Star Formation Laws and Efficiencies across 80 Nearby Galaxies

TL;DR

This study empirically examines star formation laws across 80 nearby galaxies, revealing consistent molecular gas depletion times and systematic variations in star formation efficiency depending on local conditions, with methodological choices significantly affecting results.

Contribution

It provides a comprehensive, multi-scale analysis of star formation laws in a large galaxy sample, quantifying uncertainties and the impact of methodological choices on derived relations.

Findings

Molecular Kennicutt-Schmidt relation has ~10% larger scatter than others.

The slope of the molecular gas relation is approximately 0.9-1.2, indicating roughly constant depletion time.

Star formation efficiency varies systematically with local gas and SFR surface densities.

Abstract

We measure empirical relationships between the local star formation rate (SFR) and properties of the star-forming molecular gas on 1.5 kpc scales across 80 nearby galaxies. These relationships, commonly referred to as "star formation laws," aim at predicting the local SFR surface density from various combinations of molecular gas surface density, galactic orbital time, molecular cloud free-fall time, and the interstellar medium dynamical equilibrium pressure. Leveraging a multiwavelength database built for the PHANGS survey, we measure these quantities consistently across all galaxies and quantify systematic uncertainties stemming from choices of SFR calibrations and the CO-to-H$_2$ conversion factors. The star formation laws we examine show 0.3-0.4 dex of intrinsic scatter, among which the molecular Kennicutt-Schmidt relation shows a $\sim$10% larger scatter than the other three. The slope of this relation ranges $\beta\approx0.9{-}1.2$, implying that the molecular gas depletion time remains roughly constant across the environments probed in our sample. The other relations have shallower slopes ($\beta\approx0.6{-}1.0$), suggesting that the star formation efficiency (SFE) per orbital time, the SFE per free-fall time, and the pressure-to-SFR surface density ratio (i.e., the feedback yield) may vary systematically with local molecular gas and SFR surface densities. Last but not least, the shapes of the star formation laws depend sensitively on methodological choices. Different choices of SFR calibrations can introduce systematic uncertainties of at least 10-15% in the star formation law slopes and 0.15-0.25 dex in their normalization, while the CO-to-H$_2$ conversion factors can additionally produce uncertainties of 20-25% for the slope and 0.10-0.20 dex for the normalization.