ROSAT X-RAY COLORS AND EMISSION MECHANISMS IN EARLY-TYPE GALAXIES

The X-ray colors and X-ray-to-optical luminosity ratios (L-X/L-B) of 6 1 early-type galaxies observed with the ROSAT PSPC are determined. The colors indicate that the X-ray spectral properties of galaxies vary a s a function of L-X/L-B. The brightest X-ray galaxies have colors that are consistent with thermal emission from hot gas with roughly the sa me metallicity of 50% solar. The spatial variation of the colors indic ates that the gas temperature in these galaxies increases radially. Ga laxies with medium L-X/L-B also have spectral properties consistent wi th emission from hot gas. If a simple one-component thermal model is a ssumed to describe the 0.1-2.0 keV X-ray emission in these galaxies, t hen one possible explanation for the progressive decrease in L-X/L-B a mong galaxies of this class could be the progressive decrease in metal abundance of the X-ray-emitting gas contained by the galaxies. Howeve r, stellar X-ray emission may become a complicating factor for the fai nter galaxies in this medium-L-X/L-B class. Galaxies with the lowest L -X/L-B values appear to be lacking a hot interstellar component. Their X-ray colors are consistent with those derived from the bulges of the spiral galaxies M31 and NGC 1291. In M31, the X-ray emission is resol ved into discrete sources and is apparently due primarily to low-mass X-ray binaries (LMXBs). We therefore suggest that the bulk of the X-ra y emission in the faintest elliptical galaxies is also due to LMXBs. P reviously, the X-ray spectra of X-ray-faint galaxies had been found to be described by a hard component, which was attributed to LMXB emissi on, and a very soft component of unknown origin. We show that the very soft component also likely results from LMXBs, as a very soft compone nt is seen in the X-ray spectra of the nearby LMXB Her X-1 and LMXBs i n the bulge of M31. If the X-ray emission in X-ray-faint galaxies is p rimarily from stellar sources, then a range in L-X/L-B among these gal axies suggests that the stellar X-ray luminosity does not scale with o ptical luminosity, at least for galaxies of low optical luminosities. This could be the result of a decrease in the proportion of LMXBs with decreasing optical luminosity and/or the effects of fluctuations in t he small number of LMXBs expected.