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

The relation between X-ray luminosity and near-infrared (NIR) luminosity for early-type galaxies has been examined. 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, Mg-2, H beta, H gamma line strength indices, spectroscopic age and nuclear H alpha emission. We found that galaxies with high Mg-2 index, low H beta and H gamma indices or a ` core' profile have a large scatter in LX, whereas galaxies with low Mg-2, high H beta and H gamma indices or ` power-law' profiles generally have L-X < 10(41) erg s(-1). There is no clear trend in the scatter with environment or nuclear Ha emission.