Neutral atomic and molecular clouds and star formation in the outer Carina arm

TL;DR

This study investigates the properties and star formation activity of atomic and molecular clouds in the outer Carina arm, revealing the importance of molecular gas in star formation and the regular spacing of superclouds due to gravitational instabilities.

Contribution

It provides a detailed analysis of HI and molecular clouds in the Carina arm, highlighting their gravitational binding, spatial distribution, and relation to star formation, which advances understanding of galactic structure and star formation mechanisms.

Findings

Most HI clouds are superclouds with masses over 10^6 solar masses.

Star formation is linked to molecular gas surface density, not total gas surface density.

HI superclouds are regularly spaced along the spiral arm, likely due to gravitational instabilities.

Abstract

We present a comprehensive investigation of HI (super)clouds, molecular clouds (MCs), and star formation in the Carina spiral arm of the outer Galaxy. Utilizing HI4PI and CfA CO survey data, we identify HI clouds and MCs based on the ($l$, ${v_\mathrm{LSR}}$) locations of the Carina arm. We analyzed 26 HI clouds and 48 MCs. Most of the identified HI clouds are superclouds, with masses exceeding $10^6~{\mathrm{M_\odot}}$. We find that 15 of these superclouds have associated MC(s) with ${M_\mathrm{HI}} \gtrsim 10^6~{\mathrm{M_\odot}}$ and ${\Sigma_\mathrm{HI+H_2}} \gtrsim$ 50 ${\mathrm{M_\odot}} \rm pc^{-2}$. Our virial equilibrium analysis suggests that these CO-bright HI clouds are gravitationally bound or marginally bound. We report an anti-correlation between molecular mass fractions and Galactocentric distances, and a correlation with total gas surface densities. Nine CO-bright HI superclouds are associated with HII regions, indicating ongoing star formation. We confirm the regular spacing of HI superclouds along the spiral arm, which is likely due to some underlying physical process, such as gravitational instabilities. We observe a strong spatial correlation between HII regions and MCs, with some offsets between MCs and local HI column density peaks. Our study reveals that in the context of HI superclouds, the star formation rate surface density is independent of HI and total gas surface densities but positively correlates with molecular gas surface density. This finding is consistent with both extragalactic studies of the resolved Kennicutt-Schmidt relation and local giant molecular clouds study of Lada et al. (2013), emphasizing the crucial role of molecular gas in regulating star formation processes.