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

Abstract

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.