Relationships between the Stellar, Gaseous, and Star Formation Disks in LITTLE THINGS Dwarf Irregular Galaxies: Indirect Evidence for Substantial Fractions of Dark Molecular Gas

TL;DR

This study investigates the relationships between stellar, gaseous, and star-forming disks in dwarf irregular galaxies, providing indirect evidence for significant fractions of dark molecular gas through analysis of gas and star distributions.

Contribution

It introduces a model linking molecular gas surface density to total gas density, explaining the observed disk profiles and estimating molecular fractions in dwarf galaxies.

Findings

HI profiles are flatter in the center and steeper in the outskirts compared to stellar profiles.

Molecular gas likely forms in the galaxy centers, correlating with star formation regions.

Average molecular fraction inside 3 disk radii is approximately 23%.

Abstract

The stellar, gaseous and young stellar disks in the LITTLE THINGS sample of nearby dIrrs are fitted with functions to search for correlations between the parameters. We find that the HI radial profiles are generally flatter in the center and fall faster in the outer regions than the V-band profiles, while young stars are more centrally concentrated, especially if the HI is more centrally flat. This pattern suggests that the HI is turning into molecules in the center and the molecular clouds are forming stars and FUV. A model that assumes the molecular surface density is proportional to the total gas surface density to a power of 1.5 or 2, in analogy with the Kennicutt-Schmidt relation, reproduces the relationship between the ratio of the visible to the HI scale length and the HI Sersic index. The molecular fraction is estimated as a function of radius for each galaxy by converting the FUV to a molecular surface density using conventional calibrations. The average molecular fraction inside 3R_D is 23+/-17%. However, the break in the stellar surface brightness profile has no unified tracer related to star formation.