RELICS: ICL Analysis of the z = 0.566 Merging Cluster WHL J013719.8–08284

We present a pilot study of the intracluster light (ICL) in massive clusters using imaging of the z = 0.566 cluster of galaxies WHL J013719.8–08284 observed by the RELICS project with the Hubble Space Telescope. We measure the ICL fraction in four optical ACS/WFC filters (F435W, F475W, F606W, and F814W) and five infrared WFC3/IR bands (F105W, F110W, F125W, F140W, and F160W). The ICL maps are calculated using the free-of-a-priori-assumptions algorithm CICLE, and the cluster membership is estimated from photometric properties. We find optical ICL fractions that range between ∼6% and 19%, in nice agreement with the values found in previous works for merging clusters. We also observe an ICL fraction excess between 3800 Å and 4800 Å, previously identified as a signature of merging clusters at 0.18 < z < 0.55. This excess suggests the presence of an enhanced population of young/low-metallicity stars in the ICL. All indicators thus point to WHL J013719.8–08284 as a disturbed cluster with a significant amount of recently injected stars, bluer than the average stars hosted by the cluster members and likely stripped out from infalling galaxies during the current merging event. Infrared ICL fractions are ∼50% higher than optical ones, which could be signatures of an older and/or higher-metallicity ICL population that can be associated with the buildup of the brightest cluster galaxy, passive evolution of previously injected young stars, or preprocessing in infalling groups. Finally, investigating the photometry of the cluster members, we tentatively conclude that WHL J013719.8–08284 fulfills the expected conditions for a fossil system progenitor.

Halo cluster shapes: insights from simulated galaxies and ICL with prospects for weak lensing applications

ABSTRACT We present a detailed study of the shapes and alignments of different galaxy cluster components using hydrodynamical simulations. We compute shape parameters from the dark matter (DM) distribution, the galaxy members and the intracluster light (ICL). We assess how well the DM cluster shape can be constrained by means of the identified galaxy member positions and the ICL. Further, we address the dilution factor introduced when estimating the cluster elongation using weak-lensing stacking techniques, which arises due to the misalignment between the total surface mass distribution and the distribution of luminous tracers. The dilution is computed considering the alignment between the DM and the brightest cluster galaxy, the galaxy members and the ICL. Our study shows that distributions of galaxy members and ICL are less spherical than the DM component, although both are well aligned with the semimajor axis of the latter. We find that the distribution of galaxy members hosted in more concentrated subhaloes is more elongated than the distribution of the DM. Moreover, these galaxies are better aligned with the DM component compared to the distribution of galaxies hosted in less concentrated subhaloes. We conclude that the positions of galaxy members can be used as suitable tracers to estimate the cluster surface density orientation, even when a low number of members is considered. Our results provide useful information for interpreting the constraints on the shapes of galaxy clusters in observational studies.

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.

Is diffuse intracluster light a good tracer of the galaxy cluster matter distribution?

ABSTRACT We explore the relation between diffuse intracluster light (central galaxy included) and the galaxy cluster (baryonic and dark) matter distribution using a sample of 528 clusters at 0.2 ≤ z ≤ 0.35 found in the Dark Energy Survey (DES) Year 1 data. The surface brightness of the diffuse light shows an increasing dependence on cluster total mass at larger radius, and appears to be self-similar with a universal radial dependence after scaling by cluster radius. We also compare the diffuse light radial profiles to the cluster (baryonic and dark) matter distribution measured through weak lensing and find them to be comparable. The IllustrisTNG galaxy formation simulation, TNG300, offers further insight into the connection between diffuse stellar mass and cluster matter distributions – the simulation radial profile of the diffuse stellar component does not have a similar slope with the total cluster matter content, although that of the cluster satellite galaxies does. Regardless of the radial trends, the amount of diffuse stellar mass has a low-scatter scaling relation with cluster’s total mass in the simulation, out-performing the total stellar mass of cluster satellite galaxies. We conclude that there is no consistent evidence yet on whether or not diffuse light is a faithful radial tracer of the cluster matter distribution. Nevertheless, both observational and simulation results reveal that diffuse light is an excellent indicator of the cluster’s total mass.

Stellar splashback: the edge of the intracluster light

ABSTRACT We examine the outskirts of galaxy clusters in the C-EAGLE simulations to quantify the ‘edges’ of the stellar and dark matter distribution. The radius of the steepest slope in the dark matter, commonly used as a proxy for the splashback radius, is located at $\sim \, r_{200 \rm m}$; the strength and location of this feature depends on the recent mass accretion rate, in good agreement with previous work. Interestingly, the stellar distribution (or intracluster light, ICL) also has a well-defined edge, which is directly related to the splashback radius of the halo. Thus, detecting the edge of the ICL can provide an independent measure of the physical boundary of the halo, and the recent mass accretion rate. We show that these caustics can also be seen in the projected density profiles, but care must be taken to account for the influence of substructures and other non-diffuse material, which can bias and/or weaken the signal of the steepest slope. This is particularly important for the stellar material, which has a higher fraction bound in subhaloes than the dark matter. Finally, we show that the ‘stellar splashback’ feature is located beyond current observational constraints on the ICL, but these large projected distances (≫1 Mpc) and low surface brightnesses (μ ≫ 32 mag arcsec−2) can be reached with upcoming observational facilities such as the Vera C. Rubin Observatory, the Nancy Grace Roman Space Telescope, and Euclid.

On the phase-space structure of galaxy clusters from cosmological simulations

ABSTRACT Cosmological N-body simulations represent an excellent tool to study the formation and evolution of dark matter (DM) haloes and the mechanisms that have originated the universal profile at the largest mass scales in the Universe. In particular, the combination of the velocity dispersion σv with the density ρ can be used to define the pseudo-entropy $S(r)=\sigma _\mathrm{v}^2/\rho ^{\, 2/3}$, whose profile is well described by a simple power law $S\propto \, r^{\, \alpha }$. We analyse a set of cosmological hydrodynamical re-simulations of massive galaxy clusters and study the pseudo-entropy profiles as traced by different collisionless components in simulated galaxy clusters: DM, stars, and substructures. We analyse four sets of simulations, exploring different resolution and physics (N-body and full hydrodynamical simulations) to investigate convergence and the impact of baryons. We find that baryons significantly affect the inner region of pseudo-entropy profiles as traced by substructures, while DM particles profiles are characterized by an almost universal behaviour, thus suggesting that the level of pseudo-entropy could represent a potential low-scatter mass-proxy. We compare observed and simulated pseudo-entropy profiles and find good agreement in both normalization and slope. We demonstrate, however, that the method used to derive observed pseudo-entropy profiles could introduce biases and underestimate the impact of mergers. Finally, we investigate the pseudo-entropy traced by the stars focusing our interest in the dynamical distinction between intracluster light and the stars bound to the brightest cluster galaxy: the combination of these two pseudo-entropy profiles is well described by a single power law out to almost the entire cluster virial radius.