Direct observational evidence for a large transient galaxy population in groups at 0.85 < z < 1

We introduce our survey of galaxy groups at 0.85 < z < 1, as an extension of the Group Environment and Evolution Collaboration. Here we present the first results, based on Gemini GMOS‐S nod‐and‐shuffle spectroscopy of seven galaxy groups selected from spectroscopically confirmed, extended XMM detections in COSMOS. We use photometric redshifts to select potential group members for spectroscopy, and target galaxies with r < 24.75. In total, we have over 100 confirmed group members, and four of the groups have >15 members. The dynamical mass estimates are in good agreement with the masses estimated from the X‐ray luminosity, with most of the groups having 13 < log Mdyn/M⊙ < 14. We compute stellar masses by template‐fitting the spectral energy distributions; our spectroscopic sample is statistically complete for all galaxies with Mstar≳ 1010.1 M⊙, and for blue galaxies we sample masses as low as Mstar∼ 108.8 M⊙. The fraction of total mass in galaxy starlight spans a range of 0.25–3 per cent, for the six groups with reliable mass determinations. Like lower redshift groups, these systems are dominated by red galaxies, at all stellar masses Mstar > 1010.1 M⊙. A few group galaxies inhabit the ‘blue cloud’ that dominates the surrounding field; instead, we find a large and possibly distinct population of galaxies with intermediate colours. The ‘green valley’ that exists at low redshift is …