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 $leq1times10^6$ $L_odot$/($M_odot$ 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.
Download