2011MNRAS.418.1901B

We explore the nature of unresolved X-ray emission in a broad sample of galaxies of all morphological types based on archival Chandra data. After removing bright compact sources, we study L_X/L_K_ luminosity ratios of unresolved emission, and compare them with the solar neighbourhood values. We conclude that unresolved emission is determined by four main components, three of which were known before. (i) The population of faint unresolved sources associated with old stellar population: in early-type galaxies, their 2–10 keV band luminosity scales with the stellar mass with L_X/L_K = (3.1±0.9)x10²⁷erg/s L_K^–1. (ii) The interstellar medium (ISM) with kT ∼ 0.2–0.8 keV present in galaxies of all types: because of the large dispersion in the gas content of galaxies, the size of our sample is insufficient to obtain reliable scaling law for this component. (iii) The population of unresolved young stars and young stellar objects in late-type galaxies: their 2–10 keV band luminosity scales with the star formation rate with L_X/SFR ∝(1.7±0.9)x10^38erg/s/M_☉/yr. (iv) In four old and massive Virgo ellipticals (M49, M60, M84, NGC 4636) we find anomalously high X-ray emission in the 2–10 keV band. Its presence has not been recognized before and its nature is unclear. Although it appears to be stronger in galaxies having stronger ISM component, its existence cannot be explained in terms of an extrapolation of the warm ISM spectrum. Association with Virgo cluster of galaxies suggests that the excess emission may be due to intracluster gas accreted in the gravitational well of a massive galaxy. We investigate this and other possibilities.