2026/07/20 by Prachi Prajapati, Axel Weiss, Dominik Riechers +17
#astro-ph.GA
The CO luminosity-to-H2 mass conversion factor (αCO) remains a debated uncertainty in determining molecular gas masses of high-redshift dusty star-forming galaxies (DSFGs). Dynamical mass constraints have often favored αCO=0.8~M\odot~(K~km~s-1~pc2)-1, whereas dust- and radiative-transfer-based methods imply higher values. We revisit this ``tension" using the largest homogeneous sample of 21 unlensed z∼1-4 DSFGs, with securely measured \coonezero luminosities from the VLA \vzgal survey and resolved (∼0.1′′) ALMA 1~mm dust continuum imaging. For 12 galaxies with robust modeling constraints, we derive molecular gas masses using dust spectral energy distribution modeling and the TUNER LVG framework, adopting a solar-metallicity gas-to-dust mass ratio of 100. Although not fully independent due to shared assumptions on dust properties, these approaches yield mutually consistent gas masses corresponding to αCO∼1.5-11.5, with a median near the Galactic αCO=4.3. Isotropic virial dynamical masses agree with these gas masses when realistic molecular gas sizes are adopted, while our proposed ``mixed" (rotating, pressure-supported, thick-disk) estimator systematically underestimates dynamical masses, producing low αCO limits. Using GN20 (z=4.055) as a case study, we show that resolved gas geometry and kinematics reconcile the discrepancy with LVG-derived αCO. Our results suggest that current data do not require αCO=0.8, and intermediate to near-Galactic values remain dynamically viable given uncertainties in gas geometry, dust properties, and gas-to-dust ratios. Further progress in calibrating αCO in the early universe will require resolved molecular gas observations, physically motivated ISM modeling, and stringent constraints on dust properties.