Extinction of gamma-ray burst afterglows as a diagnostic of the location of cosmic star formation

ABSTRA C T The properties of gamma-ray bursts (GRBs) and their afterglows are used to investigate the location of star formation activity through the history of the Universe. This approach is motivated by the following: (i) GRBs are thought to be associated with the deaths of massive stars and so the GRB rate ought to follow the formation rate of massive stars; (ii) GRBs are the last phase of the evolution of these stars, which do not live long enough to travel far from their place of birth, and so GRBs are located where the stars formed; and (iii) GRB afterglows occur over both X-ray and optical wavelengths, and so the differential effects of dust extinction between the two wavebands can reveal whether or not large amounts of dust are present in galaxies hosting GRBs. Recent evidence suggests that a significant fraction of stars in the Universe formed in galaxies that are bright at rest-frame submillimetre (submm) and infrared wavelengths rather than at ultraviolet wavelengths; we estimate about three quarters of the star formation in the Universe occurred in the submm-bright mode. High-redshift submm-selected galaxies are thought to have properties similar to local ultraluminous infrared galaxies (ULIGs) such as Arp 220, based on the concordance between their luminosities and spectral energy distributions. If this is the case, then GRBs in submm-bright galaxies should have their optical afterglows extinguished by internal dust absorption, but only very few should have their 2 ‐ 10 keV X-ray afterglows obscured. The value that we quote of three quarters is marginally consistent with observations of GRBs: 60 ^ 15 per cent of GRBs have no detected optical afterglow, whereas almost all have an X-ray afterglow. A more definitive statement could be made with observations of soft X-ray afterglows O0:5 ‐ 2 keVU, in which extinction should be severe for GRBs located in submm-bright galaxies with gas densities similar to those in local ULIGs. If the X-ray afterglows disappear at soft X-ray wavelengths in a large number of GRBs, then this would provide strong evidence that much of the star-formation in the Universe is heavily obscured. Far-infrared and submm follow up studies of the hosts of GRB would reveal this population. We expect about 20 per cent of GRB hosts to be detectable using the SCUBA camera at the James Clerk Maxwell Telescope (JCMT) after several hours of integration.