New Radio Constraints on the Obscured Star Formation Rates of Massive GRB Hosts at Redshifts 2–3.5

Abstract

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 ≤ 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 ∼ 2–3.5 GRB hosts preselected 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 Very Large Array 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 ≲50% for GRB 021004 in particular. These observations suggest that z ∼ 2–3.5 GRBs with UV-unobscured sightlines originate in star-forming galaxies with low overall dust obscuration, unlike the dusty starburst population.