The Star Formation Laws of Eddington-Limited Star-Forming Disks

TL;DR

This study models Eddington-limited star-forming disks to analyze the Kennicutt-Schmidt and Elmegreen-Silk laws, revealing their dependence on opacity and limitations of CO lines as tracers, with implications for galaxy evolution understanding.

Contribution

The paper presents 132 theoretical models of dense star-forming disks, highlighting the impact of opacity and dust-to-gas ratio on star formation laws and the limitations of CO lines as tracers.

Findings

The slopes of star formation laws are robust to spatial averaging.

Infrared luminosity estimates star formation rate within a factor of 2.

CO line intensities are poor proxies for gas surface density in dense disks.

Abstract

Two important avenues into understanding the formation and evolution of galaxies are the Kennicutt-Schmidt (KS) and Elmegreen-Silk (ES) laws. These relations connect the surface densities of gas and star formation (\sigmagas\ and \sigmastar, respectively) in a galaxy. To elucidate the KS and ES laws for disks where \sigmagas >~ 10^4 Msun pc-2}, we compute 132 Eddington-limited star-forming disk models with radii spanning tens to hundreds of parsecs. The theoretically expected slopes (approx. 1 for the KS law and approx. 0.5 for the ES relation) are relatively robust to spatial averaging over the disks. However, the star formation laws exhibit a strong dependence on opacity that separates the models by the dust-to-gas ratio that may lead to the appearance of a erroneously large slope. The total infrared luminosity (L_TIR) and multiple carbon monoxide (CO) line intensities were computed for each model. While L_TIR can yield an estimate of the average \sigmastar\ that is correct to within a factor of 2, the velocity-integrated CO line intensity is a poor proxy for the average \sigmagas\ for these warm and dense disks, making the CO conversion factor (\alpha_CO) all but useless. Thus, observationally derived KS and ES laws at these values of \sigmagas\ that uses any transition of CO will provide a poor measurement of the underlying star formation relation. Studies of the star formation laws of Eddington-limited disks will require a high-J transition of a high density molecular tracer, as well as a sample of galaxies with known metallicity estimates.