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Astron. Astrophys. 351, 487-494 (1999)

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6. Conclusions

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:

[FORMULA] The family of core galaxies spans the whole observed range of [FORMULA] values (two orders of magnitude in [FORMULA]) while that of power law galaxies is confined to log [FORMULA] (erg s[FORMULA]. This dichotomy in the X-ray properties holds even in the [FORMULA] range where the two families coexist.

[FORMULA] The relation between [FORMULA] and the shape of the inner profile is sharp, and is stronger than the relations of [FORMULA] 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 [FORMULA] with no exception, while the same cannot be said for the property of having disky isophotes. So, a global quantity such as [FORMULA] is surprisingly tightly connected with a nuclear galaxy property.

[FORMULA] A linear regression analysis shows that [FORMULA] [FORMULA][FORMULA] could be a good description of the [FORMULA]-[FORMULA] relation for power law galaxies, while core galaxies deviate from this relation.

[FORMULA] Different possible reasons can be argued for the origin of the dichotomy in the [FORMULA] 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 [FORMULA] with the [FORMULA] 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 [FORMULA] for the whole class of power law galaxies.

[FORMULA] 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.

[FORMULA] If a massive black hole and the environment have a fundamental role in determining [FORMULA], 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.

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999