Ultra-faint [CII] emission in a redshift = 2 gravitationally-lensed
  metal-poor dwarf galaxy
  Extreme emission-line galaxies (EELGs) at redshift z=1-2 provide a unique
view of metal-poor, starburst sources that are the likely drivers of the cosmic
reionization at z$\geq6$. However, the molecular gas reservoirs of EELGs - the
fuel for their intense star-formation - remain beyond the reach of current
facilities. We present ALMA [CII] and PdBI CO(2-1) observations of a z=1.8,
strongly lensed EELG SL2S 0217, a bright Lyman-$\alpha$ emitter with a
metallicity 0.05 $Z_\odot$. We obtain a tentative (3-4$\sigma$) detection of
the [CII] line and set an upper limit on the [CII]/SFR ratio of
$\leq1\times10^6$ $L_\odot$/($M_\odot$ yr$^{-1}$), based on the synthesized
images and visibility-plane analysis. The CO(2-1) emission is not detected.
Photoionization modelling indicates that up to 80% of the [CII] emission
originates from neutral or molecular gas, although we can not rule out that the
gas is fully ionized. The very faint [CII] emission is in line with both nearby
metal-poor dwarfs and high-redshift Lyman-$\alpha$ emitters, and predictions
from hydrodynamical simulations. However, the [CII] line is 30$\times$ fainter
than predicted by the De Looze et al. [CII]-SFR relation for local dwarfs,
illustrating the danger of extrapolating locally-calibrated relations to
high-redshift, metal-poor galaxies.
