The Critical Role of Dark Matter Halos in Driving Star Formation*

Abstract Understanding the physical mechanisms that drive star formation is crucial for advancing our knowledge of galaxy evolution. We explore the interrelationships between key galaxy properties associated with star formation, with a particular focus on the impact of dark matter (DM) halos. Given the sensitivity of atomic hydrogen (H i ) to external processes, we concentrate exclusively on central spiral galaxies. We find that the molecular-to-atomic gas mass ratio ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>M</mml:mi> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:msub> </mml:math> / M H i ) strongly depends on stellar mass and specific star formation rate (sSFR). In the star formation efficiency (SFE)–sSFR plane, most galaxies fall below the H 2 fundamental formation relation, with SFE H i being consistently lower than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>SFE</mml:mi> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:msub> </mml:math> . Using the improved halo masses derived by D. Zhao et al., for star-forming galaxies, both SFE H i and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>M</mml:mi> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:msub> </mml:math> / M H i increase rapidly and monotonically with halo mass, indicating a higher efficiency in converting H i to H 2 in more massive halos. This trend ultimately leads to the unsustainable state where SFE H i exceeds <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>SFE</mml:mi> <mml:msub> <mml:mi mathvariant="normal">H</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:msub> </mml:math> at halo mass around 10 12 M ⊙ . For halos with masses exceeding 10 12 M ⊙ , galaxies predominantly experience quenching. We propose a plausible evolutionary scenario in which the growth of halo mass regulates the conversion of H i to H 2 , star formation, and the eventual quenching of galaxies. The disk size, primarily regulated by the mass, spin and concentration of the DM halo, also significantly influences H i to H 2 conversion and star formation. These findings underscore the critical role of DM halos as a global regulator of galaxy-wide star formation, a key factor that has been largely under-appreciated in previous studies.