The Bright and Dark Sides of High-redshift Starburst Galaxies from Herschel and Subaru Observations

Abstract

We present rest-frame optical spectra from the FMOS-COSMOS survey of 12 z ~ 1.6 Herschel starburst galaxies, with star formation rate (SFR) elevated by ×8, on average, above the star-forming main sequence (MS). Comparing the Hα to IR luminosity ratio and the Balmer decrement, we find that the optically thin regions of the sources contain on average only ~10% of the total SFR, whereas ~90% come from an extremely obscured component that is revealed only by far-IR observations and is optically thick even in Hα. We measure the [N ii]_(6583)/Hα 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 [S ii]_(6732)/[S ii]_(6717) ratio from the average spectrum indicates an electron density n_e ~ 1100 cm^(−3) , larger than what was estimated for MS galaxies but only at the 1.5σ level. Our results provide supporting evidence that high-z MS outliers are analogous of local ULIRGs and are consistent with a major-merger origin for the starburst event.

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