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

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1. Introduction

With Hubble Space Telescope the central surface brightness profiles of nearby galaxies can be studied with a resolution limit of [FORMULA], which corresponds to, e.g., [FORMULA] parsecs in Virgo. So, a large number of early type galaxies have been observed with unprecedented detail in their central regions (e.g., Jaffe et al. 1994, Ferrarese et al. 1994, Lauer et al. 1995, Forbes et al. 1995; Faber et al. 1997, hereafter F97). It turns out that the most luminous galaxies show outer power law profiles that break internally to shallow inner profiles [FORMULA], with [FORMULA]. These galaxies are said to have cuspy cores 1; their break radii typically range from 50 to 1000 pc. Faint galaxies show instead steep featureless power law profiles that lack cores down to the resolution limit. Their central surface brightness slope is typically [FORMULA], with [FORMULA] on average. At intermediate luminosities core and power law galaxies coexist.

These central properties correlate with isophotal shape and amount of rotation: core galaxies tend to be boxy and slowly rotating, whereas power law galaxies tend to be disky and rapidly rotating (F97). At intermediate luminosities, where power law and core galaxies coexist, the presence of a core is a better predictor of boxiness or slow rotation than luminosity; the same can be said for the absence of a core and diskiness, or high rotation. So, it has been suggested that early type galaxies can be divided into two families: one includes those galaxies that are coreless, nearly isotropic spheroids, rotate rapidly, and have disky isophotes; the other ones that have cores, rotate slowly, are possibly moderately triaxial and boxy-distorted (Kormendy & Bender 1996).

In this paper I investigate whether power law and core early type galaxies (ellipticals and S0s) differ systematically also in their soft X-ray luminosities. I find that power law galaxies are confined below log [FORMULA](erg s[FORMULA], while core ones can reach [FORMULA] values at least one order of magnitude higher. This holds even in the range of optical luminosities where the two families coexist. So, the central properties of early type galaxies are tightly related to, and possibly at the origin of, their global X-ray emission. A key factor could be the presence of a central massive black hole. In fact, it has been suggested that nuclear massive black holes are important for explaining the dichotomy of the inner light profiles, as they should have substantial influence on the dynamics and evolution of the surrounding gas and stars (e.g., Cipollina & Bertin 1994, van der Marel 1999, Merritt 1999, Nakano & Makino 1999). I also explore the alternative possibility that the other basic properties characterizing the two families (importance of rotation and isophotal shape distortion) affect the global [FORMULA]. This hypothesis is less plausible because the trend of [FORMULA] with these other properties is less sharp. Finally, I also examine the effects on [FORMULA] that may be produced by differences in the environment.

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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