Assembly Pathways and the Growth of Massive Early-Type Galaxies

Abstract

Based on data from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, I present results on the assembly pathways, dark matter content and halo growth of massive early-type galaxies. Using galaxy starlight information we find that such galaxies had an early dissipative phase followed by a second phase of halo growth from largely minor mergers (and in rare cases major mergers). Thus our result fits in well with the two-phase scenario of galaxy formation. We also used globular cluster radial velocities to measure the enclosed mass within 5 effective radii. The resulting dark matter fractions reveal a few galaxies with very low dark matter fractions that are not captured in the latest cosmological models. Multiple solutions are possible, but none yet is convincing. Translating dark matter fractions into 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.Keywords: galaxies; formation; evolution; halos Assembly PathwaysThe SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey (Brodie et al. 2014) [1] is studying 25 nearby, massive (log stellar mass ∼11), early-type galaxies. The survey uses the DEIMOS multi-slit instrument on the Keck II telescope to obtain spectra of both the underlying starlight and surrounding globular clusters (GCs). The data have the advantage of reaching out to ∼3 R e (effective radii) for starlight and ∼10 R e for GCs. From this data, we can probe the dark matter content, assembly pathways and halo growth of these galaxies.A key aim of near-field cosmology is to determine the assembly history of an individual galaxy. The hydro-zoom cosmological simulations of Naab et al. (2014) [2] showed that the assembly histories of massive galaxies are preserved in the 2D kinematics of present day galaxies. In the Naab et al. simulations massive galaxies form in two phases-the first in-situ phase at high redshift results in a compact, massive object (a red nugget); the second phase after redshift 2 is dominated by accretion of ex-situ stars i.e., formed in external galaxies. In this picture low mass galaxies have a high in-situ formed fraction, whereas high mass galaxies are largely built by accretion. A schematic of this two-phase galaxy formation is given in Figure 1. Naab et al. used 3 kinematic diagnostics to classify galaxies into one of six assembly pathways. The SLUGGS starlight data provides similar diagnostics out to 3 R e . They are radial lambda (spin) profiles, 2D kinematic maps and higher order velocity moments h3 and h4 vs V/σ. Using these 3 diagnostics (see Figure 2 for examples) we have classified each galaxy into a Nabb et al. assembly class. We find the most common pathway (14/24) to be class A-these reveal disk-like kinematics with slowly rotating stellar halos whose mass growth is due to minor mergers. Three galaxies are classified as class E which show disturbed kinematics, including rolling, double sigma profile and a decoupled core. Galaxies with high accretion ...