High star formation rates in turbulent atomic-dominated gas in the interacting galaxies IC 2163 and NGC 2207

TL;DR

This study reveals that in interacting galaxies IC 2163 and NGC 2207, high star formation rates occur in turbulent, atomic-dominated gas regions, especially in outer galaxy parts with high velocity dispersion, challenging traditional molecular gas-centric views.

Contribution

It demonstrates that significant star formation can occur in HI-dominated regions with high turbulence, supported by combined multi-wavelength observations and theoretical turbulence models.

Findings

HI-dominated regions have excess SFRs relative to molecular gas

High turbulence correlates with atomic-dominated star formation

Star-forming clouds have envelopes at lower densities and cores at higher densities

Abstract

CO observations of the interacting galaxies IC 2163 and NGC 2207 are combined with HI, Halpha and 24 microns to study the star formation rate (SFR) surface density as a function of the gas surface density. More than half of the high SFR regions are HI dominated. When compared to other galaxies, these HI-dominated regions have excess SFRs relative to their molecular gas surface densities but normal SFRs relative to their total gas surface densities. The HI-dominated regions are mostly located in the outer part of NGC 2207, where the HI velocity dispersion is high, 40 - 50 km/s. We suggest that the star-forming clouds in these regions have envelopes at lower densities than normal, making them predominantly atomic, and cores at higher densities than normal because of the high turbulent Mach numbers. This is consistent with theoretical predictions of a flattening in the density probability distribution function for compressive, high Mach number turbulence.