster mass) when the former is computed within 0.1R500. Our results demonstrate that simulations can correctly describe the dynamics of BCGs and their surrounding stellar envelope, as determined by the past star formation and assembly histories of the most massive galaxies of the Universe.

On the Origin and Evolution of the Intra-Cluster Light: A Brief Review of the Most Recent Developments

Not all the light in galaxy groups and clusters comes from stars that are bound to galaxies. A significant fraction of it constitutes the so-called intracluster or diffuse light (ICL), a low surface brightness component of groups/clusters generally found in the surroundings of the brightest cluster galaxies and intermediate/massive satellites. In this review, I will describe the mechanisms responsible for its formation and evolution, considering the large contribution given to the topic in the last decades by both the theoretical and observational sides. Starting from the methods that are commonly used to isolate the ICL, I will address the remarkable problem given by its own definition, which still makes the comparisons among different studies not trivial, to conclude by giving an overview of the most recent works that take advantage of the ICL as a luminous tracer of the dark matter distribution in galaxy groups and clusters.