A 1.4 deg2 blind survey for C II], C III] and C IV at z ∼ 0.7-1.5 - II. Luminosity functions and cosmic average line ratios

Recently, the C III] and C IV emission lines have been observed in galaxies in the early Universe (z > 5), providing new ways to measure their redshift and study their stellar populations and active galactic nuclei (AGN). We explore the first blind C II], C III] and C IV survey (z ∼ 0.68, 1.05, 1.53, respectively) presented in Stroe et al. (2017). We derive luminosity functions (LF) and study properties of C II], C III] and C IV line emitters through comparisons to the LFs of H α and Ly α emitters, UV selected star-forming (SF) galaxies and quasars at similar redshifts. The C II] LF at z ∼ 0.68 is equally well described by a Schechter or a power-law LF, characteristic of a mixture of SF and AGN activity. The C III] LF (z ∼ 1.05) is consistent to a scaled down version of the Schechter H α and Ly α LF at their redshift, indicating a SF origin. In stark contrast, the C IV LF at z ∼ 1.53 is well fit by a power-law, quasar-like LF. We find that the brightest UV sources (M_UV < -22) will universally have C III] and C IV emission. However, on average, C III] and C IV are not as abundant as H α or Ly α emitters at the same redshift, with cosmic average ratios of ∼0.02-0.06 to H α and ∼0.01-0.1 to intrinsic Ly α. We predict that the C III] and C IV lines can only be truly competitive in confirming high-redshift candidates when the hosts are intrinsically bright and the effective Ly α escape fraction is below 1 per cent. While C III] and C IV were proposed as good tracers of young, relatively low-metallicity galaxies typical of the early Universe, we find that, at least at z ∼ 1.5, C IV is exclusively hosted by AGN/quasars, especially at large line equivalent widths.