Most Massive Group Galaxies at Intermediate Redshifts

I present a study of the most massive group galaxies in the Group Environment and Evolution Collaboration (GEEC) sample of optically- and X-ray-selected groups at redshift z~0.4. We include only those groups with sufficient completeness, ensuring that the most massive galaxy (MMG) has been observed spectroscopically. The position and dominance of the MMGs in their host groups is explored, basic properties of the MMGs are compared to those of control samples of satellites and field galaxies at similar stellar mass and redshift, and correlations of MMG properties with host group properties examined. While many MMGs are projected to lie close to the luminosity-weighted center, a broad range in the stellar mass fraction is found with some MMGs containing a large fraction of the total group stellar mass while others are not at all dominant. Compared to central group galaxies in the local universe, the SSFR of the MMG in similar mass systems is found to be significantly higher. Star formation rates, morphologies, and mass fractions of the MMGs are also compared to control samples of non-MMG galaxies with similar stellar mass and redshift. While MMGs and satellite galaxies have SSFRs that agree at the 40% level, MMGs definitely do not come from the same distribution as field galaxies. As a function of stellar mass, the fraction of passive galaxies for both MMGs and satellites is systematically higher than the field. We cannot however confirm that, at the same stellar mass, MMGs are more quenched than satellites, as has been found in the local universe. 19 GEEC MMGs were imaged with HST-ACS and many show significant disk components, but the sample is too small to conclude whether or not the MMG morphologies are the same as stellar and redshift matched control samples of satellite and field galaxies. Finally, we search for correlation of MMG and global group properties, including group masses and dynamics. Thus far MMG properties appear to be only weakly correlated with the properties of the groups they inhabit in this redshift regime.