Rosat X-ray colors and emission mechanisms in early-type galaxies

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IOP Publishing

The X-ray colors and X-ray-to-optical luminosity ratios (L-X/L-B) of 61 early-type galaxies observed with the ROSAT PSPC are determined. The colors indicate that the X-ray spectral properties of galaxies vary as 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 same metallicity of 50% solar. The spatial variation of the colors indicates that the gas temperature in these galaxies increases radially. Galaxies with medium L-X/L-B also have spectral properties consistent with emission from hot gas. If a simple one-component thermal model is assumed to describe the 0.1-2.0 keV X-ray emission in these galaxies, then one possible explanation for the progressive decrease in L-X/L-B among galaxies of this class could be the progressive decrease in metal abundance of the X-ray-emitting gas contained by the galaxies. However, stellar X-ray emission may become a complicating factor for the fainter 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 resolved into discrete sources and is apparently due primarily to low-mass X-ray binaries (LMXBs). We therefore suggest that the bulk of the X-ray emission in the faintest elliptical galaxies is also due to LMXBs. Previously, the X-ray spectra of X-ray-faint galaxies had been found to be described by a hard component, which was attributed to LMXB emission, and a very soft component of unknown origin. We show that the very soft component also likely results from LMXBs, as a very soft component is seen in the X-ray spectra of the nearby LMXB Her X-1 and LMXBs in the bulge of M31. If the X-ray emission in X-ray-faint galaxies is primarily from stellar sources, then a range in L-X/L-B among these galaxies suggests that the stellar X-ray luminosity does not scale with optical 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 the small number of LMXBs expected.

galaxies, abundances, galaxies, elliptical and lenticular, cD, galaxies, halos, galaxies, ISM, X-rays, galaxies, X-rays, ISM, ELLIPTIC GALAXIES, SPECTRAL PROPERTIES, GLOBULAR-CLUSTERS, PSPC OBSERVATIONS, DARK-MATTER, SKY SURVEY, HOT, SAMPLE, GAS, EVOLUTION, Astronomy & Astrophysics
Irwin, J., Sarazin, C. (1998): ROSAT X-Ray Colors and Emission Mechanisms in EarlyType Galaxies. The Astrophysical Journal, 499(2). DOI: 10.1086/305666