The ALMA REBELS Survey: Average [CII] $158\hat{\mathrm{I}}¼\mathrm{m}$ sizes of Star-Forming Galaxies from $z\sim 7$ to $z\sim 4$

Abstract

We present the average [CII] $158\hat{\mathrm{I}}¼\mathrm{m}$ emission line sizes of UV-bright star-forming galaxies at $z\sim 7$. Our results are derived from a stacking analysis of [CII] $158\hat{\mathrm{I}}¼\mathrm{m}$ 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 $z\sim 7$ has an effective radius $r_e$ of $2.2\pm 0.2\mathrm{kpc}$. It is $\gtrsim 2\times$ larger than the dust continuum and the rest-frame UV emission, in agreement with recently reported measurements for $z\lesssim 6$ 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 $r_{\mathrm{e}}=2.2\pm 0.2\mathrm{kpc}$ and $r_{\mathrm{e}}=2.5\pm 0.2\mathrm{kpc}$ for $z\sim 5.5$ and $z\sim 4.5$ galaxies, respectively. These measurements show that star-forming galaxies, on average, show no evolution in the size of the [CII] $158\hat{\mathrm{I}}¼\mathrm{m}$ emitting regions at redshift between $z\sim 7$ and $z\sim 4$. This finding suggest that the star-forming galaxies could be morphologically dominated by gas over a wide redshift range.

9pages, 4 figures, submitted to ApJ; minor corrections were made for typos in the author list and the title