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 October 2006. 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
Mo 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 Mo yr^-1 and at most 8% can have SFR > 500 Mo yr^-1. For the
undetected hosts the mean radio flux (<35 uJy 3sigma) corresponds to an average
SFR < 15 Mo 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, Halpha 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.
