The Effect of Environment on the X-Ray Emission from Early-Type Galaxies

In order to help understand the phenomena of X-ray emission from early-type galaxies, we obtained an optically flux-limited sample of 34 elliptical and S0 galaxies, observed with high and low angular resolution detectors on the X-ray Röntgen Satellite (ROSAT). Our analysis of this sample, discussed previously, suggested that the most X-ray luminous galaxies were in rich environments. Here we investigate environmental influences quantitatively and find a positive correlation between LB/LX and the local galaxy density. Since the local galaxy density is usually related to the density of hot intergalactic gas, we suggest that this correlation occurs because the X-ray luminosity is enhanced either through accretion of the intergalactic gas or because the ambient medium stifles galactic winds. When the ambient medium is unimportant, partial or global galactic winds can occur, reducing LB/LX . These effects lead to the large observed dispersion in LX at fixed LB. The transition from global winds to partial winds occurs in the LB range of our sample and we argue that this transition is one of the principle reasons for the steep relationship between LX and LB. This is a significant departure from the steady-state cooling flow interpretation and we discuss predictions that can be tested with future observatories. We discuss details of the data reduction not previously presented, and examine the dependence of LX on the choice of outer source radius and background location. Effects of Malmquist bias are shown not to be important for the issues addressed. Finally, we compare the temperature deduced for these galaxies from different analyses of ROSAT and ASCA data. The ASCA and ROSAT temperatures are similar, provided that the same metallicities and Galactic absorption columns are used. However, temperatures have a dependence on the metallicity and absorption column, especially for the lower temperature systems. National Research Council Research Associate at NASA/Goddard Space Flight Center, Greenbelt, MD