The impact of galaxy formation on X-ray groups

Abstract

Using hydrodynamical simulations of the LCDM cosmology, that include both radiative cooling and a phenomenological model for star formation and supernovae feedback, we investigate the impact of galaxy formation on the X-ray properties of groups at redshift zero. Motivated by the observed ``break'' in the L_x-T_x relation at kT~1-2keV, our feedback model is based on the assumption that supernovae imprint a temperature scale on the hot gas, with the star formation rate and corresponding reheated gas mass then depending only on the available energy budget. We demonstrate that a strong feedback model with a heating temperature comparable to this break (kT_=2keV), and an energy budget twice that available from supernovae (epsilon=2), raises the core entropy of groups sufficiently to produce an adequate match to their observed X-ray properties. A lower value of epsilon increases the star formation rate without significantly affecting the X-ray properties of groups, and a model with epsilon~0.1 reproduces the observed fraction of baryons in stars. However, a heating temperature which is lower than the virial temperatures of the groups leads to an excess of cooling gas that boosts their X-ray luminosities, due to the failure of the reheated material to escape from the gravitational potential. A limited study of numerical resolution effects reveals that the temperature of poorly-resolved objects is underestimated, therefore (in our case) a fully-resolved group population would lead to a steeper L_x-T_x relation, bringing our results into even better agreement with the observations.