Astronomy and Astrophysics – Astrophysics
Scientific paper
1998-04-21
Astronomy and Astrophysics
Astrophysics
49 pages, 17 embedded Postscript figures, uses aaspp4.sty, Astrophysical Journal, volume 499, in press
Scientific paper
10.1086/305666
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 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 contained by the galaxies. 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 NGC1291. 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 ellipticals 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. (Abridged)
Irwin Jimmy A.
Sarazin Craig L.
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