Ultrafaint [C II] 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 ≥ 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 [C II] and PdBI CO(2-1) observations of the z = 1.8, strongly lensed EELG SL2S 0217, a bright Lyα emitter with a metallicity 0.05 Z_⊙. We obtain a tentative (∼3σ-4σ) detection of the [C II] line and set an upper limit on the [C II]/SFR (star-forming rate) ratio of ≤1 × 10⁶ L_⊙/(M_⊙ yr^-1), based on the synthesized images and visibility-plane analysis. The CO(2-1) emission is not detected. Photoionization modeling indicates that up to 80% of the [C II] emission originates from neutral or molecular gas, although we cannot rule out that the gas is fully ionized. The very faint [C II] emission is in line with both nearby metal-poor dwarfs and high-redshift Lyα emitters, and predictions from hydrodynamical simulations. However, the [C II] line is 30× fainter than predicted by the De Looze et al. [C II]-SFR relation for local dwarfs, illustrating the danger of extrapolating locally calibrated relations to high-redshift, metal-poor galaxies.