Massive elliptical galaxies in X-rays: The role of late gas accretion

We present a new chemical evolution model meant to be a first step in the self- consistent study of both optical and X- ray properties of elliptical galaxies. Detailed cooling and heating processes in the interstellar medium ( ISM) are taken into account using a mono- phase one- zone treatment which allows a more reliable modelling of the galactic wind regime with respect to previous work. The model successfully reproduces simultaneously the mass- metallicity, the colour - magnitude, the L-X - L-B and the L-X - T relations, as well as the observed trend of the [ Mg/ Fe] ratio as a function of sigma, by adopting the prescriptions of Pipino & Matteucci ( 2004) for the gas infall and star formation timescales. We found that a late secondary accretion of gas from the environment plays a fundamental role in driving the L-X - L-B and L-X - T relations and can explain their large observational scatter. The iron discrepancy, namely the too high predicted iron abundance in X- ray haloes of ellipticals compared to observations, still persists. On the other hand, we predict [ O/ Fe] in the ISM which is in good agreement with the most recent observations. We suggest possible mechanisms acting on a galactic scale which may solve the iron discrepancy. In particular, mixing of gas driven by AGNs may preserve the gas mass ( and thus the X- ray luminosity) while diluting the iron abundance. New predictions for the amounts of iron, oxygen and energy ejected into the intracluster medium ( ICM) are presented and we conclude that type Ia supernovae ( SNe Ia) play a fundamental role in the ICM enrichment. SNe Ia activity, in fact, may power a galactic wind lasting for a considerable amount of the galactic lifetime, even in the case for which the efficiency of energy transfer into the ISM per SN Ia event is less than unity.