Evolution of central dark matter of early-type galaxies up to z ∼ 0.8

We investigate the evolution of dark and luminous matter in the central regions of early-type galaxies up to z ∼ 0.8. We use a spectroscopically selected sample of 154 cluster and field galaxies from the ESO Distant Clusters Survey (EDisCS), covering a wide range in redshifts (z ∼ 0.4-0.8), stellar masses (log M_⋆/M_⊙ ∼ 10.5-11.5 dex) and velocity dispersions (σ_⋆ ∼ 100-300 km s^-1). We obtain central dark matter (DM) fractions by determining the dynamical masses from Jeans modelling of galaxy aperture velocity dispersions and the M_⋆ from galaxy colours, and compare the results with local samples. We discuss how the correlations of central DM with galaxy size (i.e. the effective radius, R_e), M_⋆ and σ_⋆ evolve as a function of redshift, finding clear indications that local galaxies are, on average, more DM dominated than their counterparts at larger redshift. This DM fraction evolution with z can be only partially interpreted as a consequence of the size-redshift evolution. We discuss our results within galaxy formation scenarios, and conclude that the growth in size and DM content which we measure within the last 7 Gyr is incompatible with passive evolution, while it is well reproduced in the multiple minor merger scenario. We also discuss the impact of the initial mass function (IMF) on our DM inferences and argue that this can be non-universal with the look-back time. In particular, we find that the Salpeter IMF can be better accommodated by low-redshift systems, while producing stellar masses at high z which are unphysically larger than the estimated dynamical masses (particularly for lower σ_⋆ systems).