Small Bites: Star formation recipes in extreme dwarfs

TL;DR

This study investigates star formation in tiny dwarf galaxies, revealing stochastic gas-star formation relations, lower efficiencies than spirals, and a correlation between gas density and the ratio of H-alpha to FUV emission.

Contribution

It provides new insights into star formation processes in extreme dwarf galaxies, highlighting stochasticity and density-dependent star formation efficiency.

Findings

Gas with Sigma_HI >~ 10 M_sun/pc^2 always has star formation

Star formation efficiency is at least twice lower than in spiral regions

H-alpha to FUV ratio increases with gas density

Abstract

We study the relationship between the gas column density (Sigma_HI) and the star formation rate surface density (Sigma_SFR) for a sample of extremely small (M_B ~ -13, Delta V_50 ~ 30 km/s) dwarf irregular galaxies. We find a clear stochasticity in the relation between the gas column density and star formation. All gas with Sigma_HI >~ 10 M_sun/pc^2 has some ongoing star formation, but the fraction of gas with ongoing star formation decreases as the gas column density decreases, and falls to about 50% at Sigma_HI ~ 3 M_sun/pc^2. Further, even for the most dense gas, the star formation efficiency is at least a factor of ~ 2 smaller than typical of star forming regions in spirals. We also find that the ratio of H-alpha emission to FUV emission increases with increasing gas column density. This is unlikely to be due to increasing dust extinction because the required dust to gas ratios are too high. We suggest instead that this correlation arises because massive (i.e. H-alpha producing) stars are formed preferentially in regions with high gas density.