2021MNRAS.501.3238T

Deep spectroscopy of galaxies in the reionization era has revealed intense C III] and C IV line emission (equivalent width, EW >15-20 Å). In order to interpret the nebular emission emerging at z > 6, we have begun targeting rest-frame ultraviolet (UV) emission lines in galaxies with large specific star formation rates (sSFRs) at 1.3 < z < 3.7. We find that C III] reaches the EWs seen at z > 6 only in large sSFR galaxies with [O III]+Hβ EW >1500 Å. In contrast to previous studies, we find that many galaxies with intense [O III] have weak C III] emission (EW = 5-8 Å), suggesting that the radiation field associated with young stellar populations is not sufficient to power strong C III]. Photoionization models demonstrate that the spread in C III] among systems with large sSFRs ([O III]+Hβ EW >1500 Å) is driven by variations in metallicity, a result of the extreme sensitivity of C III] to electron temperature. We find that the strong C III] emission seen at z > 6 (EW >15 Å) requires metal-poor gas (≃ 0.1 Z_☉), whereas the weaker C III] emission in our sample tends to be found at moderate metallicities (≃ 0.3 Z_☉). The luminosity distribution of the C III] emitters in our z ≃ 1-3 sample presents a consistent picture, with stronger emission generally linked to low-luminosity systems (M_UV > -19.5) where low metallicities are more likely. We quantify the fraction of strong C III] and C IV emitters at z ≃ 1-3, providing a baseline for comparison against z > 6 samples. We suggest that the first UV line detections at z > 6 can be explained if a significant fraction of the early galaxy population is found at large sSFR (>200 Gyr^–1) and low metallicity (< 0.1 Z_☉).