Optical Spectroscopy of Galaxies in the CLoGS and MHONGOOSE Surveys

Abstract

The majority of observable galaxies within the local Universe can be found within group environments. Roughly at the centre of each galaxy group's gravitational potential well lies each group's brightest and most massive galaxy. The evolution of these galaxies is closely tied to their group environments, and recent studies have revealed that the dark matter content, hydrodynamics and baryonic processes of group environments are more intricate and diverse than previously thought. Therefore, the evolution of brightest group galaxies is more complex than previously expected. In this thesis, we study the spatiallyresolved stellar kinematics and stellar populations of 18 brightest group early-type galaxies (BGEs) from the Complete Local-Volume Groups Sample (CLoGS). Spatially-resolved optical spectra have been obtained for these galaxies using the Southern African Large Telescope Robert Stobie Spectrograph (SALT RSS). The stellar kinematics and stellar populations of these galaxies are measured from their spectra using full-spectrum fitting software. Some of the CLoGS BGEs, such as NGC 128, NGC 4697, NGC 5084 and NGC 5153 are found to have marginal rotation, with an anisotropy parameter of Vmax/σ0 ≳ 0.75; however all of the CLoGS BGEs studied here are found to be pressure supported systems. We further find that the CLoGS BGEs have velocity dispersion slopes similar to field earlytype galaxies in the central velocity dispersion range 2.1 ≤ log(σ0) ≤ 2.4. The diversity of velocity dispersion slopes that we measure for the CLoGS BGEs, is not exactly reflected in the latest galaxy evolution simulations, suggesting that our understanding of the evolution of BGEs is still incomplete. We encourage the use of our spectroscopic results to further build and improve these galaxy evolution models. In line with recent multi-wavelength studies, we detect young stellar populations in some of the CLoGS BGEs, such as ESO 507-25, NGC 128 and NGC 7252. We further detect possibly old stellar populations for NGC 193, NGC 1550, and NGC 5044, known to have AGN activity and large-scale radio jets. We suggest that the AGN activity within these galaxies may have suppressed any new stars from forming. Additionally, we study the spatially-resolved stellar kinematics and stellar populations of HIPASS J1153-28, an HI-rich dwarf spiral galaxy from the MHONGOOSE survey. We demonstrate that the SALT RSS can provide complementary optical spectroscopic data for the ongoing MHONGOOSE survey, which can provide insight into how HI is processed within these galaxies.