2023MNRAS.520.4037M

We present new observations of 16 bright (r = 19-21) gravitationally lensed galaxies at z ≃ 1-3 selected from the CASSOWARY survey. Included in our sample is the z = 1.42 galaxy CSWA-141, one of the brightest known reionization-era analogues at high redshift (g = 20.5), with a large specific star formation rate (31.2 Gyr^–1) and an [O III]+H β equivalent width (EW_[O iii] + H β_ = 730 Å) that is nearly identical to the average value expected at z ≃ 7-8. In this paper, we investigate the rest-frame UV nebular line emission in our sample with the goal of understanding the factors that regulate strong C III] emission. Although most of the sources in our sample show weak UV line emission, we find elevated C III] in the spectrum of CSWA-141 (EW_C iii]_ = 4.6 ± 1.9 Å) together with detections of other prominent emission lines (O III], Si III], Fe II^⋆, Mg II). We compare the rest-optical line properties of high-redshift galaxies with strong and weak C III] emission, and find that systems with the strongest UV line emission tend to have young stellar populations and nebular gas that is moderately metal-poor and highly ionized, consistent with trends seen at low and high redshift. The brightness of CSWA-141 enables detailed investigation of the extreme emission line galaxies which become common at z > 6. We find that gas traced by the C III] doublet likely probes higher densities than that traced by [O II] and [S II]. Characterization of the spectrally resolved Mg II emission line and several low-ionization absorption lines suggests neutral gas around the young stars is likely optically thin, potentially facilitating the escape of ionizing radiation.