The SLUGGS survey: Dark matter fractions at large radii and assembly epochs of early-type galaxies from globular cluster kinematics

We use globular cluster kinematics data, primarily from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, to measure the dark matter fraction (f_DM) and the average dark matter density (〈 ρDM〉 ) within the inner 5 effective radii (R_e) for 32 nearby early-type galaxies (ETGs) with stellar mass log (M*/M⊙) ranging from 10.1 to 11.8. We compare our results with a simple galaxy model based on scaling relations as well as with cosmological hydrodynamical simulations where the dark matter profile has been modified through various physical processes. We find a high f_DM (≥0.6) within 5 Re in most of our sample, which we interpret as a signature of a late mass assembly history that is largely devoid of gas-rich major mergers. However, around log (M*/M⊙) ~ 11, there is a wide range of f_DM which may be challenging to explain with any single cosmological model. We find tentative evidence that lenticulars (S0s), unlike ellipticals, have mass distributions that are similar to spiral galaxies, with decreasing f_DM within 5 R_e as galaxy luminosity increases. However, we do not find any difference between the 〈 ρDM〉 of S0s and ellipticals in our sample, despite the differences in their stellar populations.We have also used 〈 ρDM〉 to infer the epoch of halo assembly (z ~ 2- 4). By comparing the age of their central stars with the inferred epoch of halo formation, we are able to gain more insight into their mass assembly histories. Our results suggest a fundamental difference in the dominant late-phase mass assembly channel between lenticulars and elliptical galaxies.