The Absence of [CII] 158$\mu$m Emission in Spectroscopically-Confirmed
  Galaxies at $z>8$
  The scatter in the relationship between the strength of [CII] 158$\mu$m
emission and the star formation rate at high-redshift has been the source of
much recent interest. Although the relationship is well-established locally,
several intensely star-forming galaxies have been found whose [CII] 158$\mu$m
emission is either weak, absent or spatially offset from the young stars. Here
we present new ALMA data for the two most distant, gravitationally-lensed and
spectroscopically-confirmed galaxies, A2744\_YD4 at $z=$8.38 and MACS1149\_JD1
at $z=$9.11, both of which reveal intense [OIII] 88$\mu$m emission. In both
cases we provide stringent upper limits on the presence of [CII] 158$\mu$m with
respect to [OIII] 88$\mu$m. We review possible explanations for this apparent
redshift-dependent [CII] deficit in the context of our recent hydrodynamical
simulations. Our results highlight the importance of using several emission
line diagnostics with ALMA to investigate the nature of the interstellar medium
in early galaxies.
