What Sets the Slope of the Molecular Kennicutt-Schmidt Relation?

TL;DR

This study uses galaxy simulations to investigate why the molecular Kennicutt-Schmidt relation has a linear slope, finding that feedback and star-forming gas criteria are key factors in its emergence.

Contribution

The paper demonstrates that the linear slope arises from feedback regulation and specific star-forming gas criteria, providing a theoretical model for the relation's origin.

Findings

Linear slope is insensitive to the star formation law at small scales.

Star-forming gas criteria influence the slope on kiloparsec scales.

Feedback regulates and stirs dense molecular gas, shaping the relation.

Abstract

The surface densities of molecular gas, $\Sigma_{\rm H_2}$, and the star formation rate (SFR), $\dot\Sigma_\star$, correlate almost linearly on kiloparsec scales in observed star-forming (non-starburst) galaxies. We explore the origin of the linear slope of this correlation using a suite of isolated $L_\star$ galaxy simulations. We show that in simulations with efficient feedback, the slope of the $\dot\Sigma_\star$-$\Sigma_{\rm H_2}$ relation on kiloparsec scales is insensitive to the slope of the $\dot\rho_\star$-$\rho$ relation assumed at the resolution scale. We also find that the slope on kiloparsec scales depends on the criteria used to identify star-forming gas, with a linear slope arising in simulations that identify star-forming gas using a virial parameter threshold. This behavior can be understood using a simple theoretical model based on conservation of interstellar gas mass as the gas cycles between atomic, molecular, and star-forming states under the influence of feedback and dynamical processes. In particular, we show that the linear slope emerges when feedback efficiently regulates and stirs the evolution of dense, molecular gas. We show that the model also provides insights into the likely origin of the relation between the SFR and molecular gas in real galaxies on different scales.