L. Tornatore

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 transition from population III to population II-I star formation

We present results from the first cosmological simulations which study the onset of primordial, metal-free (population III), cosmic star formation and the transition to the present-day, metalrich star formation (population II-I), including molecular (H2, HD, etc.) evolution, tracing the injection of metals by supernovae (SNe) into the surrounding intergalactic medium and following the change in the initial mass function (IMF) according to the metallicity of the corresponding stellar population. Our investigation addresses the role of a wide variety of parameters (critical metallicity for the transition, IMF slope and range, SN/pair-instability SN metal yields, star formation threshold, resolution, etc.) on the metal-enrichment history and the associated transition in the star formation mode. All simulations present common trends. Metal enrichment is very patchy, with rare, unpolluted regions surviving at all redshifts, inducing the simultaneous presence of metal-free and metal-rich star formation regimes. As a result of the rapid pollution within high-density regions due to the first SN/pair-instability SN, local metallicity is quickly boosted above the critical metallicity for the transition. For this reason, population III stars dominate only during the very first stages of structure formation, with an average contribution to the total star formation rate that reaches a constant value of ∼10 −3 at redshift ∼11–13. If primordial supenovae consisted only of type II ones, the contribution would be ∼10 −1 . Interestingly, the above conclusions are independent from many poorly constrained

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.

Simulating the metal enrichment of the intracluster medium

We present results from Tree + SPH simulations of a galaxy cluster, aimed at studying the metal enrichment of the intracluster medium (ICM). The simulation code includes a fairly advanced treatment of star formation, as well as the release of energy feedback and detailed yields from both Type II and Type la supernovae, also accurately accounting for the lifetimes of different stellar populations. We perform simulations of a cluster with virial mass ≃3.9 x 10 14 M ○. , to investigate the effect of varying the feedback strength and the stellar initial mass function (IMF). Although most of the models are able to produce acceptable amounts of Fe mass, we find that the profiles of the iron abundance are always steeper than observed. The [O/Fe] ratio is found to be subsolar for a Salpeter IMF, with [O/Fe] ≃ -0.2 at R > 0.1 R 200 , whereas increasing to supersolar values in central regions, as a result of recent star formation. Using a top-heavier IMF gives a larger [O/Fe] over the whole cluster, at variance with observations. On the other hand, the adoption of a variable IMF, which becomes top-heavier at z > 2, provides a roughly solar [O/Fe] ratio. Our results indicate that our simulations still lack a feedback mechanism which should quench star formation at low redshift and transport metals away from the star-forming regions.

Baryon census in hydrodynamical simulations of galaxy clusters

We carry out an analysis of a set of cosmological SPH hydrodynamical simulations of galaxy clusters and groups aimed at studying the total baryon budget in clusters, and how this budget is shared between the hot diffuse component and the stellar component. Using the TreePM+SPHGADGET-3 code, we carried out one set of non‐radiative simulations, and two sets of simulations including radiative cooling, star form ation and feedback from supernovae (SN), one of which also accounting for the effect of feedback from active galactic nuclei (AGN). The analysis is carried out with the twofold aim of studying the implication of stellar and hot gas content on the relative role played by SN and AGN feedback, and to calibrate the cluster baryon fraction and its evolution as a cosmological tool. With respect to previous similar analysis, the simulations used in this study provid e us with a sufficient statistics of massive objects and including an efficient AGN feedback. We fi nd that both radiative simulation sets predict a trend of stellar mass fraction with clu ster mass that tends to be weaker than the observed one. However this tension depends on the particular set of observational data considered. Including the effect of AGN feedback alleviates this tension on the stellar mass and predicts values of the hot gas mass fraction and total baryon fraction to be in closer agreement with observational results. We further compute the ratio between the cluster baryon content and the cosmic baryon fraction, Yb, as a function of cluster-centric radius and redshift. At R500 we find for massive clusters with M500 > 2×10 14 h −1 M⊙ that Yb is nearly independent of the physical processes included and characterized by a negligible redshift evolution: Yb,500 = 0.85 ± 0.03 with the error accounting for the intrinsic r.m.s. scatter w ithin the set of simulated clusters. At smaller radii, R2500, the typical value of Yb slightly decreases, by an amount that depends on the physics included in the simulations, while its scatter increases by about a factor of two. These results have interesting implications for the cosmological applications of the baryon fraction in clusters.

Evolution at z 0.5 of the X-ray properties of simulated galaxy clusters : comparison with observational constraints

We analyse the X-ray properties of a sample of local and high-redshift galaxy clusters extracted from a large cosmological hydrodynamical simulation. This simulation has been realized using the tree + SPH (smoothed particle hydrodynamic) code GADGET-2 for a A-cold dark matter (ACDM) model. It includes radiative cooling, star formation and supernova feedback and allows the thermodynamic structure of clusters to be resolved radially up to redshift z = 1 in a way that is not yet completely accessible to observations. We consider only objects with T ew > 2 keV to avoid the large scatter in the physical properties present at the scale of groups and compare their properties to recent observational constraints. In our analysis, we adopt an approach that mimics observations, associating with each measurement an error comparable with recent observations and providing best-fitting results via robust techniques. Within the clusters, baryons are distributed among (i) a cold neutral phase, with a relative contribution that increases from less than 1 per cent to 3 per cent at higher redshift, (ii) stars, which contribute about 20 per cent, and (iii) the X-ray-emitting plasma, which contributes 80 (76) per cent at z = 0 (1) to the total baryonic budget. A depletion of the cosmic baryon fraction of ∼7 per cent (at z = 0) and 5 per cent (at z = 1) is measured at the virial radius, R vir , in good agreement with adiabatic hydrodynamical simulations. We confirm that, also at redshift z > 0.5, power-law relations hold between the gas temperature T, the bolometric luminosity L, the central entropy S, the gas mass M gas and the total gravitating mass M tot , and that these relations are steeper than predicted by simple gravitational collapse. A significant negative evolution in the L-T and L-M tot relations and positive evolution in the S-T relation are detected at 0.5 < z < 1 in this set of simulated galaxy clusters. This is partially consistent with recent analyses of the observed properties of z ≥ 0.5 X-ray galaxy clusters. By fixing the slope to the values predicted by simple gravitational collapse, at high redshift we measure normalizations lower than the observed estimates by 10-40 per cent in the L-T, M tot -T, M gas -T, f gas -T and L-M tot relations. This suggests either that the amount of hot X-ray-emitting plasma measured in the central regions of simulated systems is smaller than the observed one or that systematically higher values of gas temperatures and total masses than actually measured are recovered in the present simulated data set.

Damped Lyman α systems in high-resolution hydrodynamical simulations

We investigate the properties of damped Lyman α systems (DLAs) using high-resolution and large box-size cosmological hydrodynamical simulations of a A cold dark matter model. The numerical code used is a modification of GADGET-2 with a self-consistent implementation of the metal enrichment mechanism. We explore the numerical convergence of some relevant physical quantities and we vary the parameters describing the properties of galactic winds, the initial stellar mass function, the linear dark matter power spectrum and the metal enrichment pattern of the intergalactic medium (IGM) around DLAs. We focus on the properties of dark matter haloes that are likely to be the hosts of DLAs systems: we predict relatively low star formation rates (∼0.01―0.1 M ⊙ ) yr ―1 ) and metallicities around 0.1 Z ⊙ ), at least for the bulk of our haloes of masses between 10 9 and 10 10 h ―1 M ⊙ hosting DLAs. For more massive haloes metallicities and star formation rates depend on the specific wind model. We find that strong galactic winds with speed of about 600 km s ―1 , in an energy-driven wind scenario, are needed in order to match the observed column density distribution function for DLAs and the evolution of the neutral hydrogen content with redshift. The momentum-driven implementation of the galactic wind model, that relates the speed and mass load in the wind to the properties of the dark matter haloes, shows a behaviour which is intermediate between the energy-driven galactic winds of small (∼100 km s ―1 ) and large (∼600 km s ―1 ) velocities. At z = 3 the contribution of haloes of masses between 10 9 and 10 10 h ―1 M ⊙ , for DLAs below 10 20.8 g cm ―2 , to the column density distribution function, is significant. By interpolating physical quantities along line-of-sights through massive haloes we qualitatively show how different galactic wind models impact on the IGM around DLAs. Furthermore, we analyse statistics related to the velocity widths of Si II associated to DLAs: while the expanding shells of gaseous matter associated to the wind can account for the observed velocities, the metallicity in the wind seems to be rather clumpy and this produces an underestimation of the observed velocity widths. We outline possible solutions to this problem.

Lyman alpha emitter evolution in the reionization epoch

Combining cosmological smoothed particle hydrodynamics (SPH) simulations with a previously developed Lyα production/transmission model and the Early Reionization Model (ERM; reionization ends at redshift z ~ 7), we obtain Lyα and UV luminosity functions (LFs) for lyman alpha emitters (LAEs) at 5.7 ≤ z ≤ 7.6. Matching model results to observations at z ~ 5.7 requires escape fractions of Lyα, f α = 0.3, and UV (non-ionizing) continuum photons, f c = 0.22, corresponding to a colour excess, E(B - V) = 0.15. We find that (i) f c increases towards higher redshifts, due the decreasing mean dust content of galaxies, (ii) the evolution of f α /f c hints at the dust content of the interstellar medium becoming progressively inhomogeneous/clumped with decreasing redshift. Using the model assumptions, clustering of sources has little effect on the Lyα LF for a cosmic hydrogen neutral fraction χHI ≤ 10 -4 , a value attained at z ≤ 6.6 in the ERM. However, during the initial reionization phases (z ≳ 7), the clustering photoionization boost becomes important. We quantify the physical properties of observed LAEs and their redshift evolution, for which we give handy analytical fitting functions. Halo (stellar) masses are in the range 10.0 20 Myr at all redshifts, while the mean stellar metallicity increases from Z = 0.12 Z ⊙ at z ~7.6 to Z = 0.22 Z ⊙ at z 5.7; both t * and Z positively correlate with stellar mass. The brightest LAEs are all characterized by large M*and intermediate ages (≈200 Myr), while objects in the faint end of the Lyα LF show large age and star formation rate spreads. With no more free parameters, the spectral energy distributions of three LAE at z ~ 5.7 observed by Lai et al. (2007) are well reproduced by an intermediate age (182-220 Myr) stellar population and the above E(B - V) value. The model uncertainties, mostly related to the simplified treatment of dust and to the possible effects related to gas outflow/infall, are discussed along with their impact on the results.

The impact of feedback on the low-redshift intergalactic medium

We analyse the evolution of the properties of the low-redshift intergalactic medium (IGM) using high-resolution hydrodynamic simulations that include a detailed chemical evolution model. We focus on the effects that two different forms of energy feedback, strong galactic winds driven by supernova explosion and active galactic nuclei powered by gas accretion on to super-massive black holes (BHs), have on the thermo- and chemodynamical properties of the low-redshift IGM. We find that feedback associated with winds (W) and BHs leaves distinct signatures in both the chemical and thermal history of the baryons, especially at redshift z < 3. BH feedback produces an amount of gas with temperature in the range of 10 5 -10 7 K, the warm-hot intergalactic medium (WHIM), larger than that produced by the wind feedback. At z = 0, the fraction of baryons in the WHIM is about 50 per cent in the runs with BH feedback and about 40 per cent in the runs with wind feedback. The number of warm baryons ( 1 0 4 < T < 10 5 K) is instead at about the same level, ~30 per cent, in the runs with BH and wind feedback. Also, BH feedback provides a stronger and more pristine enrichment of the WHIM. We find that the metal-mass-weighted age of WHIM enrichment at z = 0 is on average a factor of ~ 1.5 smaller in the BH run than for the corresponding runs with galactic winds. We present results for the enrichment in terms of mass and metallicity distributions for the WHIM phase, both as a function of density and as a function of temperature. Finally, we compute the evolution of the relative abundances between different heavy elements, namely oxygen, carbon and iron. While both C/O and O/Fe evolve differently at high redshifts for different feedback models, their values are similar at z = 0. We also find that changing the stellar initial mass function has a smaller effect on the evolution of the above relative abundances than changing the feedback model. The sensitivity of WHIM properties on the implemented feedback scheme could be important both for discriminating between different feedback physics and for detecting the WHIM with future far-UV and X-ray telescopes.