2023ApJ...955...59P

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 10⁶M_⊙. We find that 15 of these superclouds have associated MC(s) with M_H I_ ≳ 10⁶M_⊙ and ${{\rm{\Sigma }}}_{{\rm{H}}\,{\rm\small{I}}+{{\rm{H}}}_{2}}\gtrsim $ 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.