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* ~ 2.0x10^6 Msun to 1.4x10^9 Msun) 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 lines (NIV], OIII], CIV, Si III], 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 13.5 Angstroms. 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, young stellar populations, and large ionization parameters. The young ages implied by the emission lines and continuum 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 colors, and large sSFRs of our sample are similar to those of typical z>6 galaxies. Given the strong attenuation of Ly-alpha 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 of the most efficient means of confirming z>10 galaxies with the James Webb Space Telescope.