MAXIMALLY STAR-FORMING GALACTIC DISKS. I. STARBURST REGULATION VIA FEEDBACK-DRIVEN TURBULENCE

Star formation rates in the centers of disk galaxies often vastly exceed those at larger radii, whether measured by the surface density of star formation Sigma(SFR), by the star formation rate per unit gas mass, Sigma(SFR)/Sigma, or even by total output. In this paper, we investigate the idea that central starbursts are self-regulated systems in which the momentum flux injected to the interstellar medium (ISM) by star formation balances the gravitational force confining the ISM gas in the disk. For most starbursts, supernovae are the largest contributor to the momentum flux, and turbulence provides the main pressure support for the predominantly molecular ISM. If the momentum feedback per stellar mass formed is p(*)/m(*) similar to 3000 km s(-1), the predicted star formation rate is Sigma(SFR) similar to 2 pi G Sigma(2)m(*)/p(*) similar to 0.1 M-circle dot kpc(-2) yr(-1)(Sigma/100 M-circle dot pc(-2))(2) in regions where gas dominates the vertical gravity. We compare this prediction with numerical simulations of vertically resolved disks that model star formation including feedback, finding good agreement for gas surface densities in the range Sigma similar to 10(2)-10(3) M-circle dot pc(-2). We also compare to a compilation of star formation rates and gas contents from local and high-redshift galaxies (both mergers and normal galaxies), finding good agreement provided that the conversion factor X-CO from integrated CO emission to H-2 surface density decreases modestly as Sigma and Sigma(SFR) increase. Star formation rates in dense, turbulent gas are also expected to depend on the gravitational free-fall time at the corresponding mean ISM density rho(0); if the star formation efficiency per free-fall time is epsilon(ff)(rho(0)) similar to 0.01, the turbulent velocity dispersion driven by feedback is expected to be v(z) = 0.4 epsilon(ff)(rho(0))p(*)/m(*) similar to 10 km s(-1), relatively independent of Sigma or Sigma(SFR). Turbulence-regulated starbursts (controlled by kinetic momentum feedback) are part of the larger scheme of self-regulation; primarily atomic low-Sigma outer disks may have star formation regulated by ultraviolet heating feedback, whereas regions at extremely high Sigma may be regulated by feedback of stellar radiation that is reprocessed into trapped infrared.