The internal dynamics of the Local Group dwarf elliptical galaxies NGC 147, 185 and 205

Abstract

We present three-integral dynamical models for the three Local Group dwarf elliptical galaxies: NGC 147, 185 and 205. These models are fitted to the Two-Micron All-Sky Survey (2MASS) J-band surface-brightness distribution and the major-axis kinematics (mean streaming velocity and velocity dispersion) and, in the case of NGC 205, also to the minor-axis kinematics. The kinematical information extends out to 2R e in the case of NGC 205 and out to about 1R e in the case of NGC 147 and 185. It is the first time models are constructed for the Local Group dwarf ellipticals (dEs) that allow for the presence of dark matter at large radii and that are constrained by kinematics out to at least one half-light radius. The B-band mass-to-light ratios of all the three galaxies are rather similar, (M/L) B ≈ 3-4 M ⊙ /L ⊙,B . Within the inner two half-light radii, about 40-50 per cent of the mass is in the form of dark matter, so dEs contain about as much dark matter as bright ellipticals. Based on their appreciable apparent flattening, we modelled NGC 205 and 147 as being viewed edge-on. For NGC 185, having a much rounder appearance on the sky, we produced models for different inclinations. NGC 205 and 147 have a relatively isotropic velocity dispersion tensor within the region where the internal dynamics are strongly constrained by the data. Our estimated inclination for NGC 185 is i ≈ 50° because in that case the model has an intrinsic flattening close to the peak of the intrinsic shape distribution of dEs and it, like the best-fitting models for NGC 147 and 205, is nearly isotropic. We also show that the dynamical properties of the bright nucleus of NGC 205 are not unlike those of a massive globular cluster.