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We present deep spectroscopy of 17 very low mass (M⋆ ≃ 2.0 × 106–1.4 × 109 M⊙) and low luminosity (MUV ≃ −13.7 to −19.9) gravitationally lensed galaxies in the redshift range z ≃ 1.5-3.0. Deep rest-frame ultraviolet spectra reveal large equivalent width emission from numerous emission lines (N iv], O iii], C iv, Si iii], C iii]) which are rarely seen in individual spectra of more massive star-forming galaxies. C iii] is detected in 16 of 17 low-mass star-forming systems with rest-frame equivalent widths as large as 13.5 Å. Nebular C iv emission is present in the most extreme C iii] emitters, requiring an ionizing source capable of producing a substantial component of photons with energies in excess of 47.9 eV. Photoionization models support a picture whereby the large equivalent widths are driven by the increased electron temperature and enhanced ionizing output arising from metal-poor gas and stars (0.04–0.13 Z⊙), young stellar populations (6–50 Myr), and large ionization parameters (log U = −2.16 to −1.84). The young ages implied by the emission lines and continuum spectral energy distributions (SEDs) indicate that the extreme line emitters in our sample are in the midst of a significant upturn in their star formation activity. The low stellar masses, blue UV colours, and large specific star formation rates of our sample are similar to those of typical z ≳ 6 galaxies. Given the strong attenuation of Lyα in z ≳ 6 galaxies, we suggest that C iii] is likely to provide our best probe of early star-forming galaxies with ground-based spectrographs and one off the most efficient means of confirming z ≳ 10 galaxies with the James Webb Space Telescope.
Stark et al. (Fri,) studied this question.