THE X-RAY-SPECTRA OF GALAXIES .2. AVERAGE SPECTRAL PROPERTIES AND EMISSION MECHANISMS

We have systematically investigated the X-ray spectra of normal galaxies, by using the Imaging Proportional Counter (IPC) data in the Einstein data base. We find that on the average the X-ray emission temperature of spirals is higher than that of ellipticals. This is consistent with our understanding that accreting binaries are a major source of X-rays in spirals, while a hot interstellar medium (ISM) is present in ellipticals. The X-ray spectra of Sa galaxies are intermediate between those of ellipticals and spirals, suggesting that these galaxies contain hot gaseous emission as well as emission from accreting binaries. We confirm that the X-ray to optical ratio is an important indicator of the presence of a hot gaseous component in early-type galaxies. In particular we find that the emission temperature becomes higher with a decreasing X-ray to optical luminosity ratio in E and S0 galaxies. This result is what we would expect if the emission of X-ray faint early-type galaxies consists of a large evolved stellar component, while the gaseous emission becomes dominant in X-ray brighter galaxies. The group with the lowest L(X)/L(B) does not follow this trend. In these galaxies we find a very soft (kT approximately 0.2 keV) X-ray component, amounting to about half the total X-ray emission, in addition to the hard X-ray component. Possible explanations for this component include the integrated emission of M stars and a relatively cool ISM. A very soft component is also found in several spiral galaxies. This result may indicate that some spirals contain hot gaseous components similar to those seen in NGC 253 and M82.