The host galaxy of GRB 980425 / SN1998bw: a collisional ring galaxy

We report Giant Metrewave Radio Telescope (GMRT), Very Large Telescope (VLT), and Spitzer Space Telescope observations of ESO 184-G82, the host galaxy of GRB 980425/SN 1998bw, that yield evidence of a companion dwarf galaxy at a projected distance of 13 kpc. The companion, hereafter GALJ193510-524947, is a gas-rich, star-forming galaxy with a star formation rate of 0.004 M-circle dot yr(-1), a gas mass of 10(7.1 +/- 0.1) M-circle dot, and a stellar mass of 10(7.0 +/- 0.3) M-circle dot. The interaction between ESO 184-G82 and GALJ193510-524947 is evident from the extended gaseous structure between the two galaxies in the GMRT HI 21 cm map. We find a ring of high column density HI gas, passing through the actively star-forming regions of ESO 184-G82 and the GRB location. This ring lends support to the picture in which ESO 184-G82 is interacting with GALJ193510-524947. The massive stars in GALJ193510-524947 have similar ages to those in star-forming regions in ESO 184-G82, also suggesting that the interaction may have triggered star formation in both galaxies. The gas and star formation properties of ESO 184-G82 favour a head-on collision with GALJ193510-524947 rather than a classical tidal interaction. We perform state-of-the-art simulations of dwarf-dwarf mergers and confirm that the observed properties of ESO 184-G82 can be reproduced by collision with a small companion galaxy. This is a very clear case of interaction in a gamma-ray burst host galaxy and of interaction-driven star formation giving rise to a gamma-ray burst in a dense environment.