Do the stellar populations of the brightest two group galaxies depend on the magnitude gap?

M. Trevisan, G. A. Mamon, H. G. Khosroshahi

Published 2016-04-26Version 1

We investigate how the stellar populations of first and second brightest group galaxies (respectively BGGs and SBGGs) vary as a function of the magnitude gap, {\Delta}M_12, using an SDSS-based sample of 569 groups with elliptical BGGs. The sample is complete in redshift, luminosity and for {\Delta}M_12 up to 2.5 mag, and contains 75 optical fossil groups (FGs, with {\Delta}M_12 > 2.0 mag). We determine ages, metallicities, and star formation histories (SFHs) of BGGs and SBGGs using the STARLIGHT code with two single stellar population (SSP) models, one of which (MILES) leads to significantly more extended SFHs than the other (BC03). After removing the dependence with stellar mass, there is no correlation with magnitude gap of BGG ages, metallicities, and SFHs derived with the BC03 model. However, with the MILES model, the BGGs in FGs appear to have more extended SFHs than those in regular groups. But this signature with MILES is not seen in the colours, specific star formation rates nor in the 4000 A breaks, nor in any diagnostic with the BC03 model. The stellar population properties of the SBGGs in FGs are also compatible with those of the general population of galaxies with similar stellar masses residing in normal groups. The lack of trends of BGG SFHs with magnitude gap suggests that BGGs in FGs have undergone more mergers than those in small-gap groups, but these mergers are either dry or occurred at very high redshift, which in either case would leave no detectable imprint in their spectra.