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)/(CO)-C-12(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-CI/L-FIR (7.7 +/- 4.6) x 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 X[C I]/X[H-2] similar to (8.4 +/- 3.5) x 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 similar to(2) Gyr).