2015ApJ...805..140T

We present Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging and grism spectroscopy observations of the Herschel-selected gravitationally lensed starburst galaxy HATLASJ1429-0028. The lensing system consists of an edge-on foreground disk galaxy at z = 0.218 with a nearly complete Einstein ring of the infrared luminous galaxy at z = 1.027. The WFC3 spectroscopy with G102 and G141 grisms, covering the wavelength range of 0.8-1.7 µm, resulted in detections of Hα + [Nii], Hβ, [Sii], and [Oiii] for the background galaxy from which we measure line fluxes and ratios. The Balmer line ratio Hα/Hβ of 7.5±4.4, when corrected for [Nii], results in an extinction for the starburst galaxy of E(B-V) = 0.8 ± 0.5. The Hα-based star formation rate (SFR), when corrected for extinction, is 60±50 M_☉/yr, lower than the instantaneous SFR of 390±90 M_☉/yr from the total IR luminosity. We also compare the nebular line ratios of HATLASJ1429-0028 with other star-forming and sub-millimeter bright galaxies. The nebular line ratios are consistent with an intrinsic ultra-luminous infrared galaxy with no evidence for excitation by an active galactic nucleus (AGN). We estimate the metallicity, 12 + log(O/H), of HATLASJ1429-0028 to be 8.49±0.16. Such a low value is below the average relation for stellar mass versus metallicity of galaxies at z ∼ 1 for a galaxy with a stellar mass of ∼2 x 10¹¹ M_☉. The combination of high stellar mass, the lack of AGN indicators, low metallicity, and the high SFR of HATLASJ1429-0028 suggest that this galaxy is currently undergoing a rapid formation.