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Session 90 - OJ287 and X-Ray Emission from Elliptical Galaxies.
Oral session, Wednesday, January 15
Harbour B,

[90.02] X-ray Radial Color Profiles of a Large Sample of Elliptical Galaxies Observed With ROSAT

J. A. Irwin, C. L. Sarazin (U. Virginia)

Typical X-ray luminosities for elliptical galaxies vary from L_X = 10^39 - 10^42 ergs s^-1. Studies have shown that the X-ray luminosity is correlated with the blue optical luminosity, L_B. However, there appears to be a large dispersion in this correlation. For a given L_B, L_X can vary by a factor of as much as fifty. Evidence is mounting that although the X-ray emission from ellipticals with a high L_X/L_B is dominated by emission from a hot, thermal plasma with kT \sim 1 keV, a hard (\sim 10 keV) X-ray component to the emission which seems to be present in nearly all X-ray ellipticals becomes more significant as L_X/L_B decreases. Ellipticals with a very low L_X/L_B also show evidence for a very soft (\sim 0.2 keV) component, whose origin is uncertain. In this study we investigate the spatial and spectral properties of \sim 40 elliptical and S0 galaxies with varying L_X/L_B observed with the ROSAT PSPC in order to search for evidence for the various X-ray--emitting components mentioned above. We divide the data from each observation into a soft, medium, and hard image, and create two X-ray ``color" images by dividing the medium band image by the soft band image, and by dividing the hard band image by the medium band image. We then compute the surface brightness profiles from these X-ray color images to derive the dependence of each color as a function of radius. In addition to finding that the emission-weighted X-ray colors differ significantly with L_X/L_B, we find that the slopes of the radial color profiles also depend on L_X/L_B. We use these results to test the competing environmental and hydrodynamical models to explain the large dispersion in L_X/L_B in elliptical galaxies.

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