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

## TL;DR

This study analyzes high-redshift starburst galaxies, revealing that most star formation occurs in heavily obscured regions, and suggests these galaxies are analogous to local ULIRGs, likely resulting from major mergers.

## Contribution

It provides detailed spectroscopic analysis of z~1.6 starburst galaxies, highlighting their obscured star formation and metal-rich nature, and links them to local ULIRGs and merger activity.

## Key findings

- Most star formation occurs in optically-thick, obscured regions.
- Starburst galaxies are metal-rich outliers from the typical metallicity-SFR relation.
- High electron densities suggest intense star-forming environments.

## 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$8, on average, above the star-forming Main Sequence (MS). Comparing the H$\alpha$ 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 $\sim90$ percent comes from an extremely obscured component which is revealed only by far-IR observations and is optically-thick even in H$\alpha$. We measure the [NII]$_{6583}$/H$\alpha$ 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 $n_{\rm e} \sim 1,100\ \mathrm{cm}^{-3}$, larger than what estimated for MS galaxies but only at the 1.5$\sigma$ 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.