Global X-ray emission and central properties of early type galaxies

Abstract

Hubble Space Telescope observations have revealed that the central surface brightness profiles of early type galaxies can be divided into two types: ``core" profiles and featureless power law profiles, without cores. On the basis of this and previous results, early type galaxies have been grouped into two families. One consists of coreless galaxies, which are also rapidly rotating, nearly isotropic spheroids, and with disky isophotes. The other is made of core galaxies, which are slowly rotating and boxy-distorted. Here I investigate the relationship between global X-ray emission and shape of the inner surface brightness profile, for a sample of 59 early type galaxies. I find a clear dichotomy also in the X-ray properties, in the sense that core galaxies span the whole observed range of L_X values (roughly two orders of magnitude in L_X ), while power law galaxies are confined to log L_X (erg s^-1)<41. Moreover, the relation between L_X and the shape of the central profile seems to be the strongest among the relations of L_X with the basic properties characterizing the two families of early type galaxies. As an example, L_X is more deeply connected with the shape of the central profile than with the isophotal shape distortion, or the importance of galactic rotation. So, a global property such as L_X , that measures the hot gas content on a galactic scale, turns out to be surprisingly well linked to a nuclear property. Various possible reasons are explored for the origin of the different L_X behavior of core and power law galaxies. While a few explanations can be imagined for the large spread in the X-ray luminosities of core galaxies, an open problem is why power law ones never become very X-ray bright. It is likely that the presence of a central massive black hole, and possibly also the environment, play an important role in determining L_X (i.e., the hot gas content). Therefore the problem of interpreting the X-ray properties of early type galaxies turns out to be more complex than thought so far.