Ultra-faint [CII] emission in a redshift = 2 gravitationally-lensed metal-poor dwarf galaxy

Abstract

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$\ge 6$. 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-$\hat{I}\pm$ emitter with a metallicity 0.05 $Z_{\odot }$. We obtain a tentative (3-4$\ddot{I}ƒ$) detection of the [CII] line and set an upper limit on the [CII]/SFR ratio of $\le 1\times 10^6$ $L_{\odot }$/($M_{\odot }$ 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-$\hat{I}\pm$ 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.

Corrected to match the published version. We thank Y. Harikane and I. Smail for their comments on the previous arXiv version