On the nature of nearby GRB/SN host galaxies

Abstract

We present and discuss optical diagnostics of the low redshift (z<0.2) galaxies that are known to have hosted supernovae associated with gamma-ray bursts (GRBs). The three galaxies are all actively starforming sub-luminous (L<L*) galaxies with relatively low metallicities (Z<Zsun). We find no evidence for substantial internal extinction within any of the galaxies. We derive star formation rates (SFR) based on H-alpha luminosities, as well as specific star formation rates (SFFR, star formation rate per unit luminosity). For GRB 980425 (SN 1998bw) we use photometry of the supernova environment to estimate the mass of the progentitor to > 30 Msun. These three host galaxies have global properties (luminosities, SFR, SSFR, metallicity, colour, reddening) that resemble those of more distant GRB host galaxies. We also compare the host galaxies with a sample of Blue Compact Galaxies (BCGs) in the local universe, and show that these samples have similar properties.