Ultrafaint [C ii] 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 ≥ 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 Ly α emitter with a metallicity 0.05 Z ⊙ . We obtain a tentative (∼3 σ –4 σ ) detection of the [C ii ] line and set an upper limit on the [C ii ]/SFR (star-forming rate) ratio of ≤1 × 10 6 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 Ly α 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.