New radio constraints on the obscured star formation rates of massive GRB hosts at redshifts 2-3.5

TL;DR

This study uses radio observations to assess dust obscuration in high-redshift GRB host galaxies, finding most have low overall dust obscuration despite UV sightline indications, challenging the idea that GRBs trace dusty starburst galaxies at these epochs.

Contribution

First radio-based constraints on dust obscuration in $z ext{=} 2-3.5$ GRB hosts, revealing they are generally not dusty starburst galaxies.

Findings

Most GRB hosts have less than 90% of their star formation obscured by dust.

GRB hosts with UV-unobscured sightlines tend to have low overall dust obscuration.

Results challenge the assumption that GRBs at these redshifts are unbiased tracers of dusty star formation.

Abstract

It is not clear whether gamma-ray bursts (GRBs) are unbiased tracers of cosmic star formation at $z > 2$. Since dusty starburst galaxies are significant contributors to the cosmic star formation at these redshifts, they should form a major part of the GRB host population. However, recent studies at $z \leq 2$ have shown that the majority of the star formation activity in GRB hosts is not obscured by dust. Here, we investigate the galaxy-scale dust obscuration in $z \sim 2-3.5$ GRB hosts pre-selected to have high-resolution, high signal-to-noise afterglow spectra in the rest-frame ultraviolet (UV) and thus relatively low line-of-sight dust obscuration. We present new deep VLA observations of four GRB hosts, and compare the radio-based (upper limits on the) "total" star formation rates (SFRs) to the "unobscured" SFRs derived from fits to the optical-UV spectral energy distribution. The fraction of the total SFR that is obscured by dust in these galaxies is found to be $<$ 90% in general, and $\lesssim$ 50% for GRB 021004 in particular. These observations suggest that $z \sim 2-3.5$ GRBs with UV-unobscured sightlines originate in star-forming galaxies with low overall dust obscuration, unlike the dusty starburst population.