An extended [CII] halo around a massive star-forming galaxy at z = 5.3

TL;DR

This study presents high-resolution ALMA observations of a main sequence galaxy at z=5.25, revealing an extended [CII] halo twice the size of the UV emission, indicating complex galaxy structures and potential merging activity.

Contribution

First high-redshift detection of a [CII] halo around a normal star-forming galaxy, providing insights into galaxy structure and evolution at early cosmic times.

Findings

Extended [CII] emission is twice the size of UV emission.

[CII] morphology suggests a merging event or obscured emission.

Detected [CII] halo around a main sequence galaxy at z=5.25.

Abstract

High-redshift observations are often biased towards massive and bright galaxies that are not necessarily representative of the full population. In order to accurately study galaxy evolution and mass assembly at these redshifts, observations of ``normal'' main sequence galaxies are required. Here we present Atacama Large Millimeter/Submillimeter Array (ALMA) 0.3" resolution observations of the [CII] emission line at 158$\mu$m of HZ7, a main sequence galaxy at $z=5.25$. Comparing to archival rest-frame UV observations taken by the Hubble Space Telescope (HST), we find strong evidence of the existence of extended [CII] emission, which we estimate to be twice the size of the rest-frame UV emission, yielding one of the first high-redshift objects where a clear signature of a [CII] ``Halo'' has been detected to date. For a matched S\'ersic profile with n = 1, we measured a [CII] effective radius of $0.50\pm 0.04$" (3.07$\pm 0.25$ kpc) and an average rest-frame UV effective radius of $0.2\pm0.04$" ($1.48\pm0.16$ kpc). The [CII] morphology and kinematics of the system suggest a merging event resulting in a non rotating disk system. This event could be responsible for the extended [CII] emission. Alternatively, some potential obscured emission could also explain the [CII] to UV size ratio. These results contribute to the growing consensus with respect to the existence of extended [CII] emission around galaxies.