Fine Structure Lines Of Atomic Carbon Research Articles

Observations of the [C i] (3P1–3P0) emission toward the massive star-forming region RCW 38: Further evidence for highly-clumped density distribution of the molecular gas

Abstract We present observations of the 3P1–3P0 fine-structure line of atomic carbon using the ASTE 10m sub-mm telescope towards RCW 38, the youngest super star cluster in the Milky Way. The detected [C i] emission is compared with the CO J = 1–0 image cube presented in Fukui et al. (2016, ApJ, 820, 26) which has an angular resolution of 40″ (∼0.33 pc). The overall distribution of the [C i] emission in this cluster is similar to that of the 13CO emission. The optical depth of the [C i] emission was found to be τ = 0.1–0.6, suggesting mostly optically thin emission. An empirical conversion factor from the [C i] integrated intensity to the H2 column density was estimated as X[C i]$= 6.3 \times 10 ^{20}\:$cm−2 K−1 km−1 s (for visual extinction: AV ≤ 10 mag) and 1.4 × 1021 cm−2 K−1 km−1 s (for AV of 10–100 mag). The column density ratio of the [C i] to CO (N[C i]$/N_{\rm CO}$) was derived as ∼0.1 for AV of 10–100 mag, which is consistent with that of the Orion cloud presented in Ikeda et al. (2002, ApJ, 571, 560). However, our results cover an AV regime of up to 100 mag, which is wider than the coverage found in Orion, which reaches up to ∼60 mag. Such a high [C i]$/$CO ratio in a high-AV region is difficult to explain via the plane-parallel photodissociation region model, which predicts that this ratio is close to 0 due to the heavy shielding of the ultraviolet (UV) radiation. Our results suggest that the molecular gas in this cluster is highly clumpy, allowing deep penetration of UV radiation even at averaged AV values of 100 mag. Recent theoretical works have presented models consistent with such clumped gas distribution with a sub-pc clump size (e.g., Tachihara et al. 2018, arXiv:1811.02224).

A Survey of Atomic Carbon [C i] in High-redshift Main-sequence Galaxies

Abstract We present the first results of an Atacama Large Millimeter Array survey of the lower fine-structure line of atomic carbon [C i] in far-infrared-selected galaxies on the main sequence at z ∼ 1.2 in the COSMOS field. We compare our sample with a comprehensive compilation of data available in the literature for local and high-redshift starbursting systems and quasars. We show that the [C i] (3 P 1 → 3 P 0) luminosity correlates on global scales with the infrared luminosity , similar to low-J CO transitions. We report a systematic variation of / as a function of the galaxy type, with the ratio being larger for main-sequence galaxies than for starbursts and submillimeter galaxies at fixed . The / and / mass ratios are similar for main-sequence galaxies and for local and high-redshift starbursts within a 0.2 dex intrinsic scatter, suggesting that [C i] is a good tracer of molecular gas mass as CO and dust. We derive a fraction of of the total carbon mass in the atomic neutral phase. Moreover, we estimate the neutral atomic carbon abundance, the fundamental ingredient to calibrate [C i] as a gas tracer, by comparing and available gas masses from CO lines and dust emission. We find lower [C i] abundances in main-sequence galaxies than in starbursting systems and submillimeter galaxies as a consequence of the canonical α CO and gas-to-dust conversion factors. This argues against the application to different galaxy populations of a universal standard [C i] abundance derived from highly biased samples.

Atomic carbon as a powerful tracer of molecular gas in the high-redshift Universe: perspectives for ALMA

Abstract We use a high-resolution hydrodynamic simulation that tracks the non-equilibrium abundance of molecular hydrogen within a massive high-redshift galaxy to produce mock Atacama Large Millimeter Array (ALMA) maps of the fine-structure lines of atomic carbon, C i 1–0 and C i 2–1. Inspired by recent observational and theoretical work, we assume that C i is thoroughly mixed within giant molecular clouds and demonstrate that its emission is an excellent proxy for H2. Nearly all of the H2 associated with the galaxy can be detected at redshifts z < 4 using a compact interferometric configuration with a large synthesized beam (that does not resolve the target galaxy) in less than 4 h of integration time. Low-resolution imaging of the C i lines (in which the target galaxy is resolved into three to four beams) will detect ∼80 per cent of the H2 in less than 12 h of aperture synthesis. In this case, the resulting data cube also provides the crucial information necessary for determining the dynamical state of the galaxy. We conclude that ALMA observations of the C i 1–0 and 2–1 emission are well-suited for extending the interval of cosmic look-back time over which the H2 distributions, the dynamical masses, and the Tully–Fisher relation of galaxies can be robustly probed.

C I] 809 GHz Imaging of the NGC 6334 Complex

We present Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) observations of submillimeter emissions from the NGC 6334 complex for both the $^{12}[\mathrm{C {I}}] \; {{3\atop} \mathrm{P}}_2 \rightarrow {{3\atop} \mathrm{P}}_1$ fine-structure line of atomic carbon and the ${{12\atop} \mathrm{CO}} J = 4 \rightarrow 3$ rotational transition of CO. We detected strong $^{12}[\mathrm{C {I}}] \; {{3\atop} \mathrm{P}}_2 \rightarrow {{3\atop} \mathrm{P}}_1$ emission from the entire star-forming NGC 6334 complex, and present maps of $^{12}[\mathrm{C {I}}] \; {{3\atop} \mathrm{P}}_2 \rightarrow {{3\atop} \mathrm{P}}_1$ emission and ${{12\atop} \mathrm{CO}} J = 4 \rightarrow 3$ emission. Comparing these maps with archival ASCA data, we show that, to the south of NGC 6334, the [C I] emissions are likely to be associated with strong X-ray emissions from the FIR sources. However, the calculation using the X-ray dissociation region model shows that the majority of the [C I] emission can not arise from X-ray dissociation of the cloud. Alternatively, far-ultraviolet radiation produced in the photodissociation regions is expected to contribute to dissociate the carbon monoxide in the NGC 6334 complex.

The first-light APEX submillimeter heterodyne instrument FLASH

Development of a dual-color heterodyne instrument for use with the Atacama Pathfinder EXperiment. Commissioning of the APEX began in mid 2004, and regular science operation has been performed since July 2005. Verification of the telescope required a dualchannel receiver operating at (short) submillimeter wavelengths. It was important for the characterization of the telescope to observe at the highest possible frequency at which routine observations can be performed. For pointing, focus, and tracking verification (simultaneous) operation at lower frequencies was requested. We developed FLASH operating on two channels simultaneously – at orthogonal polarizations – in the 460 GHz and 810 GHz atmospheric windows. The system performs with a wide tuning range (420–500 GHz, 780–880 GHz) and intermediate frequency bandwidths of 2 and 4 GHz, respectively. As backends, we operate two fast-Fourier transform spectrometers (FFTS) with 2 × 1 GHz bandwidth each and a maximum of 16 384 channels. The receiver has been in continuous operation since June 2004. While first used for the telescope commissioning, since the middle of last year the instrument has served as the high-frequency workhorse on APEX. Simultaneous observations of the rotational transitions of warm carbon monoxide (J = 4–3 and J = 7–6) and of the two fine-structure lines of atomic carbon are scientifically attractive. FLASH is a principal investigator instrument, available to the APEX-user community on a collaborative basis with MPIfR. A state-of-the-art dualchannel heterodyne instrument has been developed, which made timely commissioning of the APEX possible. Most of the scientific results presented in this special issue rely on data derived with FLASH.

Atomic Carbon in Galaxies

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.