The $H_2$ angular momentum -- mass relation of local disc galaxies

Abstract: We present an analysis of the molecular specific angular momentum-mass ($j_{H_2}-M_{H_2}$) 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 $j_{H_2}-M_{H_2}$ relation. We quantify the scaling law by fitting a power-law with a Bayesian framework, finding $j_{H_2} \propto M_{H_2}<sup>{0.53}$. This slope closely resembles the well-known stellar $j_{\ast}$-$M_{\ast}$ (Fall) relation, highlighting the dynamical connection between molecular gas and stars. We show that the $j_{H_2}-M_{H_2}$ 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 $j_{H_2}-M_{H_2}$ relation in testing galaxy evolution theories and setting new constraints for models and simulations which aim to 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.