A statistical analysis of the Einstein normal galaxy sample. II. Elliptical and SO galaxies.

The statistical analysis of a sample of 29 elliptical and S0 field galaxies observed with the Einstein observatory is presented. The data show a correlation between the X-ray and the optical blue luminosity of the form l_x ∝ l_B^{1.64 ±0.15}. This is significantly steeper than the correlation observed in spiral and irregular galaxies: l_x ∝ l_B^{1.08 ±0.07}. Several mechanisms for the origin of the X-ray emission in these galaxies have been investigated to explain the observed correlation. Most likely two separate mechanisms are present: thermal radiation from hot gas is required to explain the high-luminosity massive objects (M > 5 x 10¹¹ M_sun, L_x, 10⁴¹ ergs s ^-1), while a collection of low-mass binary sources can explain the low-luminosity, low-mass objects. A comparison between radio-loud and radio-quiet galaxies shows that high optical luminosity, or high mass, is required for the onset of nuclear activity, at both radio and X-ray wavelengths. However, this is not sufficient to ensure the presence of active nuclear sources, and a second parameter (at least) is needed. A comparison between radio-loud elliptical galaxies and QS0s shows that the same X-ray emission mechanism that is operating in QSQs could explain the high X-ray luminosity end of the radio-loud galaxy distribution.