Rest-frame UV spectroscopy of extreme [O III] emitters at 1.3 < z < 3.7: toward a high-redshift UV reference sample for JWST

Deep spectroscopy of galaxies in the reionization era has revealed intense CIII] and C IV line emission (equivalent width, EW >15-20 angstrom). 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]+H beta EW >1500 angstrom. In contrast to previous studies, we find that many galaxies with intense [OIII] have weak C III] emission (EW = 5-8 angstrom), 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]+H beta EW >1500 angstrom) 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 angstrom) requires metal-poor gas (similar or equal to 0.1 Z(circle dot)), whereas the weaker C III] emission in our sample tends to be found at moderate metallicities (similar or equal to 0.3 Z(circle dot)). The luminosity distribution of the C III] emitters in our z similar or equal to 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 similar or equal to 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(circle dot)).

Automated summary

Deep spectroscopy of galaxies in the reionization era has shown intense CIII] and CIV line emissions, with the strength of CIII] only reaching levels seen at z > 6 in galaxies with large specific star formation rates and high equivalent widths of oxygen and hydrogen lines. Contrary to previous studies, galaxies with strong [OIII] emissions may exhibit weak CIII] emissions, possibly due to insufficient radiation from young stellar populations. Variations in CIII] strength among systems with large specific star formation rates are primarily driven by differences in metallicity, with metal-poor gas producing stronger CIII] emissions. The distribution of CIII] emitters in the sample at z similar to 1-3 indicates that strong emissions are more common in low-luminosity systems with lower metallicities.