Search

The Euclid photometric survey of galaxy clusters stands as a powerful cosmological tool, with the capacity to significantly propel our understanding of the Universe. Despite being subdominant to dark matter and dark energy, the baryonic component of our Universe holds substantial influence over the structure and mass of galaxy clusters. This paper presents a novel model that can be used to precisely quantify the impact of baryons on the virial halo masses of galaxy clusters using the baryon fraction within a cluster as a proxy for their effect. Constructed on the premise of quasi-adiabaticity, the model includes two parameters, which are calibrated using non-radiative cosmological hydrodynamical simulations, and a single large-scale simulation from the Magneticum set, which includes the physical processes driving galaxy formation. As a main result of our analysis, we demonstrate that this model delivers a remarkable 1% relative accuracy in determining the virial dark matter-only equivalent mass of galaxy clusters starting from the corresponding total cluster mass and baryon fraction measured in hydrodynamical simulations. Furthermore, we demonstrate that this result is robust against changes in cosmological parameters and against variation of the numerical implementation of the subresolution physical processes included in the simulations. Our work substantiates previous claims regarding the impact of baryons on cluster cosmology studies. In particular, we show how neglecting these effects would lead to biased cosmological constraints for a Euclid-like cluster abundance analysis. Importantly, we demonstrate that uncertainties associated with our model arising from baryonic corrections to cluster masses are subdominant when compared to the precision with which mass–observable (i.e. richness) relations will be calibrated using Euclid and to our current understanding of the baryon fraction within galaxy clusters. [ABSTRACT FROM AUTHOR]

Aims. We studied the star formation rate (SFR) in cosmological hydrodynamical simulations of galaxy (proto-)clusters in the redshift range 0 <  z <  4, comparing them to recent observational studies; we also investigated the effect of varying the parameters of the star formation model on galaxy properties such as SFR, star-formation efficiency, and gas fraction. Methods. We analyse a set of zoom-in cosmological hydrodynamical simulations centred on 12 clusters. The simulations are carried out with the GADGET-3 Tree-PM smoothed-particle hydro-dynamics code which includes various subgrid models to treat unresolved baryonic physics, including AGN feedback. Results. Simulations do not reproduce the high values of SFR observed within protocluster cores, where the values of SFR are underpredicted by a factor ≳4 both at z ∼ 2 and z ∼ 4. The difference arises as simulations are unable to reproduce the observed starburst population and is greater at z ∼ 2 because simulations underpredict the normalisation of the main sequence (MS) of star forming galaxies (i.e. the correlation between stellar mass and SFR) by a factor of ∼3. As the low normalisation of the MS seems to be driven by an underestimated gas fraction, it remains unclear whether numerical simulations miss starburst galaxies due to overly underpredicted gas fractions or overly low star formation efficiencies. Our results are stable against varying several parameters of the star formation subgrid model and do not depend on the details of AGN feedback. Conclusions. The subgrid model for star formation, introduced to reproduce the self-regulated evolution of quiescent galaxies, is not suitable to describe violent events like high-redshift starbursts. We find that this conclusion holds, independently of the parameter choice for the star formation and AGN models. The increasing number of multi-wavelength high-redshift observations will help to improve the current star formation model, which is needed to fully recover the observed star formation history of galaxy clusters. [ABSTRACT FROM AUTHOR]

The uniformity of the intracluster medium (ICM) enrichment level in the outskirts of nearby galaxy clusters suggests that chemical elements were deposited and widely spread into the intergalactic medium before the cluster formation. This observational evidence is supported by numerical findings from cosmological hydrodynamical simulations, as presented in Biffi et al., including the effect of thermal feedback from active galactic nuclei. Here, we further investigate this picture, by tracing back in time the spatial origin and metallicity evolution of the gas residing at z = 0 in the outskirts of simulated galaxy clusters. In these regions, we find a large distribution of iron abundances, including a component of highly enriched gas, already present at z = 2. At z > 1, the gas in the present-day outskirts was distributed over tens of virial radii from the main cluster and had been already enriched within high-redshift haloes. At z=2, about 40 per cent of the most Fe-rich gas at z = 0 was not residing in any halo more massive than 1011 h-1 M⊙ in the region and yet its average iron abundance was already 0.4, w.r.t. the solar value by Anders & Grevesse. This confirms that the in situ enrichment of the ICM in the outskirts of present-day clusters does not play a significant role, and its uniform metal abundance is rather the consequence of the accretion of both low-metallicity and pre-enriched (at z > 2) gas, from the diffuse component and through merging substructures. These findings do not depend on the mass of the cluster nor on its core properties. [ABSTRACT FROM AUTHOR]

We analyse cosmological hydrodynamical simulations of galaxy clusters to study the X-ray scaling relations between total masses and observable quantities such as X-ray luminosity, gas mass, X-ray temperature, and YX. Three sets of simulations are performed with an improved version of the smoothed particle hydrodynamics GADGET-3 code. These consider the following: non-radiative gas, star formation and stellar feedback, and the addition of feedback by active galactic nuclei (AGN).We select clusters with M500 > 1014M☉E(z)-1, mimicking the typical selection of Sunyaev-Zeldovich samples. This permits to have a mass range large enough to enable robust fitting of the relations even at z ~ 2. The results of the analysis show a general agreement with observations. The values of the slope of the mass-gas mass and mass- temperature relations at z=2 are 10 per cent lowerwith respect to z=0 due to the appliedmass selection, in the former case, and to the effect of early merger in the latter. We investigate the impact of the slope variation on the study of the evolution of the normalization. We conclude that cosmological studies through scaling relations should be limited to the redshift range z = 0-1, where we find that the slope, the scatter, and the covariance matrix of the relations are stable. The scaling between mass and YX is confirmed to be the most robust relation, being almost independent of the gas physics. At higher redshifts, the scaling relations are sensitive to the inclusion of AGNs which influences low-mass systems. The detailed study of these objects will be crucial to evaluate the AGN effect on the ICM. [ABSTRACT FROM AUTHOR]

We present an implementation of smoothed particle hydrodynamics (SPH) with improved accuracy for simulations of galaxies and the large-scale structure. In particular, we implement and test a vast majority of SPH improvement in the developer version of GADGET-3. We use the Wendland kernel functions, a particle wake-up time-step limiting mechanism and a time-dependent scheme for artificial viscosity including high-order gradient computation and shear flow limiter. Additionally, we include a novel prescription for time-dependent artificial conduction, which corrects for gravitationally induced pressure gradients and improves the SPH performance in capturing the development of gas-dynamical instabilities. We extensively test our new implementation in a wide range of hydrodynamical standard tests including weak and strong shocks as well as shear flows, turbulent spectra, gas mixing, hydrostatic equilibria and self-gravitating gas clouds. We jointly employ all modifications; however, when necessary we study the performance of individual code modules. We approximate hydrodynamical states more accurately and with significantly less noise than standard GADGET-SPH. Furthermore, the newimplementation promotes the mixing of entropy between different fluid phases, also within cosmological simulations. Finally, we study the performance of the hydrodynamical solver in the context of radiative galaxy formation and non-radiative galaxy cluster formation. We find galactic discs to be colder and more extended and galaxy clusters showing entropy cores instead of steadily declining entropy profiles. In summary, we demonstrate that our improved SPH implementation overcomes most of the undesirable limitations of standard GADGET-SPH, thus becoming the core of an efficient code for large cosmological simulations. [ABSTRACT FROM AUTHOR]

We carry out an analysis of a set of cosmological smoothed particle hydrodynamics (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+SPH gadget-3 code, we carried out one set of non-radiative simulations, and two sets of simulations including radiative cooling, star formation and feedback from supernovae (SNe), 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 SNe 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 provide us with a sufficient statistics of massive objects and including an efficient AGN feedback. We find that both radiative simulation sets predict a trend of stellar mass fraction with cluster 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 clustercentric radius and redshift. At R500 we find for massive clusters with M500 > 2 × 1014 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 rms scatter within 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 2. These results have interesting implications for the cosmological applications of the baryon fraction in clusters. [ABSTRACT FROM AUTHOR]

We present an analysis of the relation between the masses of cluster- and group-sized haloes, extracted from Λ cold dark matter (ΛCDM) cosmological N-body and hydrodynamic simulations, and their velocity dispersion at different redshifts from z = 2 to 0. The main aim of this analysis is to understand how the implementation of baryonic physics in simulations affects such relations, 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: DM particles, subhaloes and galaxies. We confirm that DM particles trace a relation that is fully consistent with the theoretical expectations based on the virial theorem, σv ... Mα with α = 1/3, and with previous results presented in the literature. On the other hand, subhaloes 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 subhaloes have a -10 per cent velocity bias relative to the DM particles, which can be either positive or negative, depending on the 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 σ1D-M200 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. [ABSTRACT FROM AUTHOR]

ABSTRACT At present, 30-40 per cent of the baryons in the local Universe is still undetected. According to theoretical predictions, this gas should reside in filaments filling the large-scale structure (LSS) in the form of a warm-hot intergalactic medium (WHIM), at temperatures 105-107 K, thus emitting in the soft X-ray energies via free-free interaction and line emission from heavy elements. In this work, we characterize the properties of the X-ray emission of the WHIM, and the LSS in general, focusing on the influence of different physical mechanisms, namely galactic winds (GWs), black hole feedback and star formation, and providing estimates of possible observational constraints. To this purpose, we use a set of cosmological hydrodynamical simulations that include a self-consistent treatment of star formation and chemical enrichment of the intergalactic medium, which allows us to follow the evolution of different metal species. We construct a set of simulated light cones to make predictions of the emission in the 0.3-10 keV energy range. We obtain that GWs increase the emission of both galaxy clusters and WHIM by a factor of 2. The amount of oxygen at average temperature and, consequently, the amount of expected bright O vii and O viii lines are increased by a factor of 3 due to GWs and by 20 per cent when assuming a top-heavy initial mass function. We compare our results with current observational constraints and find that the emission from faint groups and WHIM should account for half to all of the unresolved X-ray background in the 1-2 keV band. [ABSTRACT FROM AUTHOR]

ABSTRACT We introduce a generalized scaling law, Mtot= 10 K A a B b, to look for the minimum scatter in reconstructing the total mass of hydrodynamically simulated X-ray galaxy clusters, given gas mass Mgas, luminosity L and temperature T. We find a locus in the plane of the logarithmic slopes a and b of the scaling relations where the scatter in mass is minimized. This locus corresponds to b M=−3/2 a M+ 3/2 and b L=−2 a L+ 3/2 for A= Mgas and L, respectively, and B= T. Along these axes, all the known scaling relations can be identified (at different levels of scatter), plus a new one defined as Mtot∝ ( LT)1/2. Simple formula to evaluate the expected evolution with redshift in the self-similar scenario is provided. In this scenario, no evolution of the scaling relations is predicted for the cases ( b M= 0, a M= 1) and ( b L= 7/2, a L=−1), respectively. Once the single quantities are normalized to the average values of the sample under considerations, the normalizations K corresponding to the region with minimum scatter are very close to zero. The combination of these relations allows one to reduce the number of free parameters of the fitting function that relates X-ray observables to the total mass and includes the self-similar redshift evolution. [ABSTRACT FROM AUTHOR]

ABSTRACT Using extended sets of cosmological hydrodynamical simulations of galaxy clusters, we present a detailed study of scaling relations between total cluster mass and three mass proxies based on X-ray observable quantities: temperature of the intracluster medium (ICM), gas mass and the product of the two, YX= Mgas T. Our analysis is based on two sets of high-resolution hydrodynamical simulations performed with the TreePM-SPH gadget code. The first set includes about 140 clusters with masses above 5 × 1013 h−1 M⊙, with 30 such clusters having mass above 1015 h−1 M⊙. All such clusters have been simulated in two flavours, both with non-radiative physics and including cooling, star formation, chemical enrichment and the effect of supernova feedback triggering galactic ejecta. The extensive statistics offered by this set of simulated clusters is used to quantify the robustness of the scaling relations between mass proxies and total mass, to determine their redshift evolution and to calibrate their intrinsic scatter and its distribution. Furthermore, we use a smaller set of clusters including 18 haloes with masses above 5 × 1013 h−1 M⊙, four of which are more massive than 1015 h−1 M⊙, to test the robustness of mass proxies against change in the physical processes that are included in the simulations to describe the evolution of the intracluster medium. Each cluster is simulated in seven different flavours to study the effects of (i) thermal conduction, (ii) artificial viscosity, (iii) cooling and star formation, (iv) galactic winds and (v) active galactic nucleus (AGN) feedback. As a general result, we find the M- YX scaling relation to be the least sensitive to variations in the ICM physics, its slope and redshift evolution always being very close to the predictions of the self-similar model. As regards the scatter around the best-fitting relations, its distribution is always close to a log-normal one. Mgas is the mass proxy with the smallest scatter in mass, with values of σln M≃ 0.04-0.06 depending on the physics included in the simulation and with a mild dependence on redshift. In terms of the mass-temperature relation, it is the one with the largest scatter, with σln M≳ 0.1 at z= 0 increasing to ≳0.15 at z= 1. The intrinsic scatter in the M- YX relation is slightly larger than that in the M- Mgas relation, with σln M≃ 0.06 at z= 0 and 0.08 at z= 1. These results confirm that both YX and Mgas mass proxies are well suited for cosmological applications in future large X-ray surveys. As a word of caution, we point out that the analysis presented in this paper does not include the observational effects expected when measuring temperature by fitting X-ray spectra and gas mass from X-ray surface-brightness profiles. A detailed assessment of such effects will be the subject of a forthcoming paper. [ABSTRACT FROM AUTHOR]

We present a detailed comparison between the galaxy populations within a massive cluster, as predicted by hydrodynamical smoothed particle hydrodynamics (SPH) simulations and by a semi-analytic model (SAM) of galaxy formation. Both models include gas cooling and a simple prescription of star formation, which consists in transforming instantaneously any cold gas available into stars, while neglecting any source of energy feedback. This simplified comparison is thus not meant to be compared with observational data, but is aimed at understanding the level of agreement, at the stripped-down level considered, between two techniques that are widely used to model galaxy formation in a cosmological framework and which present complementary advantages and disadvantages. We find that, in general, galaxy populations from SAMs and SPH have similar statistical properties, in agreement with previous studies. However, when comparing galaxies on an object-by-object basis, we find a number of interesting differences: (i) the star formation histories of the brightest cluster galaxies (BCGs) from SAM and SPH models differ significantly, with the SPH BCG exhibiting a lower level of star formation activity at low redshift, and a more intense and shorter initial burst of star formation with respect to its SAM counterpart; (ii) while all stars associated with the BCG were formed in its progenitors in the SAM used here, this holds true only for half of the final BCG stellar mass in the SPH simulation, the remaining half being contributed by tidal stripping of stars from the diffuse stellar component associated with galaxies accreted on the cluster halo; (iii) SPH satellites can lose up to 90 per cent of their stellar mass at the time of accretion, due to tidal stripping, a process not included in the SAM used in this paper; (iv) in the SPH simulation, significant cooling occurs on the most massive satellite galaxies and this lasts for up to 1 Gyr after accretion. This physical process is not included in the SAM used in this paper, as well as in most of the models discussed in the recent literature. Our results identify specific directions of improvements for our methods to study galaxy formation in a hierarchical universe. [ABSTRACT FROM AUTHOR]

Member galaxies within galaxy clusters nowadays can be routinely identified in cosmological, hydrodynamical simulations using methods based on identifying self bound, locally over dense substructures. However, distinguishing the central galaxy from the stellar diffuse component within clusters is notoriously difficult, and in the centre it is not even clear if two distinct stellar populations exist. Here, after subtracting all member galaxies, we use the velocity distribution of the remaining stars and detect two dynamically, well-distinct stellar components within simulated galaxy clusters. These differences in the dynamics can be used to apply an unbinding procedure which leads to a spatial separation of the two components into a cD and a diffuse stellar component (DSC). Applying our new algorithm to a cosmological, hydrodynamical simulation we find that – in line with previous studies – these two components have clearly distinguished spatial and velocity distributions as well as different star formation histories. We show that the DSC fraction – which can broadly be associated with the observed intracluster light – does not depend on the virial mass of the galaxy cluster and is much more sensitive to the formation history of the cluster. We conclude that the separation of the cD and the DSC in simulations, based on our dynamical criteria, is more physically motivated than current methods which depend on implicit assumptions on a length-scale associated with the cD galaxy and therefore represent a step forward in understanding the different stellar components within galaxy clusters. Our results also show the importance of analysing the dynamics of the DSC to characterize its properties and understand its origin. [ABSTRACT FROM AUTHOR]

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 . BH feedback produces an amount of gas with temperature in the range of , the warm-hot intergalactic medium (WHIM), larger than that produced by the wind feedback. At 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 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 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 . 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. [ABSTRACT FROM AUTHOR]

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 ), we obtain Lyα and UV luminosity functions (LFs) for lyman alpha emitters (LAEs) at . Matching model results to observations at requires escape fractions of , and UV (non-ionizing) continuum photons, , corresponding to a colour excess, . We find that (i) fc increases towards higher redshifts, due the decreasing mean dust content of galaxies, (ii) the evolution of 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 , a value attained at in the ERM. However, during the initial reionization phases , 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 ) with . The star formation rates are , mass-weighted mean ages are at all redshifts, while the mean stellar metallicity increases from at to at ; both t* and Z positively correlate with stellar mass. The brightest LAEs are all characterized by large 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 observed by Lai et al. (2007) are well reproduced by an intermediate age (182–220 Myr) stellar population and the above 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. [ABSTRACT FROM AUTHOR]

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 thesubfind 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. [ABSTRACT FROM AUTHOR]

We present a method to recover mass profiles of galaxy clusters by combining data on the thermal Sunyaev–Zeldovich (tSZ) and X-ray imaging, thereby avoiding the use of any information on X-ray spectroscopy. This method, which represents the development of the geometrical deprojection technique presented in a previous paper by Ameglio et al., implements the solution of the hydrostatic equilibrium equation. In order to quantify the efficiency of our mass reconstructions, we apply our technique to a set of hydrodynamical simulations of galaxy clusters. We propose two versions of our method of mass reconstruction. Method 1 is completely model-independent and assumes the values of gas density and total mass within different radial bins as fitting parameters. Method 2 assumes instead the analytic mass profile proposed by Navarro, Frenk & White (NFW). We find that the main source of bias in recovering the mass profiles is due to deviations from hydrostatic equilibrium, which cause an underestimation of the mass of about 10 per cent at r500 and up to 20 per cent at the virial radius. Method 1 provides a reconstructed mass which is biased low by about 10 per cent, with a 20 per cent scatter, with respect to the true mass profiles. Method 2 proves to be more stable, reducing the scatter to 10 per cent, but with a larger bias of 20 per cent, mainly induced by the deviations from equilibrium in the outskirts. To better understand the results of Method 2, we check how well it allows us to recover the relation between mass and concentration parameter. When analysing the three-dimensional mass profiles, we find that including in the fit the inner 5 per cent of the virial radius biases high the halo concentration, thus suggesting that the NFW profile is not a perfect fit in the central regions of our simulations including cooling and star formation. Also, at a fixed mass, hotter clusters tend to have larger concentration. Our procedure recovers the concentration parameter essentially unbiased but with a scatter of about 50 per cent. In general, our analysis demonstrates that combining X-ray imaging with spatially resolved tSZ data is a valid alternative to using X-ray spectroscopy to recover the mass of galaxy clusters. [ABSTRACT FROM AUTHOR]

We present results from two high-resolution hydrodynamical simulations of protocluster regions at . The simulations have been compared to observational results for the so-called Spiderweb galaxy system, the core of a putative protocluster region at , found around a radio galaxy. The simulated regions have been chosen so as to form a poor cluster with (C1) and a rich cluster with (C2) at . The simulated protoclusters show evidence of ongoing assembly of a dominating central galaxy. The stellar mass of the brightest cluster galaxy of the C2 system is in excess with respect to observational estimates for the Spiderweb galaxy, with a total star formation rate which is also larger than indicated by observations. We find that the projected velocities of galaxies in the C2 cluster are consistent with observations, while those measured for the poorer cluster C1 are too low compared with the observed velocities. We argue that the Spiderweb complex resembles the high-redshift progenitor of a rich galaxy cluster. Our results indicate that the included supernovae feedback is not enough to suppress star formation in these systems, supporting the need of introducing active galactic nuclei feedback. According to our simulations, a diffuse atmosphere of hot gas in hydrostatic equilibrium should already be present at this redshift, and enriched at a level comparable to that of nearby galaxy clusters. The presence of this gas should be detectable with future deep X-ray observations. [ABSTRACT FROM AUTHOR]

We study the degree to which non-radiative gas dynamics affect the merger histories of haloes along with subsequent predictions from a semi-analytic model (SAM) of galaxy formation. To this aim, we use a sample of dark matter only and non-radiative smooth particle hydrodynamics (SPH) simulations of four massive clusters. The presence of gas-dynamical processes (e.g. ram pressure from the hot intra-cluster atmosphere) makes haloes more fragile in the runs which include gas. This results in a 25 per cent decrease in the total number of subhaloes at . The impact on the galaxy population predicted by SAMs is complicated by the presence of ‘orphan’ galaxies, i.e. galaxies whose parent substructures are reduced below the resolution limit of the simulation. In the model employed in our study, these galaxies survive (unaffected by the tidal stripping process) for a residual merging time that is computed using a variation of the Chandrasekhar formula. Due to ram-pressure stripping, haloes in gas simulations tend to be less massive than their counterparts in the dark matter simulations. The resulting merging times for satellite galaxies are then longer in these simulations. On the other hand, the presence of gas influences the orbits of haloes making them on average more circular and therefore reducing the estimated merging times with respect to the dark matter only simulation. This effect is particularly significant for the most massive satellites and is (at least in part) responsible for the fact that brightest cluster galaxies in runs with gas have stellar masses which are about 25 per cent larger than those obtained from dark matter only simulations. Our results show that gas dynamics has only a marginal impact on the statistical properties of the galaxy population, but that its impact on the orbits and merging times of haloes strongly influences the assembly of the most massive galaxies. [ABSTRACT FROM AUTHOR]

We present a comparison between simulation results and X-ray observational data on the evolution of the metallicity of the intracluster medium (ICM). The simulations of galaxy clusters have been carried out using a version of thetree pm-smoothed particle hydrodynamics (SPH)gadget-2 code that includes a detailed model of chemical evolution by assuming three different shapes for the stellar initial mass function (IMF). Besides the Salpeter IMF, we also used the IMF proposed by Kroupa and the top-heavier IMF by Arimoto and Yoshii. We find that simulations predict significant radial gradients of the Iron abundance, , which extend over the whole cluster virialized region. Using the Salpeter IMF, the profiles of have an amplitude which is in a reasonable agreement with Chandra observations within . At larger radii, we do not detect any flattening of the metallicity profiles. As for the evolution of the ICM metal abundance out to , it turns out that the results based on the Salpeter IMF agree with observations. We find that the evolution of in simulations is determined by the combined action of (i) the sinking of already enriched gas, (ii) the ongoing metal production in galaxies and (iii) the locking of ICM metals in newborn stars. As a result, rather than suppressing the metallicity evolution, stopping star formation at has the effect of producing an even too fast evolution of the emission-weighted ICM metallicity, with too high values of at low redshift within . Finally, we compare simulations with the observed rate of Type Ia supernovae per unit B-band luminosity (SnU B). We find that our simulated clusters do not reproduce the decreasing trend of SnU B at low redshift, unless star formation is truncated at . [ABSTRACT FROM AUTHOR]

In order to study the process of cooling in dark matter haloes and assess how well simple models can represent it, we run a set of radiative smoothed particle hydrodynamics (SPH) simulations of isolated haloes, with gas sitting initially in hydrostatic equilibrium within Navarro–Frenk–White potential wells. Simulations include radiative cooling and a scheme to convert high-density cold gas particles into collisionless stars, neglecting any astrophysical source of energy feedback. After having assessed the numerical stability of the simulations, we compare the resulting evolution of the cooled mass with the predictions of the classical cooling model of White & Frenk and of the cooling model proposed in themorgana code of galaxy formation. We find that the classical model predicts fractions of cooled mass which, after about 2 central cooling times, are about one order of magnitude smaller than those found in simulations. Although this difference decreases with time, after 8 central cooling times, when simulations are stopped, the difference still amounts to a factor of 2–3. We ascribe this difference to the lack of validity of the assumption that a mass shell takes one cooling time, as computed on the initial conditions, to cool to very low temperature. Indeed, we find from simulations that cooling SPH particles take most time in travelling, at roughly constant temperature and increasing density, from their initial position to a central cooling region, where they quickly cool down to ∼104 K. We show that in this case the total cooling time is shorter than that computed on the initial conditions, as a consequence of the stronger radiative losses associated to the higher density experienced by these particles. As a consequence the mass cooling flow is stronger than that predicted by the classical model. Themorgana model, which computes the cooling rate as an integral over the contribution of cooling shells and does not make assumptions on the time needed by shells to reach very low temperature, better agrees with the cooled mass fraction found in the simulations, especially at early times, when the density profile of the cooling gas is shallow. With the addition of the simple assumption that the increase in the radius of the cooling region is counteracted by a shrinking at the sound speed, themorgana model is also able to reproduce for all simulations the evolution of the cooled mass fraction to within 20–50 per cent, thereby providing a substantial improvement with respect to the classical model. Finally, we provide a very simple fitting function which accurately reproduces the cooling flow for the first ∼10 central cooling times. [ABSTRACT FROM AUTHOR]

Using the results of a cosmological hydrodynamical simulation of the concordance Lambda cold dark matter model, we study the global properties of the Sunyaev–Zel'dovich (SZ) effects, both considering the thermal (tSZ) and the kinetic (kSZ) component. The simulation follows gravitation and gas dynamics and includes also several physical processes that affect the baryonic component, like a simple reionization scenario, radiative cooling, star formation and supernova feedback. Starting from the outputs of the simulation we create mock maps of the SZ signals due to the large structures of the Universe integrated in the range . We predict that the Compton y-parameter has an average value of (1.19 ± 0.32) × 10−6 and is lognormally distributed in the sky; half of the whole signal comes from and about 10 per cent from . The Doppler b-parameter shows approximately a normal distribution with vanishing mean value and an s.d. of 1.6 × 10−6, with a significant contribution from high-redshift gas. We find that the tSZ effect is expected to dominate the primary cosmic microwave background anisotropies for in the Rayleigh–Jeans limit, while interestingly the kSZ effect dominates at all frequencies at very high multipoles (ℓ≳ 7 × 104). We also analyse the cross-correlation between the two SZ effects and the soft (0.5–2 keV) X-ray emission from the intergalactic medium and we obtain a strong correlation between the three signals, especially between X-ray emission and tSZ effect at all angular scales. [ABSTRACT FROM AUTHOR]

We present a study of the galaxy population predicted by hydrodynamical simulations of galaxy clusters. These simulations, which are based on thegadget-2tree + sph code, include gas cooling, star formation, a detailed treatment of stellar evolution and chemical enrichment, as well as supernova energy feedback in the form of galactic winds. As such, they can be used to extract the spectrophotometric properties of the simulated galaxies, which are identified as clumps in the distribution of star particles. Simulations have been carried out for a representative set of 19 cluster-sized haloes, having mass M200 in the range . All simulations have been performed for two choices of the stellar initial mass function (IMF), namely using a standard Salpeter IMF with power-law index , and a top-heavy IMF with . In general, we find that several of the observational properties of the galaxy population in nearby clusters are reproduced fairly well by simulations. A Salpeter IMF is successful in accounting for the slope and the normalization of the colour–magnitude relation for the bulk of the galaxy population. In contrast, the top-heavy IMF produces too red galaxies, as a consequence of their exceedingly large metallicity. Simulated clusters have a relation between mass and optical luminosity, which generally agrees with observations, both in normalization and in slope. Also in keeping with observational results, galaxies are generally bluer, younger and more star forming in the cluster outskirts. However, we find that our simulated clusters have a total number of galaxies which is significantly smaller than the observed one, falling short by about a factor of 2–3. We have verified that this problem does not have an obvious numerical origin, such as lack of mass and force resolution. Finally, the brightest cluster galaxies are always predicted to be too massive and too blue, when compared to observations. This is due to gas overcooling, which takes place in the core regions of simulated clusters, even in the presence of the rather efficient supernova feedback used in our simulations. [ABSTRACT FROM AUTHOR]

We examine the systematics affecting the X-ray mass estimators applied to a set of five galaxy clusters resolved at high resolution in hydrodynamic simulations, including cooling, star formation and feedback processes. These simulated objects are processed through the X-ray Map Simulator, X-MAS, to provide Chandra-like long exposures that are analysed to reconstruct the gas temperature, density and mass profiles used as input. These clusters have different dynamic state: we consider a hot cluster with temperature , a perturbed cluster with , a merging object with , and two relaxed systems with and , respectively. These systems are located at so that their emission fits within the Chandra ACIS-S3 chip between 0.6 and 1.2 R500. We find that the mass profile obtained via a direct application of the hydrostatic equilibrium (HE) equation is dependent upon the measured temperature profile. An irregular radial distribution of the temperature values, with associated large errors, induces a significant scatter on the reconstructed mass measurements. At R2500, the actual mass is recovered within 1σ, although we notice this estimator shows high statistical errors due to high level of Chandra background. Instead, the poorness of the β-model in describing the gas density profile makes the evaluated masses to be underestimated by ∼40 per cent with respect to the true mass, both with an isothermal and a polytropic temperature profile. We also test ways to recover the mass by adopting an analytic mass model, such as those proposed by Nvarro, Frenk & White and Rasia, Tormen & Moscardini, and fitting the temperature profile expected from the HE equation to the observed one. We conclude that the methods of the HE equation and those of the analytic fits provide a more robust mass estimation than the ones based on the β-model. In the present work, the main limitation for a precise mass reconstruction is to ascribe to the relatively high level of the background chosen to reproduce the Chandra one. After artificially reducing the total background by a factor of 100, we find that the estimated mass significantly underestimates the true mass profiles. This is manly due (i) to the neglected contribution of the gas bulk motions to the total energy budget and (ii) to the bias towards lower values of the X-ray temperature measurements because of the complex thermal structure of the emitting plasma. [ABSTRACT FROM AUTHOR]

The angular-diameter distance DA of a galaxy cluster can be measured by combining its X-ray emission with the cosmic microwave background fluctuation due to the Sunyaev–Zeldovich (SZ) effect. The application of this distance indicator usually assumes that the cluster is spherically symmetric, the gas is distributed according to the isothermal β-model, and the X-ray temperature is an unbiased measure of the electron temperature. We test these assumptions with galaxy clusters extracted from an extended set of cosmological N-body/hydrodynamical simulations of a Λ cold dark matter concordance cosmology, which include the effect of radiative cooling, star formation and energy feedback from supernovae. We find that, due to the temperature gradients which are present in the central regions of simulated clusters, the assumption of isothermal gas leads to a significant underestimate of DA. This bias is efficiently corrected by using the polytropic version of the β-model to account for the presence of temperature gradients. In this case, once irregular clusters are removed, the correct value of DA is recovered with a ∼5 per cent accuracy on average, with a ∼20 per cent intrinsic scatter due to cluster asphericity. This result is valid when using either the electron temperature or a spectroscopic-like temperature. Instead When using the emission-weighted definition for the temperature of the simulated clusters, DA is biased low by ∼20 per cent. We discuss the implications of our results for an accurate determination of the Hubble constant H0 and of the density parameter Ωm. We find that, at least in the case of ideal (i.e. noiseless) X-ray and SZ observations extended out to can be potentially recovered with exquisite precision, while the resulting estimate of Ωm, which is unbiased, has typical errors . [ABSTRACT FROM AUTHOR]

We study the properties of the diffuse X-ray background by using the results of a cosmological hydrodynamical simulation of the concordance Λ cold dark matter (ΛCDM) model. The simulation follows gravitational and gas dynamics and includes a treatment of physical processes, such as radiative cooling, star formation and supernova feedback. From the simulation outputs, we produce a set of two-dimensional maps of the intergalactic medium X-ray emission integrated over redshift. We find that the signal in the soft (0.5–2 keV) band is lognormally distributed with a mean intensity of about 4 × 10−12 erg s−1 cm−2 deg−2; approximately 40 per cent of the emission originates from warm-hot gas (defined as baryons with ), and 90 per cent comes from structures at z < 0.9. Since the spectrum is soft, being mostly provided by the intergalactic medium at low temperature, the total mean intensity in the hard (2–10 keV) X-ray band is smaller by a factor of about 4. In order to constrain the physical processes included in our simulation, we compare our results with the observed upper limit of the soft X-ray emission due to diffuse gas. To this purpose, we remove the contributions of observable extended objects (groups and clusters of galaxies) from the simulated maps by adopting different detectability criteria which are calibrated on the properties of systems at intermediate redshifts observed by Chandra. We show that the simulated diffuse soft X-ray emission is consistent with the present observed upper limit. However, if future measurements will decrease the level of the unresolved X-ray background by a factor of 2, a more efficient feedback mechanism should be required to suppress the soft emission of the gas residing within filaments and group-size haloes. [ABSTRACT FROM AUTHOR]

We discuss an extended set of Tree+SPH (smoothed particle hydrodynamics) simulations of the formation of clusters of galaxies, with the goal of investigating the interplay between numerical resolution effects and star formation/feedback processes. Our simulations were all carried out in a concordance Λ cold dark matter (ΛCDM) cosmology and include radiative cooling, star formation and energy feedback from galactic winds. The simulated clusters span the mass range , with mass resolution varying by several decades. At the highest achieved resolution, the mass of gas particles is , which allows us to resolve the virial region of a Virgo-like cluster with more than two million gas particles and with at least as many dark matter (DM) particles. Our resolution study confirms that, in the absence of an efficient feedback mechanism, runaway cooling leads to about 35 per cent of baryons in clusters to be locked up in long lived stars at our highest resolution, with no evidence of convergence. However, including feedback causes the fraction of cooled baryons to converge at about 15 per cent already at modest resolution, which is much closer to the typical values inferred from observational data. Feedback also stabilizes other gas-related quantities, such as radial profiles of entropy, gas density and temperature, against variations due to changes in resolution. Besides the effects of mass resolution, we also investigate the influence of the gravitational force softening length and that of numerical heating of the gas induced by two-body encounters between DM and lighter gas particles. We also show that simulations where more DM than gas particles are used, such that , show a significantly enhanced efficiency of star formation at z≳ 3, but they accurately reproduce at z= 0 the fraction of cooled gas and the thermodynamic properties of the intracluster gas. Our results are important for establishing and delineating the regime of numerical reliability of the present generation of hydrodynamical simulations of galaxy clusters. [ABSTRACT FROM AUTHOR]

We study the baryon mass fraction in a set of hydrodynamical simulations of galaxy clusters performed using the Tree+SPH codegadget-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 , 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 of massive 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 not evolving with redshift, and (2) a fixed ratio between fstar and fgas 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 for non-radiative runs and and ≲−0.05 for radiative simulations. [ABSTRACT FROM AUTHOR]

We use hydrodynamical simulations of galaxy clusters and groups to study the effect of pre-heating on the entropy structure of the intracluster medium. Our simulations account for non-gravitational heating of the gas either by imposing a minimum entropy floor at redshift in adiabatic simulations, or by considering feedback by galactic winds powered by supernova (SN) energy in runs that include radiative cooling and star formation. In the adiabatic simulations we find that the entropy is increased out to the external regions of the simulated haloes as a consequence of the transition from clumpy to smooth accretion induced by extra heating. This result is in line with the predictions of the semi-analytical model by Voit et al. However, the introduction of radiative cooling substantially reduces this entropy amplification effect. While we find that galactic winds of increasing strength are effective in regulating star formation, they have a negligible effect on the entropy profile of cluster-sized haloes. Only in models where the action of the winds is complemented with diffuse heating corresponding to a pre-collapse entropy do we find a sizeable entropy amplification out to the virial radius of the groups. Observational evidence for entropy amplification in the outskirts of galaxy clusters and groups therefore favours a scenario for feedback that distributes heating energy in a more diffuse way than predicted by the model for galactic winds from SN explosions explored here. [ABSTRACT FROM AUTHOR]

The fluctuations in the cosmic microwave background (CMB) intensity due to the Sunyaev–Zel'dovich (SZ) effect are the sum of a thermal and a kinetic contribution. Separating the two components to measure the peculiar velocity of galaxy clusters requires radio and microwave observations at three or more frequencies, and knowledge of the temperatureTe of the intracluster medium (ICM) weighted by the electron number density. To quantify the systematics of this procedure, we extract a sample of 117 massive clusters at redshiftfrom anN-body hydrodynamical simulation, withparticles, of a cosmological volume 192 on a side of a flat cold dark matter model withand. Our simulation includes radiative cooling, star formation and the effect of feedback and galactic winds from supernovae. We find that: (i) our simulated clusters reproduce the observed scaling relations between X-ray and SZ properties; (ii) bulk flows internal to the ICM affect the velocity estimate by less than 200 km s−1 in 93 per cent of the cases; (iii) using the X-ray emission weighted temperature, as an estimate ofTe, can overestimate the peculiar velocity by 20–50 per cent, if the microwave observations do not spatially resolve the cluster. For spatially resolved clusters, the assumptions on the spatial distribution of the ICM, required to separate the two SZ components, still produce a velocity overestimate of 10–20 per cent, even with an unbiased measure ofTe. Thanks to the large size of our cluster samples, these results set a robust lower limit ofto the systematic errors that will affect upcoming measures of cluster peculiar velocities with the SZ effect. [ABSTRACT FROM AUTHOR]

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 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 and 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 . At the scale of groups, , 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 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 , 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. [ABSTRACT FROM AUTHOR]

ABSTRACT 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[sup 14] M[sub ⊙] , extracted from a cosmological simulation of a flat Λ-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. [ABSTRACT FROM AUTHOR]

Discusses the effect of non-gravitational gas heating in groups and clusters of galaxies using cold dark matter model. Range of virial temperatures; Resolution x-ray luminosities; Mass-temperature relation.