2019/07/11 by Christine D. Wilson, Bruce G. Elmegreen, Ashley Bemis +1 · 1 citation
Physics and Astronomy · #astro-ph.GA
paper · pdf · doi:10.3847/1538-4357/ab31f3
Accepted to ApJ. 13 pages, 3 figures, 1 table. Supplementary data table used to generate the plots is included
arxiv created 2019/07/11 · arxiv updated 2019/09/04
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 1.74+0.09\rm -0.07 for gas surface densities Σ\rm mol>103 M_\odot pc-2 and an assumed constant CO-to-H2 conversion factor. The velocity dispersion of the CO line, σv, scales approximately as the inverse square root of Σ\rm mol, making the empirical gas scale height determined from H∼0.5σ2/(πGΣ\rm mol) nearly constant, 150-190 pc, over 1.5 orders of magnitude in Σ\rm 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\rm mol1/2, 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\rm mol, reaching only ∼ 16 Myr at Σ\rm mol∼104 M_\odot pc-2. The variation of σv with Σ\rm mol and the constancy of H are in tension with some feedback-driven models, which predict σv 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.