Mapping metals at high redshift with far-infrared lines
  Cosmic metal enrichment is one of the key physical processes regulating
galaxy formation and the evolution of the intergalactic medium (IGM). However,
determining the metal content of the most distant galaxies has proven so far
almost impossible; also, absorption line experiments at $z\sim6$ become
increasingly difficult because of instrumental limitations and the paucity of
background quasars. With the advent of ALMA, far-infrared emission lines
provide a novel tool to study early metal enrichment. Among these, the [CII]
line at 157.74 $\mu$m is the most luminous line emitted by the interstellar
medium of galaxies. It can also resonant scatter CMB photons inducing
characteristic intensity fluctuations ($\Delta I/I_{CMB}$) near the peak of the
CMB spectrum, thus allowing to probe the low-density IGM. We compute both [CII]
galaxy emission and metal-induced CMB fluctuations at $z\sim 6$ by using
Adaptive Mesh Refinement cosmological hydrodynamical simulations and produce
mock observations to be directly compared with ALMA BAND6 data ($\nu_{obs}\sim
272$ GHz). The [CII] line flux is correlated with $M_{UV}$ as
$\log(F_{peak}/\mu{\rm Jy})=-27.205-2.253\,M_{UV}-0.038\,M_{UV}^2$. Such
relation is in very good agreement with recent ALMA observations (e.g. Maiolino
et al. 2015; Capak et al. 2015) of $M_{UV}<-20$ galaxies. We predict that a
$M_{UV}=-19$ ($M_{UV}=-18$) galaxy can be detected at $4\sigma$ in $\simeq40$
(2000) hours, respectively. CMB resonant scattering can produce $\simeq\pm
0.1\,\mu$Jy/beam emission/absorptions features that are very challenging to be
detected with current facilities. The best strategy to detect these signals
consists in the stacking of deep ALMA observations pointing fields with known
$M_{UV}\simeq-19$ galaxies. This would allow to simultaneously detect both
[CII] emission from galactic reionization sources and CMB fluctuations produced
by $z\sim6$ metals.
