The X-Ray Emission of Normal Elliptical Galaxies and their Environment

Abstract

While the total X-ray luminosities L X of early-type galaxies are strongly correlated with their optical (blue) luminosities \({L_B}({L_X}\alpha \,L_B^{1.7 - 2.2})\), the X-ray luminosities exhibit considerable dispersion for a given optical luminosity. We have searched for the cause of this scatter by correlating the residual X-ray luminosities of E/S0 galaxies with other properties of the galaxies. We can find no intrinsic property of the galaxies (color, metallicity, velocity dispersion, size, HI content, or far-infrared emission) which significantly reduces this scatter in the X-ray luminosity of early-type galaxies of a given optical luminosity. However, we do find that E/S0s with low X-ray luminosities (for a given optical luminosity) are more likely to be found in dense regions. We find that early-type galaxies with relatively low L X for a given L B have ~50% more neighboring galaxies than E/S0s with relatively high L X . We conclude that the large dispersion in X-ray luminosity (for a given L B ) may be related to the environment of the galaxy, and that X-ray faint E/S0s may have lost much of their hot gas due to ram-pressure stripping.