X-ray haloes and star formation in early-type galaxies

Abstract

High-resolution 2D hydrodynamical simulations describing the evolution of the hot interstellar medium (ISM) in axisymmetric two-component models of early-type galaxies well reproduced the observed trends of the X-ray luminosity (LX) and temperature (TX) with galaxy shape and rotation, however they also revealed the formation of an exceedingly massive cooled gas disc in rotating systems. In a follow-up of this study, here we investigate the effects of star formation in the disc, including the consequent injection of mass, momentum and energy in the pre-existing ISM. It is found that subsequent generations of stars originate one after the other in the equatorial region; the mean age of the new stars is >5 Gyr, and the adopted recipe for star formation can reproduce the empirical Kennicutt-Schmidt relation. The results of the previous investigation without star formation, concerning LX and TX of the hot gas, and their trends with galactic shape and rotation, are confirmed. At the same time, the consumption of most of the cold gas disc into new stars leads to more realistic final systems, whose cold gas mass and star formation rate agree well with those observed in the local Universe. In particular, our models could explain the observation of kinematically aligned gas in massive, fast-rotating early-type galaxies.