X-ray temperature spectroscopy of simulated cooling clusters

Abstract

Results from a large sample of hydrodynamical/N-body simulations of galaxy clusters in a ΛCDM cosmology are used to simulate cluster X-ray observations. The physical modeling of the gas includes radiative cooling, star formation, energy feedback, and metal enrichment that follow from supernova explosions. Mock cluster samples are constructed grouping simulation data according to a number of constraints which would be satisfied by a data set of X-ray measurements of cluster temperatures as expected from Chandra observations. The X-ray spectra from simulated clusters are fitted into different energy bands using the XSPEC mekal model. The biasing of spectral temperatures with respect to mass-weighted temperatures is found to be influenced by two independent processes. The first scale dependency is absent in adiabatic runs and is due to cooling, whose efficiency to transform cold gas into stars is higher for cool clusters and this in turn implies a strong dependency of the spectral versus mass-weighted temperature relation on the cluster mass. The second dependency is due to photon emission because of cool gas which is accreted during merging events and biases the spectral fits. These events have been quantified according to the power ratio method and a robust correlation is found to exist between the spectral bias and the amount of cluster substructure. The shape of the simulated temperature profiles is not universal and it is steeper at the cluster center for cool clusters than for the massive ones. This follows owing to the scale dependency introduced by cooling which implies for cool clusters higher central temperatures, in scaled units, than for massive clusters. The profiles are in good agreement with data in the radial range between ∼0.1 r vir and ∼0.4 r vir; at small radii ( r ≲ 0.1 r vir) the cooling runs fail to reproduce the shape of the observed profiles. The fit is improved if one considers a hierarchical merging scenario in which cluster cores can accrete cooler gas through merging with cluster subclumps, though the shape of the temperature profiles is modified in a significant way only in the regime where the mass of the substructure is a large fraction of the cluster mass.