The optically unbiased GRB host (TOUGH) survey. VI. Radio observations at z ≲ 1 and consistency with typical star-forming galaxies

The objective of this paper is to determine the level of obscured star formation activity and dust attenuation in a sample of gamma-ray burst (GRB) hosts, and to test the hypothesis that GRB hosts have properties consistent with those of the general star-forming galaxy populations. We present a radio continuum survey of all z < 1 GRB hosts in The Optically Unbiased GRB Host (TOUGH) sample supplemented with radio data for all (mostly pre-Swift) GRB-SN hosts discovered before 2006 October. We present new radio data for 22 objects and have obtained a detection for three of them (GRB 980425, 021211, 031203; none in the TOUGH sample), increasing the number of radio-detected GRB hosts from two to five. The star formation rate (SFR) for the GRB 021211 host of 825 M yr^-1, the highest ever reported for a GRB host, places it in the category of ultraluminous infrared galaxies. We found that at least 63% of GRB hosts have SFR < 100 M yr^-1 and at most 8% can have SFR > 500 M yr^-1. For the undetected hosts the mean radio flux (<35 μJy 3σ) corresponds to an average SFR < 15 M yr^-1. Moreover, ≳ 88% of the z ≲ 1 GRB hosts have ultraviolet dust attenuation A _UV < 6.7 mag (visual attenuation A_V < 3 mag). Hence, we did not find evidence for large dust obscuration in a majority of GRB hosts. Finally, we found that the distributions of SFRs and A _UV of GRB hosts are consistent with those of Lyman break galaxies, Hα emitters at similar redshifts, and of galaxies from cosmological simulations. The similarity of the GRB population with other star-forming galaxies is consistent with the hypothesis that GRBs, a least at z ≲ 1, trace a large fraction of all star formation, and are therefore less biased indicators than once thought.