Pressure and Star Formation in LITTLE THINGS Dwarf Irregular Galaxies

TL;DR

This study investigates the relationship between pressure, gas densities, and star formation in dwarf irregular galaxies, revealing that CO traces dense gas and star formation correlates with pressure similarly to spiral galaxies.

Contribution

It demonstrates that star formation and gas pressure correlations in dwarf irregular galaxies are resolution-independent and that CO effectively traces dense, self-gravitating gas at low metallicities.

Findings

Star formation correlates with midplane pressure in dIrrs.

CO traces dense gas and is self-gravitating in low-metallicity environments.

Correlation results are consistent across different spatial resolutions.

Abstract

The surface densities of star formation, Sigma_SFR, in 24 dwarf irregular (dIrr) galaxies from the LITTLE THINGS survey are combined with gas surface densities and midplane pressures to examine the correlations found previously for spiral galaxies. The pressure is the weight of the disk inside the gas layer, including gas, stars, and dark matter, which usually dominates disk gravity in dIrrs. We compare the results to the outer part of M33, which has similar local properties but a slightly higher metallicity, enabling the detection of CO. All the data are convolved to the HI beam, but to study the effects of resolution, the galaxies are examined first with average radial profiles, and then with maps having 1.5" pixels and 244 pc pixels. The correlations are found to be independent of resolution from 24 pc to 424 pc. The average ratio of molecular to atomic surface density is estimated to be 0.23+-0.1, from the H_2 surface density in M33 compared to the HI surface density at the same Sigma_SFR in the dIrrs. With this ratio, the average star formation rate per molecule is about the same for all the dIrrs, and a factor of 2 less than the rate in M33. The pressure in dIrrs is so low that CO is essentially a dense gas tracer, with the same surface density threshold at the low metallicities of dIrrs as HCN has in spiral galaxies. As a result, CO regions in dIrrs should be strongly self-gravitating.