A SURVEY OF ATOMIC CARBON AT HIGH REDSHIFT

Abstract

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 (3P1 → 3P0) and C i (3P2 → 3P1) 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 (3P2 → 3P1)/C i (3P1 → 3P0) and the C i (3P1 → 3P0)/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 LC i/LFIR ∼ (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 X[C i]/X[H2] ∼ (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).