2011ApJ...730...18W

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 P₁ -> ³ P₀) and C I (³ P₂ -> ³ P₁) 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 P₂ -> ³ P₁)/C I (³ P₁ -> ³ P₀) and the C I (³ P₁ -> ³ P₀)/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_/LFIR_∼ (7.7 ±4.6)x10^–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[H₂] ∼ (8.4±3.5)x10^–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).