Local starburst conditions and formation of GRB 980425 / SN 1998bw
  within a collisional ring
  We present the first spatially resolved study of molecular gas in the
vicinity of a Gamma Ray Burst, using CO(2-1) emission line observations with
the Atacama Large Millimetre Array (ALMA) at ~50 pc scales. The host galaxy of
GRB 980425 contains a ring of high column density HI gas which is likely to
have formed due to a collision between the GRB host and its companion galaxy,
within which the GRB is located. We detect eleven molecular gas clumps in the
galaxy, seven of which are within the gas ring. The clump closest to the GRB
position is at a projected separation of ~280 pc. Although it is plausible that
the GRB progenitor was ejected from clusters formed in this clump, we argue
that the in situ formation of the GRB progenitor is the most likely scenario.
We measure the molecular gas masses of the clumps and find them to be
sufficient for forming massive star clusters. The molecular gas depletion times
of the clumps show a variation of ~2 dex, comparable with the large variation
in depletion times found in starburst galaxies in the nearby Universe. This
demonstrates the presence of starburst modes of star formation on local scales
in the galaxy, even while the galaxy as a whole cannot be categorised as a
starburst based on its global properties. Our findings suggest that the
progenitor of GRB 9802425 was originated in a young massive star cluster formed
in the starburst mode of star formation.
