On the origin of the neutral hydrogen supershells: the ionized progenitors and the limitations of the multiple supernovae hypothesis

TL;DR

This study investigates whether ionized shells from giant HII regions can evolve into large neutral hydrogen supershells, concluding that multiple supernovae alone are insufficient for the largest HI shells, implying additional energy sources are needed.

Contribution

The paper challenges previous assumptions by showing that multiple supernovae cannot solely explain the formation of the largest HI supershells, highlighting the need for alternative energy sources.

Findings

HII shells are unlikely progenitors of the largest HI shells

Multiple supernovae do not match observed parameters of large HI shells

Additional energy sources are necessary for large HI shell formation

Abstract

Here we address the question whether the ionized shells associated with giant HII regions can be progenitors of the larger HI shell-like objects found in the Milky Way and other spiral and dwarf irregular galaxies. We use for our analysis a sample of 12 HII shells presented recently by Rela\~no et al. (2005, 2007). We calculate the evolutionary tracks that these shells would have if their expansion is driven by multiple supernovae explosions from the parental stellar clusters. We find, contrary to Rela\~no et al. (2007), that the evolutionary tracks of their sample HII shells are inconsistent with the observed parameters of the largest and most massive neutral hydrogen supershells. We conclude that HII shells found inside giant HII regions may represent the progenitors of small or intermediate HI shells, however they cannot evolve into the largest HI objects unless, aside from the multiple supernovae explosions, an additional energy source contributes to their expansion.