2012ApJ...749L..12W

We present the first measurement of the evolution of the galaxy group stellar mass function (GrSMF) to redshift z ≳ 1 and low masses (M_{sstarf} > 10¹² M_☉). Our results are based on early data from the Carnegie-Spitzer-IMACS Survey, utilizing low-resolution spectra and broadband optical/near-IR photometry to measure redshifts for a 3.6 µm selected sample of 37,000 galaxies over a 5.3 deg² area to z ∼ 1.2. Employing a standard friends-of-friends algorithm for all galaxies more massive than log M_{sstarf}/M_☉= 10.5, we find a total of ∼4000 groups. Correcting for spectroscopic incompleteness (including slit collisions), we build cumulative stellar mass functions for these groups in redshift bins at z > 0.35, comparing to the z = 0 and z > 0 mass functions from various group and cluster samples. Our derived mass functions match up well with z > 0.35 X-ray-selected clusters, and strong evolution is evident at all masses over the past 8 Gyr. Given the already low level of star formation activity in galaxies at these masses, we therefore attribute most of the observed growth in the GrSMF to group-group and group-galaxy mergers, in accordance with qualitative notions of hierarchical structure formation. Given the factor of 3-10 increase in the number density of groups and clusters with M_{sstarf}> 10¹² M_☉ since z = 1 and the strong anticorrelation between star formation activity and environmental density during this epoch, this late-time growth in group-sized halos may therefore be an important contributor to the structural and star formation evolution of massive galaxies over the past 8 Gyr.