Constraining the Molecular Kennicutt-Schmidt Relation with Multi-Transition CO Observations of Nearby Galaxies

TL;DR

This study analyzes multi-transition CO observations of 36 nearby galaxies to better understand how dense molecular gas influences the star formation rate, revealing a trend toward more linear relations at higher-J transitions.

Contribution

It provides new empirical constraints on the molecular Kennicutt-Schmidt relation using multi-transition CO data and advanced statistical methods, highlighting the role of dense gas in star formation.

Findings

Power-law slopes decrease from 1.26 to 1.07 across CO transitions.

Denser gas tracers show more linear star formation relations.

Results support a volume density relation with a power-law index of ~1.5.

Abstract

The relationship between the star formation rate surface density and the molecular gas surface density in galaxies is key to understanding galaxy evolution. To investigate the molecular Kennicutt-Schmidt (K-S) relation and its dependence on gas density, we analyze a uniform sample of 36 nearby galaxies from the AMISS survey, focusing on the CO(1-0), CO(2-1), and CO(3-2) transitions, which trace progressively denser and warmer molecular gas. Using statistical methods that combine binning with Markov Chain Monte Carlo (MCMC) fitting, we derive the slope, scatter, and intercept of the $\Sigma_{\mathrm{SFR}}$-$\Sigma_{\mathrm{CO}}$ relation for each transition. We find power-law slopes of 1.26, 1.14, and 1.07 for CO(1-0), CO(2-1), and CO(3-2), respectively, consistent with a trend toward increasingly linear star formation relations at higher-J transitions. This behavior supports the idea that denser gas is more directly linked to ongoing star formation and is consistent with previous findings of near-linear correlations between HCN or high-J CO luminosities and global SFR. The observed trend suggests an underlying relation between gas and SFR volume densities with a power-law index of $\sim$1.5, indicating enhanced star formation efficiency in denser environments. These findings underscore the critical role of dense gas in regulating star formation and highlight the importance of tracer selection and excitation conditions when interpreting the K-S relation across different environments.