A Survey of Atomic Carbon at High Redshift

TL;DR

This survey of atomic carbon emission in high-redshift galaxies reveals that their CI properties are similar to local galaxies, indicating early chemical enrichment and providing insights into molecular gas cooling and excitation at cosmic noon.

Contribution

First comprehensive high-redshift CI survey doubling previous observations, showing similar properties to local systems and informing on molecular gas conditions in early galaxies.

Findings

CI properties are consistent with low-redshift systems

CI lines contribute minimally to gas cooling

High-z galaxies are enriched in carbon comparable to local galaxies

Abstract

We present a survey of atomic carbon (CI) emission in high-redshift (z>2) submillimeter galaxies (SMGs) and quasar host galaxies (QSOs). Sensitive observations of the CI(3P_1->3P_0) and CI(3P_2->3P_1) lines have been obtained at the IRAM Plateau de Bure interferometer and the IRAM 30m telescope. A total of 16 CI lines have been targeted in 10 sources, leading to a total of 10 detected lines --- this doubles the number of CI observations at high redshift to date. We include previously published CI observations (an additional 5 detected sources) in our analysis. Our main finding is that the CI properties of the studied high-redshift systems do not differ significantly from what is found in low-redshift systems, including the Milky Way. The CI(3P_2->3P_1)/CI(3P_1->3P_0) and the CI(3P_1->3P_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 CI 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 starforming systems, but lower than gas temperatures typically derived for starbursts in the local universe. The carbon abundance of X_CI/X_H2~8.4+/-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 ~2 Gyr).