Ultraviolet emission lines in young low-mass galaxies at z ≃ 2: Physical properties and implications for studies at z > 7

We present deep spectroscopy of 17 very low mass (M_* ≃ 2.0 × 10⁶-1.4 × 10⁹ M_⊙) and low luminosity (M_UV ≃-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], OIII], CIV, Si III], CIII]) which are rarely seen in individual spectra of more massive star-forming galaxies. C III] is detected in 16 of 17 low-mass starforming systems with rest-frame equivalent widths as large as 13.5Å. 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 (logU = -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 Lya in z ≳ 6 galaxies, we suggest that CIII] 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.