Star formation histories of brightest group galaxies in CLoGS

Abstract

An important component of galaxy formation and evolution studies, is to accurately constrain the star formation histories (SFHs) of galaxies. While the SFHs of most massive early-type galaxies can be described using a single passively-evolving stellar component (Single Stellar Population [SSP]), there is a fraction of massive early-type galaxies in the centres of galaxy groups and galaxy clusters where recent star formation is observed and their SFHs are better described by two or more stellar components (Composite Stellar Population [CSP]). In this project we identify and constrain possible recent star formation episodes in a sample of 23 BGGs (Brightest Group Galaxies) in the centres of X-ray rich groups that are closer than 80 Mpc, and a sub-sample of the Complete Local-Volume Groups Sample (CLoGS). We use archival spatially-resolved long-slit spectroscopy from the Hobby-Eberly Telescope (HET) at the McDonald Observatory, and determine whether the BGGs are better described by a SSP or a CSP using Universite´ de Lyon Spectroscopic analysis Software (ULySS). Although the stellar population models fit the spectra of the galaxies, we find that we are not able to detect young stellar components with confidence, due to the lack of blue age-sensitive absorption features in spectra of the wavelength range of 4600 ˚A - 6200 ˚A. One feature, however, that does not fit well, is the sodium doublet NaD. This is due to an overabundance that is not predicted in the stellar population models. A detailed analysis is done to investigate the effects of different model ingredients (e.g. initial mass function [IMF], α-enhancement, etc.) on the predicted spectra, in an effort to identify a possible origin for this mismatch with observations. The stellar population results of these BGGs are also compared to those from a comparable analysis of a sample of 32 Brightest Cluster Galaxies (BCGs) in rich galaxy clusters at redshifts 0.05 < z < 0.3. In this comparison we found that the sodium overabundance in the galaxies must be a physical phenomenon, rather than an artefact of the stellar population modelling, and might show a weak correlation to mass. The analysis of the stellar population models can be improved by using [Na/Fe]-enhanced models, whereas using α-enhanced models will worsen the mismatch between the data and models. A full suite of [Na/Fe]-enhanced models, however, still need to be developed for packages such as ULySS.