The numbers of z ∼ 2 star-forming and passive galaxies in 2.5 square degrees of deep CFHT imaging

Abstract

We use an adaptation of the BzK[subscript s] technique to select ∼40 000 z ∼ 2 galaxies (to K[subscript AB] = 24), including ∼5000 passively evolving (PE) objects (to K[subscript AB] = 23), from 2.5 deg[superscript 2] of deep Canada–France–Hawaii Telescope (CFHT) imaging. The passive galaxy luminosity function (LF) exhibits a clear peak at R = 22 and a declining faint-end slope ([alpha] = −0.12[superscript +0.16][subscript −0.14), while that of star-forming galaxies is characterized by a steep faint-end slope [[alpha] = −1.43 [plus or minus] 0.02(systematic)[superscript +0.05][subscript −0.04](random)]. The details of the LFs are somewhat sensitive (at the [less than]25 per cent level) to cosmic variance even in these large (∼0.5 deg[superscript 2]) fields, with the D2 field (located in the Cosmological Evolution Survey, COSMOS field) most discrepant from the mean. The shape of the z ∼ 2 stellar mass function of passive galaxies is remarkably similar to that at z ∼ 0.9, save for a factor of ∼4 lower number density. This similarity suggests that the same mechanism may be responsible for the formation of passive galaxies seen at both these epochs. This same formation mechanism may also operate down to z ∼ 0 if the local PE galaxy mass function, known to be two-component, contains two distinct galaxy populations. This scenario is qualitatively in agreement with recent phenomenological mass-quenching models and extends them to span more than three quarters of the history of the Universe.