Further evidence for large central mass-to-light ratios in massive early-type galaxies

We studied the stellar populations, distribution of dark matter, and dynamical structure of a sample of 25 early-type galaxies in the Coma and Abell 262 clusters. We derived dynamical mass-to-light ratios and dark matter densities from orbit-based dynamical models, complemented by the ages, metallicities, and \alpha-elements abundances of the galaxies from single stellar population models. Most of the galaxies have a significant detection of dark matter and their halos are about 10 times denser than in spirals of the same stellar mass. Calibrating dark matter densities to cosmological simulations we find assembly redshifts z_{DM} \approx 1-3. The dynamical mass that follows the light is larger than expected for a Kroupa stellar initial mass function, especially in galaxies with high velocity dispersion \sigma_{eff} inside the effective radius r_{eff}. We now have 5 of 25 galaxies where mass follows light to 1-3 r_{eff}, the dynamical mass-to-light ratio of all the mass that follows the light is large (\approx 8-10 in the Kron-Cousins R band), the dark matter fraction is negligible to 1-3 r_{eff}. This could indicate a "massive" initial mass function in massive early-type galaxies. Alternatively, some of the dark matter in massive galaxies could follow the light very closely suggesting a significant degeneracy between luminous and dark matter.
Comment: 4 pages, 2 figures. To appear in the proceedings of IAU Symp. 295, The intriguing life of massive galaxies, ed. D. Thomas, A. Pasquali, and I. Ferreras, Cambridge University Press