The 300-pc Scale ALMA View of [CI] $^3P_1$-$^3P_0$, CO $J$=1-0, and 609
  $\mu$m Dust Continuum in A Luminous Infrared Galaxy
  We present high-quality ALMA Band 8 observations of the [CI] $^3P_1$-$^3P_0$
line and 609 $\mu$m dust continuum emission toward the nearby luminous infrared
galaxy (LIRG) IRAS F18293-3413, as well as matched resolution (300-pc scale)
Band 3 CO $J=$1-0 data, which allow us to assess the use of the [CI]
$^3P_1$-$^3P_0$ line as a total gas mass estimator. We find that the [CI] line
basically traces structures detected in CO (and dust), and a mean (median)
[CI]/CO luminosity ($L'_{\rm [CI]}$/$L'_{\rm CO}$) ratio of 0.17 (0.16) with a
scatter of 0.04. However, a pixel-by-pixel comparison revealed that there is a
radial $L'_{\rm [CI]}$/$L'_{\rm CO}$ gradient and a superlinear $L'_{\rm CO}$
vs. $L'_{\rm [CI]}$ relation (slope = 1.54 $\pm$ 0.02) at this spatial scale,
which can be explained by radial excitation and/or line opacity gradients.
Based on the molecular gas masses converted from the dust continuum emission,
we found that the CO-to-H$_2$ and [CI]-to-H$_2$ conversion factors are
relatively flat across the molecular gas disk with a median value of
3.5$^{+1.9}_{-1.3}$ and 20.7$^{+9.2}_{-4.9}$ $M_{\odot}$ (K km s$^{-1}$
pc$^2$)$^{-1}$, respectively. A non-LTE calculation yields that typical
molecular gas properties seen in nearby (U)LIRGs ($n_{\rm H_2}$ = 10$^{3-4}$
cm$^{-3}$, $T_{\rm kin}$ $\sim$ 50 K, and $X_{\rm CI}$ = (0.8-2.3) $\times$
10$^{-5}$) can naturally reproduce the derived [CI]-to-H$_2$ conversion factor.
However, we caution that a careful treatment of the physical gas properties is
required in order to measure H$_2$ gas mass distributions in galaxies using a
single [CI] line. Otherwise, a single [CI] line is not a good molecular gas
estimator in a spatially resolved manner.
