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
$\rm0.004\,M_{\odot}\, yr^{-1}$, a gas mass of $10^{7.1\pm0.1} M_{\odot}$, and
a stellar mass of $10^{7.0\pm0.3} M_{\odot}$. 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.
