The Bright and Dark Sides of High-Redshift starburst galaxies from {\it Herschel} and {\it Subaru} observations

Abstract

We present rest-frame optical spectra from the FMOS-COSMOS survey of twelve $z\sim 1.6$ \textit{Herschel} starburst galaxies, with Star Formation Rate (SFR) elevated by $\times$8, on average, above the star-forming Main Sequence (MS). Comparing the H$\hat{I}\pm$ to IR luminosity ratio and the Balmer Decrement we find that the optically-thin regions of the sources contain on average only $\sim 10$ percent of the total SFR whereas $\sim 90$ percent comes from an extremely obscured component which is revealed only by far-IR observations and is optically-thick even in H$\hat{I}\pm$. We measure the [NII]${}_{6583}$/H$\hat{I}\pm$ ratio, suggesting that the less obscured regions have a metal content similar to that of the MS population at the same stellar masses and redshifts. However, our objects appear to be metal-rich outliers from the metallicity-SFR anticorrelation observed at fixed stellar mass for the MS population. The [SII]${}_{6732}$/[SII]${}_{6717}$ ratio from the average spectrum indicates an electron density $n_{\mathrm{e}}\sim 1,100{\mathrm{cm}}^{-3}$, larger than what estimated for MS galaxies but only at the 1.5$\ddot{I}ƒ$ level. Our results provide supporting evidence that high-$z$ MS outliers are the analogous of local ULIRGs, and are consistent with a major merger origin for the starburst event.

6 pages, 4 figures, Accepted for publication in ApJ Letters