# First Identification of 10-kpc Scale [CII] 158um Halos around Star-Forming Galaxies at z=5-7

## TL;DR

This study reports the first detection of 10-kpc scale [CII] 158um halos around star-forming galaxies at high redshift, revealing extended gas structures and outflow remnants in the early Universe.

## Contribution

It presents the first observational evidence of extended [CII] halos at z=5-7 and compares these with simulations, highlighting discrepancies in reproducing the extended emission.

## Key findings

- Extended [CII] halos are significantly larger than stellar and dust components.
- The [CII] and Lya halo scale lengths are consistent within uncertainties.
- Simulations match dust and stellar profiles but not the extended [CII] emission.

## Abstract

We report the discovery of 10-kpc scale [CII] 158um halos surrounding star-forming galaxies in the early Universe. We choose deep ALMA data of 18 galaxies each with a star-formation rate of ~ 10-70 Msun with no signature of AGN whose [CII] lines are individually detected at z=5.153-7.142, and conduct stacking of the [CII] lines and dust-continuum in the uv-visibility plane. The radial profiles of the surface brightnesses show a 10-kpc scale [CII] halo at the 9.2sigma level significantly extended more than the HST stellar continuum data by a factor of ~5 on the exponential-profile basis, as well as the dust continuum. We also compare the radial profiles of [CII] and Lya halos universally found in star-forming galaxies at this epoch, and find that the scale lengths agree within the 1sigma level. While two independent hydrodynamical zoom-in simulations match the dust and stellar continuum properties, the simulations cannot reproduce the extended [CII] line emission. The existence of the extended [CII] halo is the evidence of outflow remnants in the early galaxies and suggest that the outflows may be dominated by cold-mode outflows expelling the neutral gas.