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