Gravitational Instabilities in Two-Component Galaxy Disks with Gas Dissipation

TL;DR

This paper investigates how gas dissipation affects gravitational instabilities in galaxy disks, revealing that dissipation extends instability to smaller scales and influences star formation and spiral structure formation.

Contribution

It introduces a model accounting for gas dissipation in thick disks, showing that dissipation significantly alters stability thresholds and the scale of gravitational instabilities.

Findings

Gas dissipation removes the minimum Jeans length, destabilizing small scales.

Disks are unstable over a wide range of scales with observed turbulence levels.

Instabilities can drive turbulence and influence star formation cycles.

Abstract

Growth rates for gravitational instabilities in a thick disk of gas and stars are determined for a turbulent gas that dissipates on the local crossing time. The scale heights are derived from vertical equilibrium. The accuracy of the usual thickness correction, 1/(1+kH), is better than 6% in the growth rate when compared to exact integrations for the gravitational acceleration in the disk. Gas dissipation extends the instability to small scales, removing the minimum Jeans length. This makes infinitesimally thin disks unstable for all Toomre-Q values, and reasonably thick disks stable at high Q primarily because of thickness effects. The conventional gas+star threshold, Qtot increases from ~1 without dissipation to 2 or 3 when dissipation has a rate equal to the crossing rate over a perturbation scale. Observations of Qtot~2-3 and the presence of supersonic turbulence suggest that disks are unstable over a wide range of scales. Such instabilities drive spiral structure if there is shear and clumpy structure if shear is weak; they may dominate the generation of turbulence. Feedback regulation of Qtot is complex because the stellar component does not cool; the range of spiral strengths from multiple arm to flocculent galaxies suggests that feedback is weak. Gravitational instabilities may have a connection to star formation even when the star formation rate scales directly with the molecular mass because the instabilities return dispersed gas to molecular clouds and complete the cycle of cloud formation and destruction. The mass flow to dense clouds by instabilities can be 10 times larger than the star formation rate.