We present a survey of atomic carbon ( C I ) emission in high–redshift ( z > 2 ) submillimeter galaxies ( SMGs ) and quasar host galaxies ( QSOs ) .
Sensitive observations of the C I ( ^ { 3 } P _ { 1 } \to ^ { 3 } P _ { 0 } ) and C I ( ^ { 3 } P _ { 2 } \to ^ { 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 5 detected sources ) in our analysis .
Our main finding is that the C I properties of high–redshift systems do not differ significantly from what is found in low–redshift systems , including the Milky Way .
The C I ( ^ { 3 } P _ { 2 } \to ^ { 3 } P _ { 1 } ) /C I ( ^ { 3 } P _ { 1 } \to ^ { 3 } P _ { 0 } ) and the C I ( ^ { 3 } P _ { 1 } \to ^ { 3 } P _ { 0 } ) / ^ { 12 } CO ( 3–2 ) line luminosity ( L ^ { \prime } ) ratios change little in our sample , with respective ratios of 0.55 \pm 0.15 and 0.32 \pm 0.13 .
The C I lines are not an important contributor to cooling of the molecular gas ( average L _ { { C } { \small I } } / L _ { FIR } \sim ( 7.7 \pm 4.6 ) \times 10 ^ { -6 } ) .
We derive a mean carbon excitation temperature of 29.1 \pm 6.3 K , broadly consistent with dust temperatures derived for high–redshift starforming systems , but lower than gas temperatures typically derived for starbursts in the local universe .
The carbon abundance of X [ C I ] /X [ H _ { 2 } ] \sim ( 8.4 \pm 3.5 ) \times 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 \sim 2 Gyr ) .