Chaotic and Clumpy Galaxy Formation in an Extremely Massive Reionization-Era Halo

TL;DR

This study presents high-resolution ALMA observations of a massive galaxy system at z=6.9, revealing chaotic, clumpy structures and complex dynamics that challenge previous notions of thin-disk formation in early galaxies.

Contribution

It provides the most detailed view to date of a reionization-era galaxy system, showing chaotic clumpy formation and complex kinematics at high resolution.

Findings

Resolved the system into at least a dozen kiloparsec-sized clumps.

Observed high turbulent velocity dispersion contrasting with thin-disk models.

Clumps have star formation rates 3-5 times higher than typical for their mass.

Abstract

The SPT0311-58 system at z=6.900 is an extremely massive structure within the reionization epoch, and offers a chance to understand the formation of galaxies in an extreme peak in the primordial density field. We present 70mas Atacama Large Millimeter/submillimeter Array observations of the dust continuum and CII 158um emission in the central pair of galaxies and reach physical resolution ~100-350pc, among the most detailed views of any reionization-era system to date. The observations resolve the source into at least a dozen kiloparsec-size clumps. The global kinematics and high turbulent velocity dispersion within the galaxies present a striking contrast to recent claims of dynamically cold thin-disk kinematics in some dusty galaxies just 800Myr later at z~4. We speculate that both gravitational interactions and fragmentation from massive parent disks have likely played a role in the overall dynamics and formation of clumps in the system. Each clump individually is comparable in mass to other 6<z<8 galaxies identified in rest-UV/optical deep field surveys, but with star formation rates elevated by ~3-5x. Internally, the clumps themselves bear close resemblance to greatly scaled-up versions of virialized cloud-scale structures identified in low-redshift galaxies. Our observations are qualitatively similar to the chaotic and clumpy assembly within massive halos seen in simulations of high-redshift galaxies.