Amplification, saturation, and Q thresholds for runaway: Growth of self-gravitating structures in models of magnetized galactic gas disks

Woong Tae Kim, Eve Charis Ostriker

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105 Scopus citations


We investigate the susceptibility of gaseous, magnetized galactic disks to the formation of self-gravitating condensations using two-dimensional, local models. We focus on two issues: (1) determining the threshold condition for gravitational runaway, taking into account nonlinear effects; and (2) distinguishing the magneto-Jeans instability (MJI) that arises under inner galaxy rotation curves from the modified swing amplification (MSA) that arises under outer galaxy rotation curves. For axisymmetric density fluctuations, instability is known to require a Toomre parameter Q < 1. For nonaxisymmetric fluctuations, any nonzero shear q = -dlnΩ/dln R winds up wave fronts such that in linear theory amplification saturates. Any Q threshold for nonaxisymmetric gravitational runaway must originate from nonlinear effects. We use numerical magnetohydrodynamic simulations to demonstrate the anticipated threshold phenomenon, to analyze the nonlinear processes involved, and to evaluate the critical value Qc for stabilization. We find Qc ∼ 1.2-1.4 for a wide variety of conditions, with the largest values corresponding to nonzero but subthermal mean magnetic fields. Our findings for Qc are similar to those 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, Qc will be larger than that in outer disks by a factor ∼(vA/qcs),1/2, where vA and cs are the respective Alfvén and sound speeds.

Original languageEnglish (US)
Pages (from-to)70-95
Number of pages26
JournalAstrophysical Journal
Issue number1 PART 1
StatePublished - Sep 20 2001

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Galaxies: ISM
  • Galaxies: kinematics and dynamics
  • Galaxies: structure
  • ISM: kinematics and dynamics
  • ISM: magnetic fields
  • Instabilities
  • MHD
  • Stars: formation


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