Volumetric star formation laws of disc galaxies

TL;DR

This study derives new empirical star formation laws for disc galaxies based on volume densities, revealing a tight correlation with less scatter than surface-based laws and highlighting the importance of disc thickness in galaxy evolution models.

Contribution

It introduces volume density-based star formation laws for disc galaxies, accounting for disc thickness and revealing a consistent slope over a wide range of densities.

Findings

Tight correlation between gas volume density and SFR with slope 1.3-1.9.

Smaller scatter in volume-based law compared to surface-based Kennicutt law.

Atomic gas density correlates strongly with SFR, especially in low-density environments.

Abstract

Star formation (SF) laws are fundamental relations between the gas content of a galaxy and its star formation rate (SFR) and play key roles in galaxy evolution models. In this paper, we present new empirical SF laws of disc galaxies based on volume densities. Following the assumption of hydrostatic equilibrium, we calculated the radial growth of the thickness of the gaseous discs in the combined gravitational potential of dark matter, stars, and gas for 12 nearby star-forming galaxies. This allowed us to convert the observed surface densities of gas and SFR into the deprojected volume densities. We found a tight correlation with slope in the range 1.3-1.9 between the volume densities of gas (HI+H$_2$) and the SFR with a significantly smaller scatter than the surface-based (Kennicutt) law and no change in the slope over five orders of magnitude. This indicates that taking into account the radial increase of the thickness of galaxy discs is crucial to reconstruct their three-dimensional density profiles, in particular in their outskirts. Moreover, our result suggests that the break in the slope seen in the Kennicutt law is due to disc flaring rather than to a drop of the SF efficiency at low surface densities. Surprisingly, we discovered an unexpected correlation between the volume densities of HI and SFR, indicating that the atomic gas is a good tracer of the cold star-forming gas, especially in low density HI-dominated environments.