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 DM ) and the average dark matter density ( ρ DM ) 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 R 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 ) ∼ 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 ρ DM of S0s and ellipticals in our sample, despite the differences in their stellar populations. We have also used ρ DM 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.

Assembly Pathways and the Growth of Massive Early-Type Galaxies

“…We correct the mass estimates for bulk rotation of the GC system, and make assumptions for the galaxy potential, tracer density slope and anisotropy (here we assume isotropic orbits). Further details can be found in Alabi et al (2016Alabi et al ( , 2017 [6,7]. We find good agreement with other mass tracers such as planetary nebulae and X-ray emission (Alabi et al 2016) [6].…”

“…Multiple solutions are possible, but none yet is convincing. See Alabi et al (2017) [7] for further details. Converting dark matter fractions into halo densities and hence epochs of halo assembly, we show that low mass galaxies tend to grow via gas-rich accretion, while high mass galaxies grow via gas-poor mergers.…”

“…From the enclosed total mass within 5 R e we subtract the stellar mass (assuming a Kroupa IMF) to calculate the dark matter fraction within 5 R e . In Figure 3 we show the derived dark matter fractions within 5 R e vs galaxy stellar mass from our analysis ) [7] and two sets of cosmological simulations. The simulations are with (Remus et al 2017) [8] and without (Wu et al 2014) [9] AGN feedback.…”

Dark matter and no dark matter: on the halo mass of NGC 1052

“…5 × 2.06 kpc) is 1.17 (± 0.26) × 10 11 M . Relaxing the isotropic assumption, to include mild radial or tangential orbits (as per Alabi et al 2017), results in masses that are within ±4% of the mass for the isotropic case. If we adopt the larger R e value of 3.2 kpc, then the dynamical mass within 5R e becomes 1.71 (± 0.32) × 10 11 M .…”

How large are the globular cluster systems of early-type galaxies and do they scale with galaxy halo properties?

“…This property has been exploited by the SLUGGS survey of 25 nearby early-type galaxies (Brodie et al 2014 and see sluggs.swin.edu.au) to investigate the structural properties (Kartha et al 2016), metallicity (Usher et al 2012), kinematics (Pota et al 2013) and dynamical mass (Alabi et al 2017) of GC systems over a range of host galaxy properties.…”