Context :

Aims : 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 ( H i ) information is provided for a host of an SN of any type in the context of the SN position .

Methods : We obtained radio observations with ATCA covering the H i line , and optical integral field unit spectroscopy observations with MUSE .
Moreover , we analysed archival carbon monoxide ( CO ) and multi-wavelength data for this galaxy .

Results : 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 star formation rate ( SFR ) .
Its specific star formation rate ( \mbox { sSFR } \equiv \mbox { SFR } / \mbox { $M _ { * } $ } ) is approximately two to three times higher than the main-sequence value .
SN 2009bb exploded close to the region with the highest SFR density and the lowest age ( \sim 5.5 Myr ) .
Assuming this timescale was the lifetime of the progenitor star , its initial mass would have been close to \sim 36 \mbox { $M _ { \odot } $ } .

Conclusions : 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 .