4.5. Gamma-ray burst (GRB) host galaxies

An interesting new development is the idea that if GRBs are likely to be associated with the deaths of massive stars, then the rate of GRBs and the global high-mass star-formation rate should be linked (Krumholz et al., 1998). By searching for submm emission from the directions of GRBs, it may be possible to test whether either submm or UV-bright galaxies are the dominant population to host high-mass stars, and what fraction of the submm galaxies are powered by non-GRB-generating AGN (Blain and Natarajan, 2000). About 10% of GRBs are expected to be in hosts with 850-µm flux densities greater than 5 mJy (Ramirez-Ruiz et al., 2002), if submm galaxies dominate the cosmic star-formation rate and are not typically powered by AGN. Two excellent candidates for submm-loud host galaxies of GRBs are now known (Berger et al., 2001; Frail et al., 2002), the first based on deep VLA radio images, the second on direct SCUBA and MAMBO mm/submm observations. Most GRB hosts appear to be associated with R 25 optical galaxies (Bloom et al., 2002), which could also be typical of the submm galaxy population. It is difficult to detect GRB host galaxies without hitting the confusion limit using SCUBA, but attempts are underway. As a byproduct of surveys for submm afterglow emission, Smith et al. (1999, 2002) imposed limits to the submm host galaxy emission from the direction of 12 GRBs. An ongoing JCMT program (Nial Tanvir et al.) is searching directly for submm emission from the host galaxies of accurately located GRBs (Barnard et al., 2002). Detecting and resolving the submm emission from GRB host galaxies should ultimately be very simple using ALMA, requiring observations of only a few minutes minutes per burst.