Structure of neutral gas: the clue to the formation of massive star explosions

The typical low mass and metallicity of SLSN hosts has been interpreted as implying that low metallicity is the primary requirement for SLSN formation (e.g., Schulze et al. 2018). This is called into question by the detection of a number of SLSNe in metal-rich galaxies, in particular in the central regions of large galaxies (Lunnan et al. 2015, Nicholl et al. 2017). High density of star formation is the characteristic of over-dense regions in both metalrich and metal-poor galaxies. This seems to be the likely driver for formation of SLSN progenitors. Investigating this hypothesis requires resolved studies of molecular gas, the fuel of star formation, in the immediate vicinity of SLSNe and in their host galaxies.

In the first study of its kind, we studied the structure of molecular gas in a metal-rich and massive host galaxy of a SLSN at z = 0:04 using ALMA observations (Arabsalmani et al. 2019b). SLSN PTF10ptz is located in the central regions of its barred, spiral host galaxy. We found the GMCs in the vicinity of the SLSN to have very high surface densities, comparable with those of GMCs in the Antennae galaxy and starburst regions in nearby galaxies. We found the distribution and kinematics of gas to be consistent with two gas lanes running from the tips of the bar toward the galaxy center. These lanes encounter a gaseous structure in the central 1 kpc of the galaxy, plausibly associated with an inner Lindblad resonance ring. The position of SLSN PTF10tpz is very close to the the intersection regions of the gas lanes and the inner structure (see Fig. 3). These findings suggest that the interaction between the large-scale gas flows in the bar and the gas at the inner resonance has led to the assembly of giant molecular complexes, and consequently to the formation of massive star clusters. This is supported by the short depletion time and high surface densities of molecular gas that we measured in the central regions of the galaxy. This study revealed in situ formation of massive star clusters (in one of which the SLSN progenitor was formed) due to the internal dynamics of the host galaxy and without any external contribution such as interaction or merger. This study demonstrated the important role that the kiloparsec-scale (hydro)dynamics of galaxies can play in the formation of massive stars and hence massive star explosions.