On the Uncertainties of the Central Density in Axisymmetric Galaxies due to Deprojection

Abstract

The deprojection of the surface brightness of axisymmetric galaxies is indeterminate unless the galaxy is seen edge-on. In practice, this problem is often circumvented by making ad hoc assumptions about the density distribution. However, one can redistribute the density and still project to the same surface brightness. This is similar to adding so-called konus densities to the assumed density distribution. In this paper we investigate the maximum konus density that one can add to elliptical galaxies. In particular we focus on the uncertainties in the central densities of axisymmetric, (cusped) elliptical galaxies due to the non-uniqueness of the deprojection. In order to estimate the dynamical influence of konus densities on the central region, we calculate the mass fraction they can add to the center. We show that this mass fraction is at most a few percent. We also investigate the dynamical effect of cusped konus densities We show that konus densities can only be moderately cusped, and that an increase in cusp slope results in a decrease of the mass fraction added to the center by the konus density. We illustrate all this by the specific example of M32, and show that the uncertainty in the central mass due to deprojection is negligible compared to the inferred mass of the central black hole (BH) in this galaxy.