The H₂ angular momentum – mass relation of local disc galaxies

¹ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
² International Centre for Radio Astronomy Research (ICRAR), M468, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
³ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), ACT 2611, Australia

^⋆ Corresponding authors: geesink@strw.leidenuniv.nl, pavel@strw.leidenuniv.nl

Received: 4 December 2024
Accepted: 7 March 2025

Abstract

We present an analysis of the molecular specific angular momentum–mass (j_H2 − M_H2) relation using a sample of 51 nearby disc galaxies from the PHANGS-ALMA survey with deep high-resolution molecular gas rotation curves and surface density profiles. For the very first time, using a statistical sample, we report the discovery of a well-defined j_H2 − M_H2 relation. We quantify the scaling law by fitting a power law with a Bayesian framework, finding j_H2 ∝ M_H2^0.53. This slope closely resembles the well-known stellar j_*–M_* (Fall) relation, highlighting the dynamical connection between molecular gas and stars. We show that the j_H2 − M_H2 relation cannot be fully explained by analytic models of disc stability, but instead is well recovered with more complex physics as implemented in the SHARK semi-analytical model. These findings demonstrate the power of our novel j_H2 − M_H2 relation in testing galaxy evolution theories and in setting new constraints for models and simulations that reproduce a realistic interstellar medium. Additionally, our findings provide a critical benchmark for upcoming high-redshift studies of molecular gas kinematics, offering a local baseline to study the evolution of cold gas dynamics across cosmic time.