  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 neighborhood 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\pm 0.9) x
10^27 erg/s/L_K,sun; (ii) The 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\pm0.9) x 10^38 (erg/s)/(M_sun/yr); (iv)
In four old and massive Virgo ellipticals (M49, M60, M84, NGC4636) 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.
