Correlations of near-infrared, optical and X-ray luminosity for early-type galaxies

Abstract

The relation between X-ray luminosity and near-infrared luminosity for early-type galaxies has been examined. Near-infrared (NIR) luminosities should provide a superior measure of stellar mass compared to optical luminosities used in previous studies, especially if there is significant star-formation or dust present in the galaxies. However, we show that the X-ray-NIR relations are remarkably consistent with the X-ray-optical relations. This indicates that the large scatter of the relations is dominated by scatter in the X-ray properties of early-type galaxies, and is consistent with early-types consisting of old, quiescent stellar populations. We have investigated scatter in terms of environment, surface brightness profile, Mg2, H_beta, H_gamma line strength indices, spectroscopic age, and nuclear H_alpha emission. We found that galaxies with high Mg2 index, low H_beta and H_gamma indices or a `core&#39; profile have a large scatter in Lx, whereas galaxies with low Mg2, high H_beta and H_gamma indices or `power-law&#39; profiles, generally have Lx<10^41 erg/s. There is no clear trend in the scatter with environment or nuclear H_alpha emission.