Spatially-resolved relation between [CI] $^{3}P_{1}$-$^{3}P_{0}$ and
  $^{12}$CO (1-0) in Arp 220
  We present $\sim$0."3 (114 pc) resolution maps of [CI]
$^{3}P_{1}$-$^{3}P_{0}$ (hereafter [CI] (1-0)) and $^{12}$CO (1-0) obtained
toward Arp 220 with the Atacama Large Millimeter/submillimeter Array. The
overall distribution of the [CI] (1-0) emission is consistent with the CO
(1-0). While the [CI] (1-0) and CO (1-0) luminosities of the system follow the
empirical linear relation for the unresolved ULIRG sample, we find a sublinear
relation between [CI] (1-0) and CO (1-0) using the spatially-resolved data. We
measure the [CI] (1-0)/CO (1-0) luminosity ratio per pixel in star-forming
environments of Arp 220 and investigate its dependence on the CO (3-2)/CO (1-0)
ratio ($R_{\rm CO}$). On average, the [CI] (1-0)/CO (1-0) luminosity ratio is
almost constant up to $R_{\rm CO} \simeq 1$ and then increases with $R_{\rm
CO}$. According to the radiative transfer analysis, a high CI/CO abundance
ratio is required in regions with high [CI] (1-0)/CO (1-0) luminosity ratios
and $R_{\rm CO} > 1$, suggesting that the CI/CO abundance ratio varies at
$\sim$100 pc scale in Arp 220. The [CI] (1-0)/CO (1-0) luminosity ratio depends
on multiple factors and may not be straightforward to interpret. We also find
the high-velocity components traced by [CI] (1-0) in the western nucleus,
likely associated with the molecular outflow. The [CI] (1-0)/CO (1-0)
luminosity ratio in the putative outflow is 0.87 $\pm$ 0.28, which is four
times higher than the average ratio of Arp 220. While there is a possibility
that the [CI] (1-0) and CO (1-0) emission traces different components, we
suggest that the high line ratios are likely because of elevated CI/CO
abundance ratios based on our radiative transfer analysis. A CI-rich and
CO-poor gas phase in outflows could be caused by the irradiation of the cosmic
rays, the shock heating, and the intense radiation field.
