Giuseppe Murante

Substructures in hydrodynamical cluster simulations

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 numerical nuisance parameters such as artificial viscosity parameterizations. We find that diffuse hot gas is efficiently stripped from subhaloes when they enter the highly pressurized cluster atmosphere. This has the effect of decreasing the subhalo mass function relative to a corresponding dark-matter-only simulation. These effects are mitigated in radiative runs, where baryons condense in the central subhalo regions and form compact stellar cores. However, in all cases, only a very small fraction, of the order of one per cent, of subhaloes within the cluster virial radii preserve a gravitationally bound hot gaseous atmosphere. The fraction of mass contributed by gas in subhaloes is found to increase with the cluster-centric distance. Interestingly, this trend extends well beyond the virial radii, thus showing that galaxies feel the environment of the pressurized cluster gas over fairly large distances. The compact stellar cores (i.e. galaxies) are generally more resistant against tidal disruption than pure dark matter subhaloes. Still, the fraction of star-dominated substructures within our simulated clusters is only ~10 per cent. We expect that the finite resolution in our simulations makes the galaxies overly susceptible to tidal disruption, hence the above fraction of star-dominated galaxies should represent a lower limit for the actual fraction of galaxies surviving the disruption of their host dark matter subhalo.

The Aquila comparison project: the effects of feedback and numerical methods on simulations of galaxy formation

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.

X-ray properties of galaxy clusters and groups from a cosmological hydrodynamical simulation

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 A cold dark matter cosmological model within a box of 192 h -1 Mpc on a side, 480 3 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 Ω m = 0.3 and σ 8 = 0.8 for matter density and power spectrum normalization, respectively, the resulting X-ray temperature functton 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 ≥ 0.2r vir , 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.

The importance of mergers for the origin of intracluster stars in cosmological simulations of galaxy clusters

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.

The diffuse light in simulations of galaxy clusters

We study the properties of the diffuse light in galaxy clusters forming in a large hydrodynamical cosmological simulation of the Λ 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 M > 1014 h-1 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.

Cooling and heating the intracluster medium in hydrodynamical simulations

We discuss tree+SPH (smoothed-particle hydrodynamics) simulations of galaxy clusters and groups, aimed at studying the effect of cooling and non-gravitational heating on observable properties of the intracluster medium (ICM). We simulate at high resolution four group- and cluster-sized haloes, with virial masses in the range (0.2‐4) × 10 14 M� , extracted from a cosmological simulation of a flat �-cold dark matter model. We discuss the effects of using different SPH implementations and show that high resolution is mandatory to correctly follow the cooling pattern of the ICM. Our recipes for non-gravitational heating release energy to the gas either in an impulsive way, at some heating redshift, or by modulating the heating as a function of redshift according to the star formation history predicted by a semi-analytic model of galaxy formation. Our simulations demonstrate that cooling and non-gravitational heating exhibit a rather complex interplay in determining the properties of the ICM: results on the amount of star formation and on the X-ray properties are sensitive not only to the amount of heating energy, but also depend on the redshift at which it is assigned to gas particles. All of our heating schemes that correctly reproduce the X-ray scaling properties of clusters and groups do not succeed in reducing the fraction of collapsed gas below a level of 20 (30) per cent at the cluster (group) scale, which appears to be in excess of observational constraints. Finally, gas compression in cooling cluster regions causes an increase of the temperature and a steepening of the temperature profiles, independent of the presence of non-gravitational heating processes. This is inconsistent with recent observational evidence for a decrease of gas temperature towards the centre of relaxed clusters. Provided these discrepancies persist even for a more refined modelling of energy feedback from supernova or active galactic nuclei, they may indicate that some basic physical process is still missing in hydrodynamical simulations.

The baryon fraction in hydrodynamical simulations of galaxy clusters

We study the baryon mass fraction in a set of hydrodynamical simulations of galaxy clusters performed using the Tree+SPH code GADGET-2. We investigate the dependence of the baryon fraction upon radiative cooling, star formation, feedback through galactic winds, conduction and redshift. Both the cold stellar component and the hot X-ray-emitting gas have narrow distributions that, at large cluster-centric distances r R 500, are nearly independent of the physics included in the simulations. Only the non-radiative runs reproduce the gas fraction inferred from observations of the inner regions (r ≈ R 2500 )o fmassive clusters. When cooling is turned on, the excess star formation is mitigated by the action of galactic winds, yet not by the amount required by observational data. The baryon fraction within a fixed overdensity increases slightly with redshift, independent of the physical processes involved in the accumulation of baryons in the cluster potential well. In runs with cooling and feedback, the increase in baryons is associated with a larger stellar mass fraction that arises at high redshift as a consequence of more efficient gas cooling. For the same reason, the gas fraction appears less concentrated at higher redshift. We discuss the possible cosmological implications of our results, and find that two assumptions generally adopted, i.e. (1) mean value of Y b = f b/(� b/� m) not evolving with redshift, and (2) a fixed ratio between f star and f gas independent of radius and redshift, might not be valid. In the estimate of the cosmic matter density parameter, this implies some systematic effects of the order of �� m/� m +0.15 for non-radiative runs and �� m/� m ≈ +0.05 and −0.05 for radiative simulations. Ke yw ords: methods: numerical ‐ galaxies: clusters: general ‐ cosmology: miscellaneous ‐ X-rays: galaxies.

The relation between velocity dispersion and mass in simulated clusters of galaxies: dependence on the tracer and the baryonic physics

[Abridged] We present an analysis of the relation between the masses of cluster- and group-sized halos, extracted from

Simulated X-ray galaxy clusters at the virial radius: slopes of the gas density, temperature and surface brightness profiles

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Baryon census in hydrodynamical simulations of galaxy clusters

CDM cosmological N-body and hydrodynamic simulations, and their velocity dispersions, at different redshifts from