Decoupled hot gas flows in elliptical galaxies

Abstract

We present the results of a new set of hydrodynamical simulations of hot gas flows in model elliptical galaxies with the following characteristics: the spatial luminosity distribution approaches a power law form at small radii, in accordance with the results of recent ground based observations, and with the Hubble Space Telescope; the dark matter has a peaked profile too, as indicated by high resolution numerical simulations of dissipationless collapse; the dark to luminous mass ratio spans a large range of values, including low values found by optical studies confined to within two effective radii; finally, the type Ia supernova rate is that given by the latest estimates of optical surveys, or zero, as suggested by the iron abundances recently measured in the hot gas. We find that the resulting gas flows are strongly decoupled: an inflow develops in the central region of the galaxies, while the external parts are still degassing, i.e., the flows are mostly partial winds. This behavior can be explained in terms of the local energy balance of the hot gas. A large spread in the X-ray luminosity Lx at fixed optical luminosity Lb can be produced as in previous simulations that used King models plus massive quasi-isothermal dark halos, and higher supernova rates; the key factor causing large Lx variations is now the size of the central inflow region. The highest Lx observed correspond to global inflows. Finally, non negligible amounts of cold gas can be produced by the partial winds; this could be an explanation for the possible discovery of cold matter at the center of elliptical galaxies, an alternative to the presence of a steady state cooling flow.