Relativistic supernova 2009bb exploded close to an atomic gas cloud
  The potential similarity of the powering mechanisms of relativistic SNe and
GRBs allowed us to make a prediction that relativistic SNe are born in
environments similar to those of GRBs, that is, ones which are rich in atomic
gas. Here we embark on testing this hypothesis by analysing the properties of
the host galaxy NGC 3278 of the relativistic SN 2009bb. This is the first time
the atomic gas properties of a relativistic SN host are provided and the first
time resolved 21 cm-hydrogen-line (HI) information is provided for a host of an
SN of any type in the context of the SN position. We obtained radio
observations with ATCA covering the HI line, and optical integral field unit
spectroscopy observations with MUSE. The atomic gas distribution of NGC 3278 is
not centred on the optical galaxy centre, but instead around a third of atomic
gas resides in the region close to the SN position. This galaxy has a few times
lower atomic and molecular gas masses than predicted from its SFR. SN 2009bb
exploded close to the region with the highest SFR density and the lowest age
(~5.5 Myr, corresponding to the initial mass of the progenitor star ~36 Mo). As
for GRB hosts, the gas properties of NGC 3278 are consistent with a recent
inflow of gas from the intergalactic medium, which explains the concentration
of atomic gas close to the SN position and the enhanced SFR. Super-solar
metallicity at the position of the SN (unlike for most GRBs) may mean that
relativistic explosions signal a recent inflow of gas (and subsequent star
formation), and their type (GRBs or SNe) is determined either i) by the
metallicity of the inflowing gas, so that metal-poor gas results in a GRB
explosion and metal-rich gas results in a relativistic SN explosion without an
accompanying GRB, or ii) by the efficiency of gas mixing, or iii) by the type
of the galaxy.
