Cloud formation by combined instabilities in galactic gas layers: evidence for a Q threshold in the fragmentation of shearing wavelets

Abstract

The growth of shearing wavelets in thick galactic gas disks is studied, including the magnetic Rayleigh-Taylor instability perpendicular to the plane, various degrees of thermal instability, and the gravitational instability. Growth rates are calculated numerically for a wide range of parameter values, giving an effective dispersion relation and mass distribution function, and an approximate dispersion relation is derived analytically for the epoch of peak growth. An extensive coverage of parameter space illustrates the relative insensitivity of the gaseous shear instability to the axisymmetric stability parameter Q. The fragmentation of shearing wavelets by self-gravitational collapse parallel to the wave crest is also considered. Such fragmentation is sensitive to Q, requiring Q ≲ 1-2 for the growth of parallel perturbations to overcome shear inside the wavelet. Fragmentation instabilities may provide the link between shear instabilities and the formation of individual clouds. They are much more sensitive to Q than shear instabilities, and may regulate star formation so that Q ∼ 1.