The disk averaged star formation relation for Local Volume dwarf galaxies

TL;DR

This study develops a method to estimate atomic gas surface density from global HI parameters in dwarf galaxies, confirming a strong correlation with star formation rate and providing insights into the molecular fraction and pressure balance in these low-mass systems.

Contribution

The paper introduces an empirical approach to estimate atomic gas surface density from global HI data, enabling analysis of star formation relations in dwarf galaxies without resolved HI maps.

Findings

Atomic gas surface density correlates with star formation rate surface density, offset from the Kennicutt-Schmidt relation.

The molecular-to-atomic gas mass fraction in dwarf galaxies is estimated to be 5-15%.

Thermal pressure models better fit the data but do not fully explain the observed relationships.

Abstract

Spatially resolved HI studies of dwarf galaxies have provided a wealth of precision data. However these high-quality, resolved observations are only possible for handful of dwarf galaxies in the Local Volume. Future HI surveys are unlikely to improve the current situation. We therefore explore a method for estimating the surface density of the atomic gas from global HI parameters, which are conversely widely available. We perform empirical tests using galaxies with resolved HI maps, and find that our approximation produces values for the surface density of atomic hydrogen within typically 0.5dex of the true value. We apply this method to a sample of 147 galaxies drawn from modern near-infrared stellar photometric surveys. With this sample we confirm a strict correlation between the atomic gas surface density and the star formation rate surface density, that is vertically offset from the Kennicutt-Schmidt relation by a factor of 10-30, and significantly steeper than the classical N=1.4 of Kennicutt (1998). We further infer the molecular fraction in the sample of low surface brightness, predominantly dwarf galaxies by assuming that the star formation relationship with molecular gas observed for spiral galaxies also holds in these galaxies, finding a molecular-to-atomic gas mass fraction within the range of 5-15%. Comparison of the data to available models shows that a model in which the thermal pressure balances the vertical gravitational field captures better the shape of the $\Sigma_{\rm SFR}-\Sigma_{\rm gas}$ relationship. However, such models fail to reproduce the data completely, suggesting that thermal pressure plays an important role in the disks of dwarf galaxies.