Ly-α and ultraviolet emission from high-redshift gamma-ray burst hosts: to what extent do gamma-ray bursts trace star formation?
Östlin, G.; Jóhannesson, G.; Watson, D.; Fynbo, J. P. U.; Gorosabel, J.; Hjorth, J.; Jakobsson, P.; Møller, P.; Björnsson, G.; Thomsen, B.; Jensen, B. L.; Gudmundsson, E. H.
Denmark, Iceland, Germany, Sweden, Spain, United States
Abstract
We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts (GRBs) 030226 (z= 1.986), 021004 (z= 2.335) and 020124 (z= 3.198). We find that the host galaxy of GRB 021004 is an extended (around 8 kpc) strong Lyα emitter with a rest-frame equivalent width (EW) of 68+12-11Å, and a star formation rate of 10.6 +/- 2.0 Msolar yr-1. We do not detect the hosts of GRB 030226 and GRB 020124, but the upper limits on their Lyα fluxes do not rule out large rest-frame EWs. In the fields of GRB 021004 and GRB 030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyα-emitting galaxies in the environment of the host galaxies. We have also compiled a list of all z>~ 2 GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the z= 3 luminosity function (LF) under this assumption results in a characteristic luminosity of R*= 24.6 and a faint-end slope of α=-1.55, consistent with the LF measured for Lyman-break galaxies.