Do stars still form in molecular gas within CO-dark dwarf galaxies?

TL;DR

This study uses ultra high-resolution simulations to show that stars in CO-dark dwarf galaxies form in small molecular cores, despite large-scale observations indicating a lack of observable CO emission.

Contribution

The paper demonstrates that star formation occurs in molecular gas within CO-dark dwarf galaxies, revealing the small-scale molecular environment despite large-scale CO non-detections.

Findings

Stars form in small molecular cores in CO-dark dwarf galaxies.

High dark gas fractions coexist with ongoing star formation.

Star-forming regions are smaller than observational beam sizes.

Abstract

In the Milky Way and other main-sequence galaxies, stars form exclusively in molecular gas, which is traced by CO emission. However, low metallicity dwarf galaxies are often `CO-dark' in the sense that CO emission is not observable even at the high resolution and sensitivities of modern observing facilities. In this work we use ultra high-resolution simulations of four low-metalicity dwarf galaxies (which resolve star formation down to the scale of star-forming cores, 0.01 pc) combined with a time-dependent treatment of the chemistry of the interstellar medium, to investigate the star formation environment in this previously hidden regime. By generating synthetic observations of our models we show that the galaxies have high to extremely high dark gas fractions (0.13 to 1.00 dependent on beam size and conditions), yet despite this form stars. However, when examined on smaller scales, we find that the stars still form in regions dominated by molecular gas, it is simply that these are far smaller than the scale of the beam (1.5"). Thus, while stars in CO-dark dwarf galaxies form in small molecular cores like larger galaxies, their cloud-scale environment is very different.