X-ray luminosities for a magnitude-limited sample of early-type galaxies from the ROSAT All-Sky Survey

Abstract

For a magnitude-limited optical sample (BT≤ 13.5 mag) of early-type galaxies, we have derived X-ray luminosities from the ROSAT All-Sky Survey. The results are 101 detections and 192 useful upper limits in the range from 1036 to 1044 erg s-1. For most of the galaxies no X-ray data have been available until now. On the basis of this sample with its full sky coverage, we find no galaxy with an unusually low flux from discrete emitters. Below log (LB)≈ 9.2 L⊗ the X-ray emission is compatible with being entirely due to discrete sources. Above log (LB)≈ 11.2 Lo˙ no galaxy with only discrete emission is found. We further confirm earlier findings that Lx is strongly correlated with LB. Over the entire data range the slope is found to be 2.23 (± 0.12). We also find a luminosity dependence of this correlation. Below log Lx = 40.5 erg s-1 it is consistent with a slope of 1, as expected from discrete emission. Above this value the slope is close to 2, as expected from gaseous emission. Comparing the distribution of X-ray luminosities with the models of Ciotti et al. leads to the conclusion that the vast majority of early-type galaxies are in the wind or outflow phase. Some of the galaxies may have already experienced the transition to the inflow phase. They show X-ray luminosities in excess of the value predicted by cooling flow models with the largest plausible standard supernova rates. A possible explanation for these super X-ray-luminous galaxies is suggested by the smooth transition in the Lx--LB plane from galaxies to clusters of galaxies. Gas connected to the group environment might cause the X-ray overluminosity.