A submillimeter and radio survey of gamma-ray burst host galaxies: A glimpse into the future of star formation studies

We present the first comprehensive search for submillimeter and radio emission from the host galaxies of 20 well-localized gamma-ray bursts (GRBs). With the exception of a single source, all observations were undertaken months to years after the GRB explosions to ensure negligible contamination from the after-glows. We detect the host galaxy of GRB 000418 in both the submillimeter and radio, and the host galaxy of GRB 000210 in only the submillimeter. These observations, in conjunction with the previous detections of the host galaxies of GRB 980703 and GRB 010222, indicate that about 20% of GRB host galaxies are ultra-luminous (L > 10(12) L-circle dot) and have star formation rates of about 500 M-circle dot yr(-1). As an ensemble, the non-detected hosts have a star formation rate of about 100 M-circle dot yr(-1) (5 sigma) based on their radio emission. This, in conjunction with an average luminosity for the entire sample that is approximately 20% fainter than the local starburst galaxy Arp 220, indicates that GRB hosts probe a more representative population of star-forming galaxies than those uncovered in blank submillimeter and radio surveys. The detected and ensemble star formation rates exceed the values determined from various optical estimators by an order of magnitude, indicating significant dust obscuration. In the same vein, the ratio of bolometric dust luminosity to UV luminosity for the hosts detected in the submillimeter and radio bands ranges from about similar to30 to 500 and follows the known trend of increasing obscuration with increasing bolometric luminosity. We also show that the GRB host sample as a whole, and the submillimeter- and radio-detected hosts individually, have significantly bluer R-K colors as compared with galaxies selected in the submillimeter and radio in the same redshift range. This possibly indicates that the stellar populations in the GRB hosts are on average younger, supporting the massive stellar progenitor scenario for GRBs, but it is also possible that GRB hosts are on average less dusty. For the nondetected GRB hosts, the difference in R-K color may also be a manifestation of their more representative bolometric luminosities relative to the highly luminous submillimeter- and radio-selected galaxies. Beyond the specific results presented in this paper, the submillimeter and radio observations serve as an observational proof-of-concept in anticipation of the upcoming launch of the Swift GRB mission and SIRTF. These new facilities will possibly bring GRB host galaxies into the forefront of star formation studies.