Amplification, Saturation, andQThresholds for Runaway: Growth of Self‐Gravitating Structures in Models of Magnetized Galactic Gas Disks

Abstract

We investigate the susceptibility of gaseous, magnetized galactic disks to the formation of selfgravitating condensations using two-dimensional, local models. We focus on two issues : (1) determining the threshold condition for gravitational runaway, taking into account nonlinear e †ects ; and (2) distinguishing the magneto-Jeans instability (MJI) that arises under inner galaxy rotation curves from the modiÐed swing ampliÐcation (MSA) that arises under outer galaxy rotation curves. For axisymmetric density Ñuctuations, instability is known to require a Toomre parameter Q \ 1. For nonaxisymmetric Ñuctuations, any nonzero shear q 4 [d ln )/d ln R winds up wave fronts such that in linear theory ampliÐcation saturates. Any Q threshold for nonaxisymmetric gravitational runaway must originate from nonlinear e †ects. We use numerical magnetohydrodynamic simulations to demonstrate the anticipated threshold phenomenon, to analyze the nonlinear processes involved, and to evaluate the critical value Q c for stabilization. We Ðnd for a wide variety of conditions, with the largest values corre-Q c D 1.2È1.4 sponding to nonzero but subthermal mean magnetic Ðelds. Our Ðndings for are similar to those Q c inferred from thresholds for active star formation in the outer regions of spiral galaxies. MJI is distinct from MSA in that opposition to Coriolis forces by magnetic tension, rather than cooperation of epicyclic motion with kinematic shear, enables nonaxisymmetric density perturbations to grow. We suggest that under low-shear inner disk conditions, will be larger than that in outer disks by a factor Q c D(v A /qc s )1@2, where and are the respective and sound speeds. v A c s Alfven

Control of Star Formation in Galaxies by Gravitational Instability

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Equilibrium Sequences and Gravitational Instability of Rotating Isothermal Rings

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The Abundance of Molecular Hydrogen and Its Correlation With Midplane Pressure in Galaxies: Non-Equilibrium, Turbulent, Chemical Models

“…This has led to the suspicion that the formation of stars depends on the formation of molecular hydrogen (Schaye 2004;Krumholz & McKee 2005;Elmegreen 2007;Krumholz et al 2009b). However, the opposite view has also been argued: that molecular clouds and then stars form from converging flows (Ballesteros-Paredes et al 1999;Koyama & Inutsuka 2000;Hartmann et al 2001;Vázquez-Semadeni et al 2006; Ballesteros-Paredes e 2007; Hennebelle et al 2008;Heitsch & Hartmann 2008;Inoue & Inutsuka 2009), primarily driven by large-scale gravitational instability (Rafikov 2001;Kim & Ostriker 2001;Elmegreen 2002;Kravtsov 2003;Dalcanton et al 2004;Mac Low & Klessen 2004;Li et al 2005Li et al , 2006Shetty & Ostriker 2008;Ostriker et al 2010). In this picture, the formation of molecules is a consequence, not a cause, of the conditions required to form stars.…”