The ALMA REBELS Survey: Average [CII] $158\,\rm{\mu m}$ sizes of Star-Forming Galaxies from $z\sim 7$ to $z\sim 4$

TL;DR

This study measures the sizes of [CII] emission regions in star-forming galaxies from redshift 7 to 4, revealing no significant size evolution and suggesting gas-dominated morphologies across this epoch.

Contribution

It provides the first average [CII] size measurements at z~7 and compares sizes across z~7 to 4, demonstrating size stability over this redshift range.

Findings

[CII] sizes are ~2.2 kpc at z~7

No significant evolution in [CII] size from z~7 to 4

Star-forming galaxies are likely gas-dominated morphologically

Abstract

We present the average [CII] $158\,\rm{\mu m}$ emission line sizes of UV-bright star-forming galaxies at $z\sim7$. Our results are derived from a stacking analysis of [CII] $158\,\rm{\mu m}$ emission lines and dust continua observed by ALMA, taking advantage of the large program Reionization Era Bright Emission Line Survey (REBELS). We find that the average [CII] emission at $z\sim7$ has an effective radius $r_e$ of $2.2\pm0.2\,\rm{kpc}$. It is $\gtrsim2\times$ larger than the dust continuum and the rest-frame UV emission, in agreement with recently reported measurements for $z\lesssim6$ galaxies. Additionally, we compared the average [CII] size with $4<z<6$ galaxies observed by the ALMA Large Program to INvestigate [CII] at Early times (ALPINE). By analysing [CII] sizes of $4<z<6$ galaxies in two redshift bins, we find an average [CII] size of $r_{\rm e}=2.2\pm0.2\,\rm{kpc}$ and $r_{\rm e}=2.5\pm0.2\,\rm{kpc}$ for $z\sim5.5$ and $z\sim4.5$ galaxies, respectively. These measurements show that star-forming galaxies, on average, show no evolution in the size of the [CII] $158\,{\rm \mu m}$ emitting regions at redshift between $z\sim7$ and $z\sim4$. This finding suggest that the star-forming galaxies could be morphologically dominated by gas over a wide redshift range.