VERTICAL EQUILIBRIUM, ENERGETICS, and STAR FORMATION RATES in MAGNETIZED GALACTIC DISKS REGULATED by MOMENTUM FEEDBACK from SUPERNOVAE

Abstract

Recent hydrodynamic (HD) simulations have shown that galactic disks evolve to reach well-defined statistical equilibrium states. The star formation rate (SFR) self-regulates until energy injection by star formation feedback balances dissipation and cooling in the interstellar medium (ISM), and provides vertical pressure support to balance gravity. In this paper, we extend our previous models to allow for a range of initial magnetic field strengths and configurations, utilizing three-dimensional, magnetohydrodynamic (MHD) simulations. We show that a quasisteady equilibrium state is established as rapidly for MHD as for HD models unless the initial magnetic field is very strong or very weak, which requires more time to reach saturation. Remarkably, models with initial magnetic energy varying by two orders of magnitude approach the same asymptotic state. In the fully saturated state of the fiducial model, the integrated energy proportions Eturb: Eth: dEmag: Emag are 0.35:0.39:0.15:0.11, while the proportions of midplane support Pturb: Pth: dPmag: Pmag are 0.49:0.18:0.18:0.15. Vertical profiles of total effective pressure satisfy vertical dynamical equilibrium with the total gas weight at all heights. We measure the "feedback yields" hc/Pc ∑SFR (in suitable units) for each pressure component, finding that hturb ∼ 3.5-4 and hth ∼ 1.1 - 1.4 are the same for MHD as in previous HD simulations, and dhmag ∼ 1.3-1.5. These yields can be used to predict the equilibrium SFR for a local region in a galaxy based on its observed gas and stellar surface densities and velocity dispersions. As the ISM weight (or dynamical equilibrium pressure) is fixed, an increase in η from turbulent magnetic fields reduces the predicted ∑SFR by ∼20-30% relative to the HD case.