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

Abstract

Star formation rates in the centers of disk galaxies often vastly exceed\nthose at larger radii. We investigate the idea that these central starbursts\nare self-regulated, with the momentum flux injected to the ISM by star\nformation balancing the gravitational force confining the gas. For most\nstarbursts, supernovae are the largest contributor to the momentum flux, and\nturbulence provides the main pressure support for the predominantly-molecular\nISM. If the momentum feedback per stellar mass formed is p_*/m_* ~ 3000 km/s,\nthe predicted star formation rate is Sigma_SFR=2 pi G Sigma^2 m_*/p_*\n~0.1(Sigma/100Msun/pc^2)^2 Msun/kpc^2/yr in regions where gas dominates the\nvertical gravity. We compare this prediction with numerical simulations of\nvertically-resolved disks that model star formation including feedback, finding\ngood agreement for gas surface densities Sigma ~ 10^2-10^3 Msun/pc^2. We also\ncompare to a compilation of star formation rates and gas contents from local\nand high-redshift galaxies (both mergers and normal galaxies), finding good\nagreement provided that X_CO decreases weakly as Sigma and Sigma_SFR increase.\nStar formation rates in dense, turbulent gas are also expected to depend on the\ngravitational free-fall time; if the efficiency per free-fall time is\nepsilon_ff ~ 0.01, the turbulent velocity dispersion driven by feedback is\nexpected to be v_z = 0.4 epsilon_ff p_*/m_* ~ 10 km/s, relatively independent\nof Sigma or Sigma_SFR. Turbulence-regulated starbursts (controlled by kinetic\nmomentum feedback) are part of the larger scheme of self-regulation;\nprimarily-atomic low-Sigma outer disks may have star formation regulated by UV\nheating feedback, whereas regions at extremely high Sigma may be regulated by\nfeedback of radiation that is reprocessed into trapped IR.\n