Further Evidence for Large Central Mass-to-Light Ratios in Early-Type Galaxies: The Case of Ellipticals and Lenticulars in the a262 Cluster

We present new radially resolved spectroscopy of eight early-type galaxies in the A262 cluster. The measurements include stellar rotation, velocity dispersion, H 3 and H 4 coefficients of the line-of-sight velocity distribution along the major and minor axes and an intermediate axis as well as line-strength index profiles of Mg, Fe, and Hβ. The ionized-gas velocity and velocity dispersion is measured for six sample galaxies along different axes. We derive dynamical mass-to-light ratios and dark matter densities from orbit-based dynamical models, complemented by the galaxies\u27 ages, metallicities, and α-element abundances from single stellar-population models. The ionized-gas kinematics give a valuable consistency check for the model assumptions about orientation and intrinsic shape of the galaxies. Four galaxies have a significant detection of dark matter and their halos are about 10 times denser than in spirals of the same stellar mass. By calibrating dark matter densities to cosmological simulations we find assembly redshifts of z DM ≈ 1-3, as previously reported for the Coma Cluster. The dynamical mass that follows the light is larger than expected for a Kroupa stellar initial mass function (IMF), especially in galaxies with high velocity dispersion σeff inside the effective radius r eff. This could indicate a massive IMF in massive galaxies. Alternatively, some of the dark matter in massive galaxies could follow the light very closely. In combination with our comparison sample of Coma early-type galaxies, we now have 5 of 24 galaxies where (1) mass follows light to 1-3 r eff, (2) the dynamical mass-to-light ratio of all the mass that follows the light is large (≈8-10 in the Kron-Cousins R band), and (3) the dark matter fraction is negligible to 1-3 r eff. Unless the IMF in these galaxies is particularly massive and somehow coupled to the dark matter content, there seems to be a significant degeneracy between luminous and dark matter in at least some early-type galaxies. The role of violent relaxation is briefly discussed