Atomic carbon

Abstract: We examine the fine structure lines 3 P 1 → 3 P 0 (492 GHz) and 3 P 2 → 3 P 1 (809 GHz) of neutral atomic carbon as bulk molecular gas mass tracers and find that they can be good and on many occasions better than 12 CO transitions, especially at high redshifts. The notion of C I emission as an H 2 gas mass tracer challenges the long-held view of its distribution over only a relatively narrow layer in the C II/C I/CO transition zone in far-ultraviolet (FUV) illuminated molecular clouds. Past observations have indeed consistently pointed towards a more extended C I distribution but it was only recently, with the advent of large-scale imaging of its 3 P 1 → 3 P 0 transition, that its surprising ubiquity in molecular clouds has been fully revealed. In the present work we show that under typical interstellar medium conditions such a ubiquity is inevitable because of well-known dynamic and non-equilibrium chemistry processes maintaining a significant [C]/[ 12 CO] abundance throughout giant molecular clouds during their lifetime. These processes are more intense in star-forming environments where a larger ambient cosmic ray flux will also play an important role in boosting [C]/[ 12 CO]. The resulting C I lines can be bright and effective H 2 mass tracers especially for diffuse (∼10 2 -10 3 cm -3 ) gas while in UV-intense and/or metal-poor environments their H 2 -tracing capability diminishes because of large-scale CII production but nevertheless remains superior to that of 12 CO. The best place to take full advantage of the capacity of C I to trace H 2 is not in the low-z Universe, where large atmospheric absorption at 492 and 809 GHz precludes routine observations, but at high redshifts (z ≥ 1).

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 ( 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 CI ( 3 P 2 → 3 P 1 )/CI ( 3 P 1 → Po) and the CI ( 3 P 1 → 3 P 0 )/ 12 CO(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[CI]/X[H 2 ] ∼ (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).

Abstract: We present new measurements of the ground-state fine-structure line of atomic carbon at 492 GHz in a variety of nearby external galaxies, ranging from spiral to irregular, interacting, and merging types. In comparison with CO (1-0) emission observed at the same spatial resolution, the C I (1-0) line intensity stays fairly comparable in the different environments, with an average value of the ratio of the line-integrated areas in K km s-1 of C I (1-0)/CO (1-0) = 0.2 ± 0.2. However, some variations can be found within galaxies or between galaxies. Relative to CO lines (J = 2-1, 3-2, 4-3), C I (1-0) is weaker in galactic nuclei but stronger in disks, particularly outside star-forming regions. Also, in NGC 891, the C I (1-0) emission follows the dust continuum emission at 1.3 mm extremely well along the full length of the major axis where molecular gas is more abundant than atomic gas. Atomic carbon therefore appears to be a good tracer of molecular gas in external galaxies, possibly more reliable than CO. Atomic carbon can contribute significantly to the thermal budget of interstellar gas. The cooling due to C and CO are of the same order of magnitude for most galaxies. However, CO is generally a more important coolant in starburst galaxies. Cooling due to C and CO amounts typically to 2 × 10-5 of the far-IR (FIR) continuum, or 5% of the C II line. However, C and CO cooling reaches ~30% of the gas total in ultraluminous infrared galaxies such as Arp 220, where C II is abnormally faint. Together with C II/FIR, the emissivity ratio C I (1-0)/FIR can be used as a measure of the nonionizing UV radiation field in galaxies. The plots of C II/C I or C II/FIR versus C I/FIR show good correlations, in agreement with photodissociation region (PDR) models, except for two remarkable galaxies, Arp 220 and Mrk 231, where high opacities of the C II line and possibly the dust thermal emission may be factors reducing the C II strength below the predictions of the current PDR models.

Abstract: We have imaged CO(J = 7→6) and C i(^3P_2→^3P_1) emission in the host galaxy of the z = 6.42 quasar SDSS J114816.64+525150.3 (hereafter J1148+5251) through observations with the Plateau de Bure Interferometer. The region showing CO(J = 7→6) emission is spatially resolved, and its size of 5 kpc is in good agreement with earlier CO(J = 3→2) observations. In combination with a revised model of the collisional line excitation in this source, this indicates that the highly excited molecular gas traced by the CO J = 7→6 line is subthermally excited (showing only 58% ± 8% of the CO J = 3→2 luminosity), but not more centrally concentrated. We also detect Ci(^3P_2→^3P_1) emission in the host galaxy of J1148+5251, but the line is too faint to enable a reliable size measurement. From the C i(^3P_2→^3P_1) line flux, we derive a total atomic carbon mass of M_(Ci) = 1.1 ×10^7 M_⊙, which corresponds to ~5 × 10^(−4) times the total molecular gas mass. We also searched for H_2O(J_(K_aK_c) = 212→101) emission, and obtained a sensitive line luminosity limit of L'_(H_2O) < 4.4 × 10^9 K km s^(−1) pc^2, i.e., <15% of the CO(J = 3→2) luminosity. The warm, highly excited molecular gas, atomic gas and dust in this quasar host at the end of cosmic reionization maintain an intense starburst that reaches surface densities as high as predicted by (dust opacity) Eddington limited star formation over kiloparsec scales.