The abundance and structure of dark matter subhaloes have been analysed extensively in recent studies of dark-matter-only simulations, but comparatively little is known about the impact of baryonic physics on halo substructures. We here extend the SUBFIND algorithm for substructure identification such that it can be reliably applied to dissipative hydrodynamical simulations that include star formation. This allows, in particular, the identification of galaxies as substructures in simulations of clusters of galaxies and a determination of their content of gravitationally bound stars, dark matter and hot and cold gas. Using a large set of cosmological cluster simulations, we present a detailed analysis of halo substructures in hydrodynamical simulations of galaxy clusters, focusing in particular on the influence both of radiative and non-radiative gas physics and of non-standard physics such as thermal conduction and feedback by galactic outflows. We also examine the impact of numerica

We present results on the X-ray properties of clusters and groups of galaxies, extracted from a large cosmological hydrodynamical simulation. We used the TREE+SPH code GADGET to simulate a concordance Λ cold dark matter cosmological model within a box of 192 h-1 Mpc on a side, 4803 dark matter particles and as many gas particles. The simulation includes radiative cooling assuming zero metallicity, star formation and supernova feedback. The very high dynamic range of the simulation allows us to cover a fairly large interval of cluster temperatures. We compute X-ray observables of the intracluster medium (ICM) for simulated groups and clusters and analyse their statistical properties. The simulated mass-temperature relation is consistent with observations once we mimic the procedure for mass estimates applied to real clusters. Also, with the adopted choices of Ωm= 0.3 and σ8= 0.8 for matter density and power spectrum normalization, respectively, the resulting X-ray temperature function agrees with the most recent observational determinations. The luminosity-temperature relation also agrees with observations for clusters with T>~ 2 keV. At the scale of groups, T<~ 1 keV, we find no change of slope in this relation. The entropy in central cluster regions is higher than predicted by gravitational heating alone, the excess being almost the same for clusters and groups. We also find that the simulated clusters appear to have suffered some overcooling. We find f*~= 0.2 for the fraction of baryons in stars within clusters, thus approximately twice as large as the value observed. Interestingly, temperature profiles of simulated clusters are found to increase steadily toward cluster centres. They decrease in the outer regions, much like observational data do at r>~ 0.2rvir, while not showing an isothermal regime followed by a smooth temperature decline in the innermost regions. Our results thus demonstrate the need for yet more efficient sources of energy feedback and/or the need to consider additional physical process which may be able to further suppress the gas density at the scale of poor clusters and groups, and, at the same time, to regulate the cooling of the ICM in central regions

We compare the results of various cosmological gas-dynamical codes used to simulate the formation of a galaxy in the Λ cold dark matter structure formation paradigm. The various runs (13 in total) differ in their numerical hydrodynamical treatment [smoothed particle hydrodynamics (SPH), moving mesh and adaptive mesh refinement] but share the same initial conditions and adopt in each case their latest published model of gas cooling, star formation and feedback. Despite the common halo assembly history, we find large code-to-code variations in the stellar mass, size, morphology and gas content of the galaxy at z= 0, due mainly to the different implementations of star formation and feedback. Compared with observation, most codes tend to produce an overly massive galaxy, smaller and less gas rich than typical spirals, with a massive bulge and a declining rotation curve. A stellar disc is discernible in most simulations, although its prominence varies widely from code to code. There is a well-defined trend between the effects of feedback and the severity of the disagreement with observed spirals. In general, models that are more effective at limiting the baryonic mass of the galaxy come closer to matching observed galaxy scaling laws, but often to the detriment of the disc component. Although numerical convergence is not particularly good for any of the codes, our conclusions hold at two different numerical resolutions. Some differences can also be traced to the different numerical techniques; for example, more gas seems able to cool and become available for star formation in grid-based codes than in SPH. However, this effect is small compared to the variations induced by different feedback prescriptions. We conclude that state-of-the-art simulations cannot yet uniquely predict the properties of the baryonic component of a galaxy, even when the assembly history of its host halo is fully specified. Developing feedback algorithms that can effectively regulate the mass of a galaxy without hindering the formation of high angular momentum stellar discs remains a challenge

We study the origin of the diffuse stellar component (DSC) in 117 galaxy clusters extracted from a cosmological hydrodynamical simulation. We identify all galaxies present in the simulated clusters at 17 output redshifts, starting with z = 3.5, and then build the family trees for all the z = 0 cluster galaxies. The most massive cluster galaxies show complex family trees, resembling the merger trees of dark matter haloes, while the majority of other cluster galaxies experience only one or two major mergers during their entire life history. Then, for each diffuse star particle identified at z = 0, we look for the galaxy to which it once belonged at an earlier redshift, thus linking the presence of the DSC to the galaxy formation history.The main results of our analysis are as follows. (i) On average, half of the DSC star particles come from galaxies associated with the family tree of the most massive galaxy (bright cluster galaxy -hereafter BCG), one quarter comes from the family trees of other massive galaxies and the remaining quarter from dissolved galaxies. That is, the formation of the DSC is parallel to the build-up of the BCG and other massive galaxies. (ii) Most DSC star particles become unbound during mergers in the formation history of the BCGs and of other massive galaxies, independent of cluster mass. Our results suggest that the tidal stripping mechanism is responsible only for a minor fraction of the DSC. (iii) At cluster radii larger than 250 h −1 kpc, the DSC fraction from the BCG is reduced and the largest contribution comes from the other massive galaxies; in the cluster outskirts, galaxies of all masses contribute to the DSC. (iv) The DSC does not have a preferred redshift of formation: however, most DSC stars are unbound at z < 1. (v) The amount of DSC stars at z = 0 does not correlate strongly with the global dynamical history of clusters, and increases weakly with cluster mass.

We present an analysis of the relation between the masses of cluster-and group-sized halos, extracted from ΛCDM cosmological N-body and hydrodynamic simulations, and their velocity dispersions, at different redshifts from z = 2 to z = 0. The main aim of this analysis is to understand how the implementation of baryonic physics in simulations affects such relation, i.e. to what extent the use of the velocity dispersion as a proxy for cluster mass determination is hampered by the imperfect knowledge of the baryonic physics. In our analysis we use several sets of simulations with different physics implemented: one DM-only simulation, one simulation with non-radiative gas, and two radiative simulations, one of which with feedback from Active Galactic Nuclei. Velocity dispersions are determined using three different tracers, dark matter (DM hereafter) particles, subhalos, and galaxies.We confirm that DM particles trace a relation that is fully consistent with the theoretical expectations based on the virial theorem, σ v ∝ M α with α = 1/3, and with previous results presented in the literature. On the other hand, subhalos and galaxies trace steeper relations, with velocity dispersion scaling with mass with α > 1/3, and with larger values of the normalization. Such relations imply that galaxies and subhalos have a ∼ 10 per cent velocity bias relative to the DM particles, which can be either positive or negative, depending on halo mass, redshift and physics implemented in the simulation.We explain these differences as due to dynamical processes, namely dynamical friction and tidal disruption, acting on substructures and galaxies, but not on DM particles. These processes appear to be more or less effective, depending on the halo masses and the importance of baryon cooling, and may create a non-trivial dependence of the velocity bias and the σ 1D -M 200 relation on the tracer, the halo mass and its redshift.These results are relevant in view of the application of velocity dispersion as a proxy for cluster masses in ongoing and future large redshift surveys.

Aims. We study the efficiency and reliability of cluster mass estimators that are based on the projected phase-space distribution of galaxies in a cluster region. Methods. We analyse a data-set of 62 clusters extracted from a concordance ΛCDM cosmological hydrodynamical simulation. We consider both dark matter (DM) particles and simulated galaxies as tracers of the clusters gravitational potential. Two cluster mass estimators are considered: the virial mass estimator, corrected for the surface-pressure term, and a mass estimator (that we call M σ ) based entirely on the velocity dispersion estimate of the cluster. In order to simulate observations, galaxies (or DM particles) are first selected in cylinders of given radius (from 0.5 to 1.5h −1 Mpc) and 200h −1 Mpc length. Cluster members are then identified by applying a suitable interloper removal algorithm. Results. The virial mass estimator overestimates the true mass by 10% on average, for sample sizes of > ∼ 60 cluster members. For similar sample sizes, M σ underestimates the true mass by 15%, on average. For smaller sample sizes, the bias of the virial mass estimator substantially increases, while the M σ estimator becomes essentially unbiased. The dispersion of both mass estimates increases by a factor ∼2 as the number of cluster members decreases from ∼400 to ∼20. It is possible to reduce the bias in the virial mass estimates either by removing clusters with significant evidence for subclustering or by selecting early-type galaxies, which substantially reduces the interloper contamination. Early-type galaxies cannot however be used to improve the M σ estimates since their intrinsic velocity distribution is slightly biased relative to that of the DM particles. Radially-dependent incompleteness can drastically affect the virial mass estimates, but leaves the M σ estimates almost unaffected. Other observational effects, like centering and velocity errors and different observational apertures, have little effect on the mass estimates.

We present a study of the effect of active galactic nuclei (AGN) feedback on metal enrichment and thermal properties of the intracluster medium (ICM) in hydrodynamical simulations of galaxy clusters. The simulations are performed using a version of the TreePM–sphgadget‐2 code, which also follows chemodynamical evolution by accounting for metal enrichment contributed by different stellar populations. We carry out cosmological simulations for a set of galaxy clusters, covering the mass range M200≃ (0.1–2.2) × 1015 h−1 M⊙. Besides runs not including any efficient form of energy feedback, we carry out simulations including three different feedback schemes: (i) kinetic feedback in the form of galactic winds triggered by supernova explosions; (ii) AGN feedback from gas accretion on to supermassive black holes (BHs) and (iii) AGN feedback in which a ‘radio mode’ is included with an efficient thermal coupling of the extracted energy, whenever BHs enter in a quiescent accretion phase. Besides investigating the resulting thermal properties of the ICM, we analyse in detail the effect that these feedback models have on the ICM metal enrichment. We find that AGN feedback has the desired effect of quenching star formation in the brightest cluster galaxies at z < 4 and provides correct temperature profiles in the central regions of galaxy groups. However, its effect is not yet sufficient to create ‘cool cores’ in massive clusters while generating an excess of entropy in central regions of galaxy groups. As for the pattern of metal distribution, AGN feedback creates a widespread enrichment in the outskirts of clusters, thanks to its efficiency in displacing enriched gas from galactic haloes to the intergalactic medium. This turns into profiles of iron abundance, ZFe, which are in better agreement with observational results, and into a more pristine enrichment of the ICM around and beyond the cluster virial regions. Following the pattern of the relative abundances of silicon and iron, we conclude that a significant fraction of the ICM enrichment is contributed in simulations by a diffuse population of intracluster stars. Our simulations also predict that profiles of the ZSi/ZFe abundance ratio do not increase at increasing radii, at least out to 0.5Rvir. Our results clearly show that different sources of energy feedback leave distinct imprints in the enrichment pattern of the ICM. They further demonstrate that such imprints are more evident when looking at external regions, approaching the cluster virial boundaries.

We study the properties of the diffuse light in galaxy clusters forming in a large hydrodynamical cosmological simulation of the L cold dark matter cosmology. The simulation includes a model for radiative cooling, star formation in dense cold gas, and feedback by Type II supernova explosions. We select clusters having mass 14 Ϫ1M 1 10 h M , and study the spatial distribution of their star particles. While most stellar light is concentrated in gravitationally bound galaxies orbiting in the cluster potential, we find evidence for a substantial diffuse component, which may account for the extended halos of light observed around central cD galaxies. We find that more massive simulated clusters have a larger fraction of stars in the diffuse light than the less massive ones. The intracluster light is more centrally concentrated than the galaxy light, and the stars in the diffuse component are on average older than the stars in cluster galaxies, supporting the view that the diffuse light is not a random sampling of the stellar population in the cluster galaxies. We thus expect that at least ∼10% of the stars in a cluster may be distributed as intracluster light, largely hidden thus far because of its very low surface brightness.