Interpreting the sub-linear Kennicutt-Schmidt relationship: The case for diffuse molecular gas

TL;DR

This paper explores the sub-linear Kennicutt-Schmidt relationship, proposing that diffuse molecular gas significantly influences star formation measurements and challenges the assumption that CO only traces dense star-forming regions.

Contribution

It introduces the idea that diffuse molecular gas explains the sublinear KS relationship, highlighting the importance of considering hierarchical ISM structure and variable XCO factors.

Findings

Diffuse gas constitutes at least 30% of molecular content.

CO-bright diffuse gas can account for the sublinear KS relationship.

The fraction of diffuse gas increases with molecular surface density.

Abstract

Recent statistical analysis of two extragalactic observational surveys strongly indicate a sublinear Kennicutt-Schmidt (KS) relationship between the star formation rate (Sigsfr) and molecular gas surface density (Sigmol). Here, we consider the consequences of these results in the context of common assumptions, as well as observational support for a linear relationship between Sigsfr and the surface density of dense gas. If the CO traced gas depletion time (tau_mol) is constant, and if CO only traces star forming giant molecular clouds (GMCs), then the physical properties of each GMC must vary, such as the volume densities or star formation rates. Another possibility is that the conversion between CO luminosity and Sigmol, the XCO factor, differs from cloud-to-cloud. A more straightforward explanation is that CO permeates the hierarchical ISM, including the filaments and lower density regions within which GMCs are embedded. A number of independent observational results support this description, with the diffuse gas comprising at least 30% of the total molecular content. The CO bright diffuse gas can explain the sublinear KS relationship, and consequently leads to an increasing tau_mol with Sigmol. If Sigsfr linearly correlates with the dense gas surface density, a sublinear KS relationship indicates that the fraction of diffuse gas fdiff grows with Sigmol. In galaxies where Sigmol falls towards the outer disk, this description suggests that fdiff also decreases radially.