The star formation laws of Eddington-limited star-forming disks

Abstract

Two important avenues into understanding the formation and evolution of galaxies are the Kennicutt-Schmidt (K-S) and Elmegreen-Silk (E-S) laws. These relations connect the surface densities of gas and star formation (Σgas and Σ*, respectively) in a galaxy. To elucidate the K-S and E-S laws for disks where Σgas ≳ 104 M ⊙ pc-2, we compute 132 Eddington-limited star-forming disk models with radii spanning tens to hundreds of parsecs. The theoretically expected slopes (≈1 for the K-S law and ≈0.5 for the E-S 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 Σ* that is correct to within a factor of two, the velocity-integrated CO line intensity is a poor proxy for the average Σgas for these warm and dense disks, making the CO conversion factor (αCO) all but useless. Thus, observationally derived K-S and E-S laws at these values of Σgas 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. © 2013. The American Astronomical Society. All rights reserved.