Atomic Carbon [CI]$(^3P_1-^3P_0)$ 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]$(^3P_1-^3P_0)$
(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$ \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,
$T_{\rm dust}$, and dust mass surface density, $\Sigma_{\rm dust}$, by fitting
the IR continuum-spectrum distribution with a single-temperature modified
blackbody. The distribution of $\Sigma_{\rm dust}$ 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, $T_{\rm ex}$, was
examined using the intensity ratio of the two [CI] transitions. An appropriate
$T_{\rm ex}$ at the central, bar, arm, and inter-arm regions yields a constant
[C]/[H$_2$] abundance ratio of $\sim7 \times 10^{-5}$ 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 $T_{\rm dust}$. 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.
