The impact of galaxy formation on X-ray groups

Using hydrodynamical simulations of the Λ-cold dark matter 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 zero redshift. Motivated by the observed ‘break’ in the Lx–Tx relation at kTx∼ 1–2 keV, 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 (kTSN= 2 keV), and an energy budget twice that available from supernovae (e= 2), raises the core entropy of groups sufficiently to produce an adequate match to their observed X-ray properties. A lower value of e increases the star formation rate without significantly affecting the X-ray properties of groups, and a model with e∼ 0.1 reproduces the observed fraction of baryons in stars. However, a heating temperature that 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 Lx–Tx relation, bringing our results into even better agreement with the observations.