ALMA hints at the presence of turbulent disk galaxies at z > 5

TL;DR

This study uses ALMA data to analyze the gas kinematics of galaxies at z > 5, revealing that they are rotating but highly turbulent, with velocity dispersions much higher than local galaxies, indicating early disk formation with significant turbulence.

Contribution

The paper provides the first systematic kinematic analysis of 22 z > 5 galaxies using ALMA, demonstrating that early galaxies have turbulent rotating disks with higher velocity dispersions than local counterparts.

Findings

High-z galaxies exhibit rotating but turbulent gas disks.

Velocity dispersion is about four times higher than in local galaxies.

Dynamically colder disks are found in dusty, massive galaxies compared to dust-poor ones.

Abstract

High-redshift galaxies are expected to be more turbulent than local galaxies because of their smaller size and higher star formation and thus stronger feedback from star formation, frequent mergers events, and gravitational instabilities. However, this scenario has recently been questioned by the observational evidence of a few galaxies at z~4-5 with a gas velocity dispersion similar to what is observed in the local population. Our goal is to determine whether galaxies in the first Gyrs of the Universe have already formed a dynamically cold rotating disk similar to the local counterparts. We studied the gas kinematic of 22 main-sequence star-forming galaxies at z > 5 and determined their dynamical state by estimating the ratio of the rotational velocity and of the gas velocity dispersion. We mined the ALMA archive and exploited the [CII] and [OIII] observations to perform a kinematic analysis of the cold and warm gas of z>5 main-sequence galaxies. The gas kinematics of the high-z galaxies is consistent within the errors with rotating but turbulent disks. We infer a velocity dispersion that is systematically higher by 4 times than the local galaxy population and the z~5 dust-obscured galaxies reported in the literature. The difference between our results and those reported at similar redshift can be ascribed to the systematic difference in the galaxy properties in the two samples: the disks of massive dusty galaxies are dynamically colder than the disks of dust-poor galaxies. The comparison with the theoretical predictions suggests that the main driver of the velocity dispersion in high-z galaxies is the gravitational energy that is released by the transport of mass within the disk. Finally, we stress that future deeper ALMA high-angular resolution observations are crucial to constrain the kinematic properties of high-z galaxies and to distinguish rotating disks from kpc-scale mergers.