The mystery of the closest known GRB host

The famous GRB 980425, the closest known GRB, is one of the first GRBs detected in association with a supernova, SN1998bw (Galama et al. 1998). It occurred in a dwarf galaxy at z = 0:0087, 37 Mpc away, providing a unique opportunity to study the close environment of a GRB (Fynbo et al. 2000). Detailed studies of the host galaxy showed nothing particular, but an unusually bright star forming region, about 800 pc from the GRB location, dominating the emission of the host galaxy in several bands (Hammer et al. 2006; Le Floc’h et al. 2006, 2012). The multi-wavelength investigations over two decades failed to provide any explanation for the presence of such a rare region in a typical dwarf galaxy.

We carried out the spatially resolved observations of HI 21 cm emission line from this GRB host, the first study of its kind, and found the gas disk to be disturbed (Arabsalmani et al. 2015b). We detected an HI knot about 12 kpc from the main body of the galaxy, towards which much of the kinematically disturbed gas is extended. With further studies we identified a galaxy associated with this HI knot, and visible in Hα, 4.5 um, and optical images (Arabsalmani et al. 2019a). Our HI 21 cm emission line observations also revealed the presence of a high column density HI ring around the galaxy centre, passing through the actively star forming regions and the GRB location. This ring, with large velocity gradients, is an ideal site for formation of massive giant molecular clouds (GMCs) and hence young and massive clusters, the birth-place of the most massive stars (see also Roychowdhury, Arabsalmani & Kanekar 2019).

We furthermore performed ALMA observations of this host galaxy to study the individual GMCs and detected massive GMCs within the HI ring and in the vicinity of the GRB (Arabsalmani et al. 2020). The molecular gas depletion times of these GMCs show a variation of ~2 dex, demonstrating the presence of starburst modes of star formation on local scales in the galaxy. This is while the global gas and star-formation properties of the host do not categorize it as a starburst galaxy. Through state-of-the-art simulations of dwarf-dwarf mergers, we showed that the collision between the GRB host and its companion has led to the formation of the gas ring and the enhancement of star formation within the ring in which the GRB was formed. The detailed studies of the distribution and kinematics of neutral gas in this GRB host galaxy unveiled that the host galaxy is a collisional-ring galaxy and resolved the puzzle of the formation of the bright region and the GRB in an otherwise unexceptional galaxy. This study demonstrated the important role that interactions can play in the formation of massive star explosions.