Extinction and nebular line properties of a Herschel-selected lensed dusty starburst AT z=1.027

Abstract

We present Hubble Space Telescope (HST) WFC3 imaging and grism spectroscopy observations of the {\it 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 to 1.7 $\hat{I}¼$m, resulted in detections of H$\hat{I}\pm$+[NII], H${\hat{I}}^2$, [SII], and [OIII] for the background galaxy from which we measure line fluxes and ratios. The Balmer line ratio H$\hat{I}\pm$/H${\hat{I}}^2$ of $7.5\pm 4.4$, when corrected for [NII], results in an extinction for the starburst galaxy of $E(B-V)=0.8\pm 0.5$. The $H\hat{I}\pm$ based star-formation rate, when corrected for extinction, is $60\pm 50$ M${}_{\odot }$ yr${}^{-1}$, lower than the instantaneous star-formation rate of 390 $\pm$ 90 M${}_{\odot }$ yr${}^{-1}$ from the total IR luminosity. We also compare the nebular line ratios of HATLASJ1429-0028 with other star-forming and sub-mm 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 $\pm$ 0.16. Such a low value is below the average relations for stellar mass vs. metallicity of galaxies at $z\sim 1$ for a galaxy with stellar mass of $\sim 2\times 10^{11}$ M${}_{\odot }$. The combination of high stellar mass, lack of AGN indicators, low metallicity, and the high star-formation rate of HATLASJ1429-0028 suggest that this galaxy is currently undergoing a rapid formation.

7 pages, 8 figures, Accepted for publication in the Astrophysical Journal