Neutral Atomic and Molecular Clouds and Star Formation in the Outer Carina Arm

Abstract

We present a comprehensive investigation of H i (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 H i clouds and MCs based on the (l, v LSR) locations of the Carina arm. We analyzed 26 H i clouds and 48 MCs. Most of the identified H i clouds are superclouds, with masses exceeding 106 M ⊙. We find that 15 of these superclouds have associated MC(s) with M H I ≳ 106 M ⊙ and Σ H I + H 2 ≳ 50 M ⊙ pc−2. Our virial equilibrium analysis suggests that these CO-bright H i clouds are gravitationally bound or marginally bound. We report an anticorrelation between molecular mass fractions and Galactocentric distances, and a correlation with total gas surface densities. Nine CO-bright H i superclouds are associated with H ii regions, indicating ongoing star formation. We confirm the regular spacing of H i 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 H ii regions and MCs, with some offsets between MCs and local H i column density peaks. Our study reveals that, in the context of H i superclouds, the star formation rate surface density is independent of H i 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.