Title: The SLUGGS Survey: Dark matter fractions at large radii and assembly epochs of early-type galaxies from globular cluster kinematics,
Abstract: We use globular cluster kinematics data, primarily from the SLUGGS survey, to
measure the dark matter fraction ($f_{\rm DM}$) and the average dark matter
density ($\left< \rho_{\rm DM} \right>$) within the inner 5 effective radii
($R_{\rm e}$) for 32 nearby early--type galaxies (ETGs) with stellar mass log
$(M_*/\rm M_\odot)$ 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 $f_{\rm DM}$ ($\geq0.6$) within 5~$R_{\rm e}$ 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 $(M_*/M_\odot)
\sim 11$, there is a wide range of $f_{\rm DM}$ 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 $f_{\rm DM}$ within 5~$R_{\rm e}$ as galaxy
luminosity increases. However, we do not find any difference between the
$\left< \rho_{\rm DM} \right>$ of S0s and ellipticals in our sample, despite
the differences in their stellar populations. We have also used $\left<
\rho_{\rm DM} \right>$ to infer the epoch of halo assembly ($z{\sim}2-4$). 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.