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-1, lower than the
instantaneous SFR of 390 ± 90 {{M}⊙ }
yr-1 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× {{10}11} {{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.
