A 158 μm [C II] LINE SURVEY OF GALAXIES AT z∼ 1-2: AN INDICATOR OF STAR FORMATION IN THE EARLY UNIVERSE

Abstract

We have detected the 158 μm [C ii ] line from 12 galaxies at z ∼ 1–2. This is the first survey of this important star formation tracer at redshifts covering the epoch of maximum star formation in the universe and quadruples the number of reported high- z [C ii ] detections. The line is very luminous, between <0.024% and 0.65% of the far-infrared (FIR) continuum luminosity of our sources, and arises from photodissociation regions on molecular cloud surfaces. An exception is PKS 0215+015, where half of the [C ii ] emission could arise from X-ray-dominated regions near the central active galactic nucleus (AGN). The L [C ii ] / L FIR ratio in our star formation-dominated systems is ∼8 times larger than that of our AGN-dominated systems. Therefore this ratio selects for star formation-dominated systems. Furthermore, the L [C ii ] / L FIR and L [C ii ] / L (CO(1–0)) ratios in our star-forming galaxies and nearby starburst galaxies are the same, so that luminous star-forming galaxies at earlier epochs ( z ∼ 1–2) appear to be scaled-up versions of local starbursts entailing kiloparsec-scale starbursts. Most of the FIR and [C ii ] radiation from our AGN-dominated sample (excepting PKS 0215+015) also arises from kiloparsec-scale star formation, but with far-UV radiation fields ∼8 times more intense than in our star formation-dominated sample. We speculate that the onset of AGN activity stimulates large-scale star formation activity within AGN-dominated systems. This idea is supported by the relatively strong [O iii ] line emission, indicating very young stars, that was recently observed in high- z composite AGN/starburst systems. Our results confirm the utility of the [C ii ] line, and in particular, the L [C ii ] / L (FIR) and L [C ii ] / L CO(1–0) ratios as tracers of star formation in galaxies at high redshifts.