Atomic Carbon [CI]${(}^3{P}_1{-}^3{P}_0)$ Mapping of the Nearby Galaxy M83

Abstract

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](1-0)) mapping observations of the northern part of the nearby spiral galaxy M83 with the ASTE telescope and compared the distributions of [CI](1-0) with CO lines (CO(1-0), CO(3-2), and ${}^{13}$CO(1-0)), HI, and infrared (IR) emission (70, 160, and 250$\hat{I}¼$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, $T_{\mathrm{dust}}$, and dust mass surface density, $\hat{I}{\pounds }_{\mathrm{dust}}$, by fitting the IR continuum-spectrum distribution with a single-temperature modified blackbody. The distribution of $\hat{I}{\pounds }_{\mathrm{dust}}$ 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, $T_{\mathrm{ex}}$, was examined using the intensity ratio of the two [CI] transitions. An appropriate $T_{\mathrm{ex}}$ 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](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 $T_{\mathrm{dust}}$. 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.

14 pages, 11 figures, Accepted for publication in PASJ