We study the cluster mass function in mixed dark matter (MDM) models, using two COBE-normalized simulations with Ωh = 0.26, n = 1.2 and Ωh = 0.14, n = 1.05, both with two massive neutrinos (models MDM1 and MDM2, respectively). For the sake of comparison, we also simulate a tilted cold dark matter model with spectral index n = 0.8 (TCDM), also COBE normalized. We argue that, in our nonhydrodynamical simulations, where cold dark matter (CDM) particles describe both actual CDM and baryons, the galaxy distribution traces CDM particles. Therefore, we use them to define clusters and their velocities to work out cluster masses. Since CDM particles are more clustered than hot dark matter (HDM) and therefore have, on average, greater velocities, this leads to significant differences from Press & Schechter (PS) predictions. Such predictions agree with simulations if both HDM and CDM are used to define clusters. If this criterion is adopted, however, we see that (1) MDM corresponds to δc values slightly but systematically greater than CDM; and (2) such δc exhibit a scale dependence: on scales ~1014 M☉, we find δc ~ 1.7 or 1.8 for CDM or MDM, respectively, while at greater scales the required δc decreases, and a substantial cluster excess is found at the large-mass end (M > 1015 M☉). Clusters defined through CDM in MDM models, on the other hand, are less numerous than PS estimates by a factor of ~0.3 at the low-mass end; the factor becomes ~0.6-0.8, depending on the mix, on intermediate-mass scales (~4-5 h-1 1014 M☉), and almost vanishes on the high-mass end. Therefore, (1) MDM models expected to overproduce clusters over intermediate scales are viable; (2) the greater reduction factor at small scales agrees with the observational data dependence on the cluster mass M (which, however, may be partially due to sample incompleteness); (3) the higher spectral normalization is felt at large scales, where MDM models produce more objects (hence, large clusters) than CDM. MDM1 even exceeds the findings of Donahue et al., while MDM2 is consistent with them. Simulations are performed using a parallel algorithm worked out from the Couchman AP3M serial code, but allowing for different particle masses and used with variable time steps. This allowed us to simulate a cubic box with sides of 360 h-1 Mpc, reaching a Plummer resolution of 40.6 h-1 kpc, using (3×)1803 particles.
The direct comparison of observations to numerical hydro-N-body simulations, although simple in principle, is not always trivial because of possible artificial effects produced by the instrument response and by instrumental and sky background. To overcome this problem we build the software package X-MAS (X-ray MAp Simulator) devoted to simulate X-ray observations of galaxy clusters obtained from hydro-N-body simulations.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html
A software package able to simulate imaging observations of galaxy clusters by the Chandra X-ray telescope is here presented. We start from high resolution N-body hydrodynamical simulations of galaxy clusters and assign to each gas particle a spectrum of emissivity, after assuming the MeKaL model. We then construct spatial images of the source differential flux which are used to create lists of incoming X-ray photons, preserving information on photon direction and energy. The photon lists are passed on to the Chandra simulator (MARX) to produce the final observation events. Background events are added to complete the simulation. Data analysis is currently in progress and simulated observations by other telescopes will become available in the future.
We review the general properties of the intracluster medium (ICM) in clusters that host a cooling flow, and in particular the effects on the ICM of the injection of hot plasma by a powerful active galactic nucleus (AGN). It is observed that, in some cases, the hot plasma produces cavities in the ICM that finally detach and rise, perhaps buoyantly. The gas dynamics induced by the rising bubbles can help in explaining the absence of a cooled gas component in clusters with a cooling flow. This scenario is explored using numerical simulations.
Aims.Over the past several years, numerous examples of X-ray cavities coincident with radio sources have been observed in so-called “cool core” clusters of galaxies. Motivated by these observations, we explore the evolution and the effect of cavities on a cooling intracluster medium (ICM) numerically, adding relevant physics step by step.
Although trivial in principle, direct comparison of galaxy clusters X-ray observations to numerical hydro–N-body simulations is not always simple, because of many possible artefacts introduced by the instrument response, sky background and instrumental noise. To address these problems, we constructed the software package x-mas (X-ray Map Simulator), a tool devoted to simulate X-ray observations of galaxy clusters obtained from hydro–N-body simulations. One of the main features of x-mas is the ability to generate event files following the same standards used for real observations. This implies that its simulated observations can be analysed in the same way as – and with the same tools of – real observations. In this paper we present how the x-mas package works, and discuss its application to the simulation of Chandra ACIS-S3 observations. Using the results of high-resolution hydro–N-body simulations, we generate fake Chandra observations of a number of simulated clusters. We then compare some of the main physical properties of the input data to those derived from the simulated observations after performing a standard imaging and spectral analysis. We find that, because of the sky background, the lower surface brightness spatial substructures, which can be easily identified in the simulations, are no longer detected in the simulated observations. We also show that, when a cluster has a complex (i.e. not isothermal) thermal structure along the line of sight, then the projected spectroscopic temperature obtained from the observation is significantly lower than the emission-weighed value inferred directly from hydrodynamical simulation. This implies that much attention should be paid in the theoretical interpretation of observed temperatures.
Over the past several years, numerous examples of X-ray cavities coincident with radio sources have been observed in so-called cool core clusters of galaxies. Motivated by these observations, we explore the evolution and the effect of cavities on a cooling intracluster medium (ICM) numerically, adding relevant physics step by step. In this paper we present a first set of hydrodynamical, high resolution (1024^3 effective grid elements), three-dimensional simulations, together with two-dimensional test cases. The simulations follow the evolution of radio cavities, modeled as bubbles filled by relativistic plasma, in the cluster atmosphere while the ICM is subject to cooling. We find that the bubble rise retards the development of a cooling flow by inducing motions in the ICM which repeatedly displace the material in the core. Even bubbles initially set significantly far from the cluster center affect the cooling flow, although much later than the beginning of the simulation. The effect is, however, modest: the cooling time is increased by at most only 25%. As expected, the overall evolution of pure hydrodynamic bubbles is at odds with observations, showing that some additional physics has to be considered in order to match the data.
