Star Formation Efficiency in Nearby Galaxies Revealed with a New CO-to-H₂ Conversion Factor Prescription

Abstract

Determining how the galactic environment, especially the high gas densities and complex dynamics in bar-fed galaxy centers, alters the star formation efficiency (SFE) of molecular gas is critical to understanding galaxy evolution. However, these same physical or dynamical effects also alter the emissivity properties of CO, leading to variations in the CO-to-H2 conversion factor (α CO) that impact the assessment of the gas column densities and thus of the SFE. To address such issues, we investigate the dependence of α CO on the local CO velocity dispersion at 150 pc scales using a new set of dust-based α CO measurements and propose a new α CO prescription that accounts for CO emissivity variations across galaxies. Based on this prescription, we estimate the SFE in a sample of 65 galaxies from the PHANGS–Atacama Large Millimeter/submillimeter Array survey. We find increasing SFE toward high-surface-density regions like galaxy centers, while using a constant or metallicity-based α CO results in a more homogeneous SFE throughout the centers and disks. Our prescription further reveals a mean molecular gas depletion time of 700 Myr in the centers of barred galaxies, which is overall three to four times shorter than in nonbarred galaxy centers or the disks. Across the galaxy disks, the depletion time is consistently around 2–3 Gyr, regardless of the choice of α CO prescription. All together, our results suggest that the high level of star formation activity in barred centers is not simply due to an increased amount of molecular gas, but also to an enhanced SFE compared to nonbarred centers or disk regions.

Resolved Measurements of the CO-to-H2 Conversion Factor in 37 Nearby Galaxies

“…The advantage in this direction is that the physical quantities at the cloud scale are more strongly linked to the fundamental physics of CO emission and dynamics of molecular clouds. For instance, based on the α CO measurements in this work, Teng et al (2024) have reported an α CO dependence with the cloud-scale velocity dispersion, which likely traces CO opacity change. We also refer the readers to Teng et al (2022Teng et al ( , 2023 for more details.…”

The Resolved Star Formation Law in NGC 7469 from JWST, ALMA, and VLA

“…Still, local variations in gas properties, including composition, turbulence, and metallicity, significantly influence the CO-to-H 2 conversion factor (Leroy et al 2011;Narayanan et al 2012;Papadopoulos et al 2012;Bolatto et al 2013;Sandstrom et al 2013;Teng et al 2024). Oxygen abundances compatible with solar metallicity were reported for the NGC 7469 SB ring and outer region by Cazzoli et al (2020) and Zamora & Díaz (2023) using optical nebular lines.…”