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

Abstract

Recently, the CIII] and CIV 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 AGN. We explore the first blind CII], CIII] and CIV survey ($z\sim0.68, 1.05, 1.53$, respectively) presented in Stroe et al. (2017). We derive luminosity functions (LF) and study properties of CII], CIII] and CIV line emitters through comparisons to the LFs of H$\alpha$ and Ly$\alpha$ emitters, UV selected star forming (SF) galaxies and quasars at similar redshifts. The CII] LF at $z\sim0.68$ is equally well described by a Schechter or a power-law LF, characteristic of a mixture of SF and AGN activity. The CIII] LF ($z\sim1.05$) is consistent to a scaled down version of the Schechter H$\alpha$ and Ly$\alpha$ LF at their redshift, indicating a SF origin. In stark contrast, the CIV LF at $z\sim1.53$ is well fit by a power-law, quasar-like LF. We find that the brightest UV sources ($M_{UV}<-22$) will universally have CIII] and CIV emission. However, on average, CIII] and CIV are not as abundant as H$\alpha$ or Ly$\alpha$ emitters at the same redshift, with cosmic average ratios of $\sim0.02-0.06$ to H$\alpha$ and $\sim0.01-0.1$ to intrinsic Ly$\alpha$. We predict that the CIII] and CIV lines can only be truly competitive in confirming high redshift candidates when the hosts are intrinsically bright and the effective Ly$\alpha$ escape fraction is below 1 per cent. While CIII] and CIV were proposed as good tracers of young, relatively low-metallicity galaxies typical of the early Universe, we find that, at least at $z\sim1.5$, CIV is exclusively hosted by AGN/quasars, especially at large line equivalent widths.