1. Gamma-ray Bursts in Starburst Galaxies Introduction: At least some long duration GRBs are caused by exploding stars, which could be reflected by colours of their their host galaxies. We investigate if they have large star formation rates, perhaps even larger than for field galaxies. The purpose is to investigate if a special environment is required to form a GRB. Lise Christensen (AIP, Germany), Jens Hjorth (NBI-AO, Denmark), and Javier Gorosabel (IAA, Spain) SED fitting: We study the broad band spectral energy distribution (SED) by comparing the broad band magnitudes with spectra of template galaxies. We derive for each host following properties: Type of galaxy Star formation rate based on rest frame UV flux Intrinsic extinction Age of the dominant stellar population in the hosts Fig1: GRB 981226 host observed with STIS Fig. 2: One example of a SED fit of the GRB 981226 host. Distributions of specific SFRs Properties of GRB hosts from SED fits: All GRB hosts are starburst galaxies All GRB hosts are young (< 0.3 Gyr) GRB hosts do not have extreme SFRs (0.5 - 13 M s yr -1 ) based on their UV flux Specific SFRs between 5 and 12 M s yr -1 (L/L*) -1 Moderate to low extinction (0 < A v < 2) Conclusions and outlook: All GRB hosts show similar properties in terms of age, extinctions, and specific star-formation rates Hosts of ‘dark bursts’ are no different than the ones which have detected optical afterglows GRB hosts are younger than the average field galaxy, but have similar ages as field starburst galaxies GRB hosts have larger specific SFRs than field galaxies in general which suggests efficient star formation The specific SFR distribution is similar to the youngest starburst galaxies Hence, it is clear that young starburst galaxies make GRBs. We do not find any significant different properties which distinguish them from other young field starburst galaxies. The investigation and results outlined here suffer from small number statistics. A larger, uniformly selected sample of GRBs will soon be found by the Swift satellite. With multi band follow-up observations of the hosts, we can obtain an unbiased sample and investigate, with a better statistical significance, if GRB hosts are different from young field starburst galaxies. Fig 3.: Comparison of specific SFRs of 10 GRB hosts with those of field galaxies in the Hubble Deep Fields. GRB hosts have spectroscopic redshifts; field galaxies have photometric redshifts. (Credits: Holland et al.) Fig.4: Cumulative distribution of specific SFRs for GRB hosts and young field galaxies with ages less than 0.1 Gyr. Only when comparing the specific SFRs with the youngest starbursts do we se a similar distribution. GRB host sample selection: Our current sample consists of > 10 GRB hosts with redshifts: 0.1 < z < 2.0 and magnitudes 20 < R < 25.5. These hosts are observed in 5 to 9 bands from UV to near-IR. Specific star formation rates Comparison with field galaxies We estimate for field galaxies from the Hubble Deep Fields the same properties as for the GRB hosts, i.e. galaxy type, redshift, age, extinction, and SFRs. GRB hosts and field galaxies have similar redshift distributions Age distribution is similar as for field starburst galaxies Intrinsic extinctions are similar as for field starburst galaxies GRB hosts have lager than average specific SFRs Contact: lchristensen@aip.de lchristensen@aip.de Field galaxies GRB hosts