ALMA as a Redshift Machine: Using [CII] to Efficiently Confirm Galaxies
  in the Epoch of Reionization
  The [CII]$_{158\mu m}$ line has long been proposed as a promising line to
spectroscopically confirm galaxies in the epoch of reionization. In this paper
we present the results of new ALMA observations spectral scanning for [CII] in
six particularly luminous Lyman Break Galaxies at $z\sim7$. The six sources
were drawn from a sample of bright $z\sim7$ galaxies identified using the
wide-area optical, near-IR, and Spitzer/IRAC data over the COSMOS/UltraVISTA
field and were targeted on the basis of tight constraints on their redshifts
from their IRAC [3.6]-[4.5] colors. We detect significant ($>9\sigma$) [CII]
lines in three of our six targets ($50\%$) co-spatial with the rest-$UV$
emission from the ground/space-based near-IR imaging. The luminosities of the
[CII] lines lie in the range $5.6$ to $8.8\times10^{8}L_{\odot}$, consistent
with the local [CII]-SFR relation. Meanwhile, their
[CII]/$L_{IR}\sim1-3\times10^{-3}$ ratios are slightly elevated compared to
local (U)LIRGS. This could be due to lower dust-to-gas or dust-to-metal ratios.
We also find that our sources display a large kinematic diversity, with one
source showing signs of rotation, one source a likely major merger and one
dispersion dominated source that might contain a bright star-forming clump. Our
results highlight the effectiveness of spectral scans with ALMA in
spectroscopically confirming luminous galaxies in the epoch of reionization,
something that is being be applied on a significantly larger sample in the
on-going REBELS large program.
