Accretion-Driven Turbulence and the Transition to Global Instability in Young Galaxy Disks

TL;DR

This paper presents a model where gas accretion initially drives turbulence in young galaxy disks, but later disk instabilities and star formation feedback become the dominant turbulence sources, explaining observed high turbulence in early galaxies.

Contribution

The paper introduces a simple model quantifying how accretion energy drives turbulence and transitions to other sources in young galaxy disks, linking turbulence to galaxy evolution stages.

Findings

Accretion-driven turbulence lasts for a limited time (~2 t_acc).

High turbulence in high-redshift galaxies likely not driven by accretion during starburst phase.

Early turbulence could be powered by accretion before star formation feedback dominates.

Abstract

A simple model of gas accretion in young galaxy disks suggests that fast turbulent motions can be driven by accretion energy for a time t_acc~2(epsilon^{0.5} GM^2/xi V^3)^{0.5} where epsilon is the fraction of the accretion energy going into disk turbulence, M and V are the galaxy mass and rotation speed, and xi is the accretion rate. After t_acc, accretion is replaced by disk instabilities as a source of turbulence driving, and shortly after that, energetic feedback by young stars should become important. The star formation rate equilibrates at the accretion rate after 1 to 2 t_acc, depending on the star formation efficiency per dynamical time. The fast turbulence that is observed in high redshift starburst disks is not likely to be driven by accretion because the initial t_acc phase is over by the time the starburst is present. However, the high turbulent speeds that must have been present earlier, when the observed massive clumps first formed, could have been driven by accretion energy. The combined observations of a high relative velocity dispersion in the gas of z~2 clumpy galaxies and a gas mass comparable to the stellar mass suggests that either the star formation efficiency is fairly high, perhaps 10x higher than in local galaxies, or the observed turbulence is powered by young stars.