Star formation drivers across the M33 disk

¹ INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
² Katonah, NY 10536, USA
³ Department of Physics & Astronomy, University of Victoria, Finnerty Road, Victoria, British Columbia V8P 1A1, Canada

^⋆ Corresponding authors: edvige.corbelli@inaf.it; belmegreen@gmail.com; sarae@uvic.ca

Received: 6 May 2025
Accepted: 16 June 2025

Abstract

Aims. We investigate the star formation process across M33, which is characterized by a low molecular content and can be sampled with high spatial resolution out to regions where star formation activity drops.

Methods. We used a multiwavelength dataset and disk dynamics to extract the local physical parameters across the M33 disk, such as the atomic, molecular, stellar and dust mass surface densities, dark matter densities, and hydrostatic pressure. We computed numerically equilibrium values of gas densities and scale heights across the disk, testing several analytic approximations that are often used to estimate these variables. Orthogonal regressions and hierarchical Bayesian models, as well as random forest (RF) analyses, were used to establish the fundamental relations at physical scales from 160 pc to 1 kpc.

Results. The gas hydrostatic pressure, P_hy, which balances the local weight, is the main driver of the star formation rate surface density, Σ_SFR, throughout the whole star-forming disk of M33. High-pressure regions enhance the atomic-to-molecular gas conversion, with the molecular hydrogen mass surface density, Σ_H2, being tightly correlated to P_hy and a uniform scaling law throughout the M33 disk. The P_hy–Σ_SFR relation differs, showing a change in slope from the inner to the outer disk. Our use of an accurate analytic expression and database to compute P_hy for a multicomponent disk minimizes observational scatter. This points to scaling laws that do not depend on the physical scale and brings out an intrinsic scatter linked to variations in the efficiency and relative age of the molecular gas-to-stars conversion. In the inner disk, where spiral arms are present and the stellar surface density dominates gravity, P_hy and Σ_SFR establish an almost linear correlation with a smaller dispersion than in the Σ_H2–Σ_SFR relation. In the atomic gas-dominated outer disk, Σ_SFR has a steeper dependence on P_hy, which we propose could be the result of an increasing fraction of diffuse molecular gas that does not form stars.