2014MNRAS.445.3200S

We present deep spectroscopy of 17 very low mass (M{sstarf} ≃ 2.0x10⁶-1.4x10⁹ 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 (Niv], Oiii], Civ, Siiii], Ciii]) which are rarely seen in individual spectra of more massive star-forming galaxies. Ciii] is detected in 16 of 17 low-mass star-forming systems with rest-frame equivalent widths as large as 13.5Å. Nebular Civ emission is present in the most extreme Ciii] 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 (0.04-0.13Z_☉), young stellar populations (6-50Myr), 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 Lyα 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.