Atomic carbon [C I](³P₁-³P₀) mapping of the nearby galaxy M 83

Abstract

Atomic carbon (C I) 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 C I as the tracer, we performed [C I](³P₁-³P₀) [hereinafter [C I](1-0)] mapping observations of the northern part of the nearby spiral galaxy M 83 with the Atacama Submillimeter Telescope Experiment (ASTE) telescope and compared the distributions of [C I](1-0) with CO lines [CO(1-0), CO(3-2), and ¹³CO(1-0)], H I, and infrared (IR) emission (70, 160, and 250 μm). The [C I](1-0) distribution in the central region is similar to that of the CO lines, whereas [C I](1-0) in the arm region is distributed outside the CO. We examined the dust temperature, T_dust, and dust mass surface density, Σ_dust, by fitting the IR continuum-spectrum distribution with a single-temperature modified blackbody. The distribution of Σ_dust shows a much better consistency with the integrated intensity of CO(1-0) than with that of [C I](1-0), indicating that CO(1-0) is a good tracer of the cold molecular gas. The spatial distribution of the [C I] excitation temperature, T_ex, was examined using the intensity ratio of the two [C I] transitions. An appropriate T_ex at the central, bar, arm, and inter-arm regions yields a constant [C]$/$[H₂] abundance ratio of ~7 × 10^-5 within a range of 0.1 dex in all regions. We successfully detected weak [C I](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 [C I](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), [C I](1-0) is less reliable in tracing the bulk of "cold" molecular gas in the galactic disk.