Atomic Carbon [CI]${(}^3{P}_1{-}^3{P}_0)$ 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]${(}^3{P}_1{-}^3{P}_0)$ (hereinafter CI) mapping observations of the northern part of the nearby spiral galaxy M83 with the ASTE telescope and compared the distributions of CI with CO lines (CO(1-0), CO(3-2), and ${}^{13}$CO(1-0)), HI, and infrared (IR) emission (70, 160, and 250$\mu$m). The CI distribution in the central region is similar to that of the CO lines, whereas CI in the arm region is distributed outside the CO. We examined the dust temperature, $T_{\mathrm{d}\mathrm{u}\mathrm{s}\mathrm{t}}$, and dust mass surface density, ${\mathrm{\Sigma }}_{\mathrm{d}\mathrm{u}\mathrm{s}\mathrm{t}}$, by fitting the IR continuum-spectrum distribution with a single-temperature modified blackbody. The distribution of ${\mathrm{\Sigma }}_{\mathrm{d}\mathrm{u}\mathrm{s}\mathrm{t}}$ shows a much better consistency with the integrated intensity of CO(1-0) than with that of CI, indicating that CO(1-0) is a good tracer of the cold molecular gas. The spatial distribution of the [CI] excitation temperature, $T_{\mathrm{e}\mathrm{x}}$, was examined using the intensity ratio of the two [CI] transitions. An appropriate $T_{\mathrm{e}\mathrm{x}}$ at the central, bar, arm, and inter-arm regions yields a constant [C]/[H${}_2$] abundance ratio of $\sim 7\times {10}^{-5}$ within a range of 0.1 dex in all regions. We successfully detected weak CI 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 is found to be strongly correlated with $T_{\mathrm{d}\mathrm{u}\mathrm{s}\mathrm{t}}$. Our results indicate that the atomic carbon is a photodissociation product of CO, and consequently, compared to CO(1-0), CI is less reliable in tracing the bulk of “cold” molecular gas in the galactic disk.