Emission Mechanisms in X‐Ray–faint Galaxies

Hot gas dominates the emission in X-ray luminous early-type galaxies, but inrelatively X-ray faint systems, integrated X-ray emission from discretestellar-like sources is thought to be considerable, although the amount of thecontribution is controversial. To help resolve this issue, we examine theradial X-ray surface brightness distribution of 17 X-ray faint galaxiesobserved with the ROSAT HRI and PSPC. We assume that the stellar contributionfollows a de Vaucouleurs law while the hot gas component follows a King betamodel. For some galaxies, both models fit equally well, but for a number ofsystems, a dual component model yields the best fit, from which upper boundsare placed on the stellar contribution. Best-fit values for the stellarcontribution are inconsistent with (lower than) that suggested by Fabbiano,Gioia, & Trinchieri (1989) and estimated from the bulge of M31, but areconsistent with the Forman, Jones, & Tucker (1985) estimate of the stellarfraction in X-ray faint elliptical and S0 galaxies. Our results indicate anupper limit to discrete sources of L_X/L_B = 1.6 x 10^29 ergs/s/L(solar).Comment: 18 pages including 5 embedded Postscript figures. Accepted by the Astrophysical Journa