Atomic carbon [Ci](3P1-3P0) mapping of the nearby galaxy M83

Atomic carbon (Ci) has been proposed to be a global tracer of the molecular gas as a substitute for CO, however, its utility remains unproven. To evaluate the suitability of Ci as the tracer, we performed [Ci](P-3(1)-P-3(0)) [hereinafter [Ci](1-0)] mapping observations of the northern part of the nearby spiral galaxy M83 with the Atacama Submillimeter Telescope Experiment (ASTE) telescope and compared the distributions of [Ci](1-0) with CO lines [CO(1-0), CO(3-2), and (CO)-C-13(1-0)], Hi, and infrared (IR) emission (70, 160, and 250 mu m). The [Ci](1-0) distribution in the central region is similar to that of the CO lines, whereas [Ci](1-0) in the arm region is distributed outside the CO. We examined the dust temperature, T-dust, and dust mass surface density, Sigma(dust), by fitting the IR continuum-spectrum distribution with a single-temperature modified blackbody. The distribution of Sigma(dust) shows a much better consistency with the integrated intensity of CO(1-0) than with that of [Ci](1-0), indicating that CO(1-0) is a good tracer of the cold molecular gas. The spatial distribution of the [Ci] excitation temperature, T-ex, was examined using the intensity ratio of the two [Ci] transitions. An appropriate T-ex at the central, bar, arm, and inter-arm regions yields a constant [C][H-2] abundance ratio of similar to 7 x 10(-5) within a range of 0.1dex in all regions. We successfully detected weak [Ci](1-0) emission, even in the inter-arm region, in addition to the central, arm, and bar regions, using spectral stacking analysis. The stacked intensity of [Ci](1-0) is found to be strongly correlated with T-dust. Our results indicate that the atomic carbon is a photodissociation product of CO, and consequently, compared to CO(1-0), [Ci](1-0) is less reliable in tracing the bulk of "cold" molecular gas in the galactic disk.