ALMA hints at the presence of turbulent disk galaxies at z > 5 (2304.00036v1)

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.

• The paper’s main contribution highlights that high-redshift galaxies exhibit gas velocity dispersions 4-5 times higher than those in local galaxies.
• It employs both 2D and 1D kinematic modeling of ALMA spectral data ([CII] and [OIII]) to analyze galaxy dynamics.
• The study reveals that gravitational instabilities and variations in dust content influence the early stabilization of disk structures.

Overview of Turbulent Disk Galaxies at High Redshifts from ALMA Observations

Introduction

This paper investigates the dynamics of high-redshift galaxies, specifically those at $z > 5$, using data from the Atacama Large Millimeter/submillimeter Array (ALMA). The conventional model assumes that these early galaxies are more turbulent compared to their local counterparts due to factors such as frequent mergers, high star formation rates, and gravitational instabilities. The research attempts to discern whether galaxies in these formative periods had evolved into dynamically cold rotating disks similar to modern galaxies.

Methodology

The paper comprises a detailed kinematic analysis of 22 main-sequence star-forming galaxies at $z > 5$. It utilizes observational data from ALMA, focusing on spectral lines such as [CII] and [OIII]. The analysis involves investigating the gas velocity dispersion and comparing the rotational velocity to gas dispersion ratios to determine the dynamical state of these galaxies. The paper adopts two primary methods for kinematic modeling:

1. 2D Kinematic Fitting: This method involves fitting moment maps derived from the spectral data to model galaxy kinematics.
2. 1D Kinematic Fitting: This approach is used for galaxies with less discernible velocity gradients, utilizing integrated spectra to infer kinematic properties.

Results

The analysis shows high gas velocity dispersions in high-redshift galaxies, 4-5 times higher than that observed in local galaxies. The $V_{\rm max}/\sigma_{\rm gas}$ ratios suggest that most galaxies are rotating but are more turbulent than local universe counterparts. Interestingly, galaxies with significant dust presence showed lower velocity dispersions, hinting at differences in internal dynamics based on dust content.

Discussion

The results suggest that the velocity dispersion in these galaxies is primarily influenced by gravitational instabilities rather than just star formation feedback. The paper suggests that the kinematic properties hinge on multiple factors, including stellar mass, dust mass, and gas fraction. A surprising observation is the presence of dynamically colder disks amidst expectedly turbulent high-redshift galaxies, indicating that some mechanisms promote early disk stabilization.

Implications and Future Work

This research has significant implications for the understanding of galaxy evolution. It postulates the existence of two populations of high-redshift galaxies: dynamically cold starbursts and turbulent main-sequence galaxies. The results advocate for more comprehensive future surveys incorporating higher resolution and sensitivity to elucidate the dynamics of early galaxies more clearly. Understanding these dynamics provides foundational insight into galaxy formation and the evolution of the universe's structure over cosmic time.

In conclusion, the paper broadens the understanding of galaxy dynamics at high redshifts, illustrating a complex interplay of processes influencing their evolution. Further observations with advanced instruments could refine these insights, advancing the frontier of astrophysics regarding the early universe.