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

Abstract

We present deep spectroscopy of 17 very low mass (M* ~ 2.0x10^6 Msun to\n1.4x10^9 Msun) and low luminosity (M_UV ~ -13.7 to -19.9) gravitationally\nlensed galaxies in the redshift range z~1.5-3.0. Deep rest-frame ultraviolet\nspectra reveal large equivalent width emission from numerous lines (NIV],\nOIII], CIV, Si III], CIII]) which are rarely seen in individual spectra of more\nmassive star forming galaxies. CIII] is detected in 16 of 17 low mass star\nforming systems with rest-frame equivalent widths as large as 13.5 Angstroms.\nNebular CIV emission is present in the most extreme CIII] emitters, requiring\nan ionizing source capable of producing a substantial component of photons with\nenergies in excess of 47.9 eV. Photoionization models support a picture whereby\nthe large equivalent widths are driven by the increased electron temperature\nand enhanced ionizing output arising from metal poor gas and stars, young\nstellar populations, and large ionization parameters. The young ages implied by\nthe emission lines and continuum SEDs indicate that the extreme line emitters\nin our sample are in the midst of a significant upturn in their star formation\nactivity. The low stellar masses, blue UV colors, and large sSFRs of our sample\nare similar to those of typical z>6 galaxies. Given the strong attenuation of\nLy-alpha in z>6 galaxies we suggest that CIII] is likely to provide our best\nprobe of early star forming galaxies with ground-based spectrographs and one of\nthe most efficient means of confirming z>10 galaxies with the James Webb Space\nTelescope.\n