I have investigated the relationship between X-ray emission and shape of the inner surface brightness profile, for a sample 59 early type galaxies. I have found that:
The family of core galaxies spans the whole observed range of values (two orders of magnitude in ) while that of power law galaxies is confined to log (erg s. This dichotomy in the X-ray properties holds even in the range where the two families coexist.
The relation between and the shape of the inner profile is sharp, and is stronger than the relations of with the other basic properties characterizing the two families of early type galaxies. For example, being a power law galaxy is connected with a low with no exception, while the same cannot be said for the property of having disky isophotes. So, a global quantity such as is surprisingly tightly connected with a nuclear galaxy property.
A linear regression analysis shows that could be a good description of the - relation for power law galaxies, while core galaxies deviate from this relation.
Different possible reasons can be argued for the origin of the dichotomy in the behavior. The central profile shape itself should not be the main factor, given the results of previous numerical simulations of the hot gas evolution and the absence of a trend of with the values. A higher degree of rotation of power law galaxies could produce an effect, but not large enough, and, much like the possibility of a higher degree of flattening of the mass distribution, does not seem to be a general explanation of the reduction in for the whole class of power law galaxies.
It has been suggested that nuclear massive black holes and environmental differences are important for explaining the dichotomy of the inner light profiles; both aspects are likely to influence the hot gas content. While a few explanations can be imagined for the large spread in the X-ray luminosities of core galaxies, a clearly open problem is why power law galaxies never become X-ray bright, even when they are the brightest objects of the groups where they reside.
If a massive black hole and the environment have a fundamental role in determining , the problem of interpreting the X-ray properties of early type galaxies becomes much more complex than thought so far, when the input ingredients were just stellar mass loss, supernova heating and gas potential energy.
© European Southern Observatory (ESO) 1999
Online publication: November 3, 1999