The GEEC2 spectroscopic survey of Galaxy groups at 0.8 < z < 1

Abstract

We present the data release of the Gemini-South GMOS spectroscopy in the fields of 11 galaxy groups at 0.8 ă z ă 1, within the COSMOS field. This forms the basis of the Galaxy Environment Evolution Collaboration 2 (GEEC2) project to study galaxy evolution in haloes with M " 10 13 M d across cosmic time. The final sample includes 162 spectroscopically-confirmed members with R ă 24.75, and is ą 50 per cent complete for galaxies within the virial radius, and with stellar mass M star ą 10 10.3 M d . Including galaxies with photometric redshifts we have an effective sample size of " 400 galaxies within the virial radii of these groups. We present group velocity dispersions, dynamical and stellar masses. Combining with the GCLASS sample of more massive clusters at the same redshift we find the total stellar mass is strongly correlated with the dynamical mass, with log M 200 " 1.20 plog M star´1 2q`14.07. This stellar fraction of " 1 per cent is lower than predicted by some halo occupation distribution models, though the weak dependence on halo mass is in good agreement. Most groups have an easily identifiable most massive galaxy (MMG) near the centre of the galaxy distribution, and we present the spectroscopic properties and surface brightness fits to these galaxies. The total stellar mass distribution in the groups, excluding the MMG, compares well with an NFW profile with concentration 4, for galaxies beyond " 0.2R 200 . This is more concentrated than the number density distribution, demonstrating that there is some mass segregation.

Evidence for a change in the dominant satellite galaxy quenching mechanism atz = 1

“…Thus we consider the main result, or an increasing t delay with decreasing stellar mass, to be quite robust, although quantitatively it is sensitive to having the correct probability distribution function for the photometric redshifts. We showed in Balogh et al (2014) that, at least for massive galaxies, these probabilities do agree well with the spectroscopic results. It would be worth repeating these analyses with photometric redshifts from Ultravista; however spectroscopic confirmation will ultimately be required to support our conclusions.…”

“…Halo masses are computed from the velocity dispersions, within R 200 and within R rms , the rms position of all spectroscopic members. As argued in Balogh et al (2014) we choose the larger of these two radii, R max , as some poorly populated systems have few members within R 200 . The corresponding dynamical mass within this radius shown, as a function of group redshift, with red circles in Fig.…”

“…This procedure is described in detail in Balogh et al (2014), and includes a correction for the bias that results from targeting known overdensities. To summarize, we first compute, from the distribution function, the probability p(z group , 3σ ) that a galaxy lies within 3σ of a group redshift, z group , where σ is the group velocity dispersion.…”

“…Stellar masses for all galaxies with spectra were computed as described in Mok et al (2013), based upon Bruzual & Charlot (2003) templates (using the more recent models of Bruzual 2007) with a Chabrier (2003) initial mass function. For the photometric sample, stellar masses are computed from a fit to the [3.6]µm fluxes and (r − i) colours of the spectroscopic sample, as described in Balogh et al (2014). The stellar mass limit for passive galaxies at the IRAC 5σ limit AB = 24.0 and our maximum redshift of z = 1 corresponds to 10 9.5 M , though we caution that some incompleteness may set in at M star < 10 10 M as a small fraction of these galaxies will be undetected in i.…”

“…Probably more significant for us is the fact that the actual p(z) will be modified by the prior knowledge that there is an overdensity of galaxies in the field. In Balogh et al (2014) we attempt to make a global correction for this, based on the spectroscopic sample. It is possible, and even likely, that the correction should depend on galaxy properties like stellar mass, apparent magnitude, and colour.…”