Unstable disks at high redshift: Evidence for smooth accretion in galaxy formation

TL;DR

High-redshift galaxies are characterized by unstable, clumpy disks likely formed through smooth gas accretion rather than mergers, indicating a different galaxy formation process in the early universe.

Contribution

This study provides evidence that the formation of clumpy, irregular high-redshift galaxies is driven by smooth gas accretion, challenging merger-based models.

Findings

High-redshift clumpy galaxies lack stabilizing bulges or stellar halos.

Mergers tend to produce spheroids, not clumpy disks.

Smooth accretion explains the formation of unstable, gas-rich disks.

Abstract

Galaxies above redshift 1 can be very clumpy, with irregular morphologies dominated by star complexes as large as 2 kpc and as massive as a few 10^8 or 10^9 Mo. Their co-moving densities and rapid evolution suggest that most present-day spirals could have formed through a clumpy phase. The clumps may form by gravitational instabilities in gas-rich turbulent disks; they do not appear to be separate galaxies merging together. We show here that the formation of the observed clumps requires initial disks of gas and stars with almost no stabilizing bulge or stellar halo. This cannot be achieved in models where disk galaxies grow by mergers. Mergers tend to make stellar spheroids even when the gas fraction is high, and then the disk is too stable to make giant clumps. The morphology of high-redshift galaxies thus suggests that inner disks assemble mostly by smooth gas accretion, either from cosmological flows or from the outer disk during a grazing interaction.