A Survey of Atomic Carbon at High Redshift

We present a survey of atomic carbon (C I) emission in high-redshift (z > 2) submillimeter galaxies and quasar host galaxies. Sensitive observations of the C I (3 P 1 → 3 P 0) and C I (3 P 2 → 3 P 1) lines have been obtained at the IRAM Plateau de Bure interferometer and the IRAM 30 m telescope. A total of 16 C I lines have been targeted in 10 sources, leading to a total of 10 detected lines—this doubles the number of C I observations at high redshift to date. We include previously published C I observations (an additional five detected sources) in our analysis. Our main finding is that the C I properties of the high-redshift galaxies studied here do not differ significantly from what is found in low-redshift systems, including the Milky Way. The C I (3 P 2 → 3 P 1)/C I (3 P 1 → 3 P 0) and the C I (3 P 1 → 3 P 0)/12CO(3-2) line luminosity (L') ratios change little in our sample, with respective ratios of 0.55 ± 0.15 and 0.32 ± 0.13. The C I lines are not an important contributor to cooling of the molecular gas (average L C I /L FIR ~ (7.7 ± 4.6) × 10-6). We derive a mean carbon excitation temperature of 29.1 ± 6.3 K, broadly consistent with dust temperatures derived for high-redshift star-forming systems, but lower than gas temperatures typically derived for starbursts in the local universe. The carbon abundance of XC I/XH2 ~ (8.4±3.5)×10-5 is of the same order as found in the Milky Way and nearby galaxies. This implies that the high-z galaxies studied here are significantly enriched in carbon on galactic scales, even though the look-back times are considerable (the average redshift of the sample sources corresponds to an age of the universe of ~2 Gyr).