The Kennicutt-Schmidt Law and Gas Scale Height in Luminous and Ultra-Luminous Infrared Galaxies

Christine D. Wilson; Bruce G. Elmegreen; Ashley Bemis; Nathan Brunetti

doi:10.3847/1538-4357/ab31f3

paper

The Kennicutt-Schmidt Law and Gas Scale Height in Luminous and Ultra-Luminous Infrared Galaxies

arXiv:1907.05432 · doi:10.3847/1538-4357/ab31f3

Abstract

A new analysis of high-resolution data from the Atacama Large Millimeter/submillimeter Array (ALMA) for 5 luminous or ultra-luminous infrared galaxies gives a slope for the Kennicutt-Schmidt (KS) relation equal to $1.7 4_{- 0.07}^{+ 0.09}$ for gas surface densities $Σ_{mol} > 1 0^{3} M_{⊙}$ pc $^{- 2}$ and an assumed constant CO-to-H $_{2}$ conversion factor. The velocity dispersion of the CO line, $σ_{v}$ , scales approximately as the inverse square root of $Σ_{mol}$ , making the empirical gas scale height determined from $H \sim 0.5 σ^{2} / (π G Σ_{mol})$ nearly constant, 150-190 pc, over 1.5 orders of magnitude in $Σ_{mol}$ . This constancy of $H$ implies that the average midplane density, which is presumably dominated by CO-emitting gas for these extreme star-forming galaxies, scales linearly with the gas surface density, which, in turn, implies that the gas dynamical rate (the inverse of the free-fall time) varies with $Σ_{mol}^{1/2}$ , thereby explaining most of the super-linear slope in the KS relation. Consistent with these relations, we also find that the mean efficiency of star formation per free-fall time is roughly constant, 5%-7%, and the gas depletion time decreases at high $Σ_{mol}$ , reaching only $\sim 16$ Myr at $Σ_{mol} \sim 1 0^{4} M_{⊙}$ pc $^{- 2}$ . The variation of $σ_{v}$ with $Σ_{mol}$ and the constancy of $H$ are in tension with some feedback-driven models, which predict $σ_{v}$ to be more constant and $H$ to be more variable. However, these results are consistent with simulations in which large-scale gravity drives turbulence through a feedback process that maintains an approximately constant Toomre $Q$ instability parameter.

Accepted to ApJ. 13 pages, 3 figures, 1 table. Supplementary data table used to generate the plots is included

The Kennicutt-Schmidt Law and Gas Scale Height in Luminous and Ultra-Luminous Infrared Galaxies

Abstract

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