Hunting for the Primordial Quenched Population in 2 < z < 2.5 COSMOS Protoclusters

Abstract

We present an analysis of the galaxy stellar mass function (SMF) of 14 known protoclusters between 2.0 < z < 2.5 in the COSMOS field, down to a mass limit of 10⁹ᐧ⁵ M⊙. We use existing photometric redshifts with a statistical background subtraction, and consider star−forming and quiescent galaxies identified from (NUV−r) and (r−J) colours separately. Our fiducial sample includes galaxies within 1 Mpc of the cluster centres. The shape of the protocluster SMF of star−forming galaxies is indistinguishable from that of the general field at this redshift. Quiescent galaxies, however, show a flatter SMF than in the field, with an upturn at low mass, though this is only significant at ∼2σ. There is no strong evidence for a dominant population of quiescent galaxies at any mass, with a fraction <15% at 1σ confidence for galaxies with łog(M⋆/M⊙)<10.5. We compare our results with a sample of galaxy groups at 1<z<1.5, and demonstrate that a significant amount of environmental quenching must take place between these epochs, increasing the relative abundance of high−mass (M⋆ > 10¹⁰ᐧ⁵ M⊙) quiescent galaxies by a factor ≳ 2. However, we find that at lower masses (M⋆ < 10¹⁰ᐧ⁵ M⊙), no additional environmental quenching is required.