Further evidence for large central mass-to-light ratios in early-type galaxies: the case of ellipticals and lenticulars in the Abell~262 cluster

TL;DR

This study provides detailed dynamical analysis of early-type galaxies in Abell 262, revealing high mass-to-light ratios and significant dark matter in some, suggesting possible variations in the stellar initial mass function or dark matter distribution.

Contribution

It offers new evidence for large central mass-to-light ratios and dark matter densities in early-type galaxies, using orbit-based models and spectroscopic data, expanding understanding of galaxy mass composition.

Findings

Four galaxies have dense dark matter halos about 10 times denser than in spirals.

Some galaxies show mass following light with high mass-to-light ratios (~8-10).

Degeneracy exists between luminous and dark matter in certain early-type galaxies.

Abstract

We present radially resolved spectroscopy of 8 early-type galaxies in Abell~262, measuring rotation, velocity dispersion, $H_3$ and $H_4$ coefficients along three axes, and line-strength index profiles of Mg, Fe and H$\beta$. Ionized-gas velocity and velocity dispersion is included for 6 galaxies. We derive dynamical mass-to-light ratios and dark matter densities from orbit-based dynamical models, complemented by the galaxies' ages, metallicities, and $\alpha$-elements abundances. Four galaxies have significant dark matter with halos about 10 times denser than in spirals of the same stellar mass. Using dark matter densities and cosmological simulations, assembly redshifts $\zdm\approx 1-3$, which we found earlier for Coma. The dynamical mass following the light is larger than expected for a Kroupa stellar IMF, especially in galaxies with high velocity dispersion $\sigeff$ inside the effective radius $\reff$. This could indicate a `massive' IMF in massive galaxies. Alternatively, some dark matter in massive galaxies could follow the light closely. Combining with our comparison sample of Coma early-types, we now have 5 of 24 galaxies where (1) mass follows light to $1-3\,\reff$, (2) 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), (3) the dark matter fraction is negligible to $1-3\,\reff$. Unless the IMF in these galaxies is particularly `massive' and somehow coupled to the dark matter content, there seems a significant degeneracy between luminous and dark matter in some early-type galaxies. The role of violent relaxation is briefly discussed.