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

TL;DR

This study analyzes the distribution and properties of star-forming and passive galaxies at z~2 over 2.5 square degrees, revealing their luminosity functions, mass functions, and potential formation mechanisms, with implications for galaxy evolution models.

Contribution

It provides the first large-area analysis of z~2 galaxy populations using adapted BzKs, revealing detailed luminosity and mass functions and their evolution, supporting mass-quenching models.

Findings

Passive galaxy LF peaks at R=22 with a shallow faint-end slope.

Star-forming galaxy LF has a steep faint-end slope ({e4} = -1.43).

Passive galaxy mass function at z~2 resembles that at z~0.9, scaled by a factor of 4.

Abstract

We use an adaptation of the BzKs technique to select ~40,000 z~2 galaxies (to K(AB) = 24), including ~5,000 passively evolving (PE) objects (to K(AB) = 23), from 2.5 deg^2 of deep CFTH imaging. The passive galaxy luminosity function exhibits a clear peak at R = 22 and a declining faint-end slope ({\alpha} = - 0.12 [+0.16 -0.14]),while that of star-forming galaxies is characterized by a steep faint-end slope ({\alpha} = -1.43 +- [0.02] (systematic) [+0.05 -0.04] (random)). The details of the LFs are somewhat sensitive (at <25% level) to cosmic variance even in these large(~0.5 deg^2) fields, with the D2 field (located in the 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.