2022PASJ...74..407U

We present ∼0."3 (114 pc) resolution maps of [C I] ³P₁-³P₀ (hereafter [C I] (1-0)) and ¹²CO (1-0) obtained toward Arp 220 with the Atacama Large Millimeter/submillimeter Array. The overall distribution of the [C I] (1-0) emission is consistent with the CO (1-0). While the [C I] (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 [C I] (1-0) and CO (1-0) using the spatially-resolved data. We measure the [C I] (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_CO). On average, the [C I] (1-0)/CO (1-0) luminosity ratio is almost constant up to R_CO ≃ 1 and then increases with R_CO. According to the radiative transfer analysis, a high C I/CO abundance ratio is required in regions with high [C I] (1-0)/CO (1-0) luminosity ratios and R_CO > 1, suggesting that the C I/CO abundance ratio varies at ∼100 pc scale in Arp 220. The [C I] (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 [C I] (1-0) in the western nucleus, likely associated with the molecular outflow. The [C I] (1-0)/CO (1-0) luminosity ratio in the putative outflow is 0.87 ± 0.28, which is four times higher than the average ratio of Arp 220. While there is a possibility that the [C I] (1-0) and CO (1-0) emission traces different components, we suggest that the high line ratios are likely to be because of elevated C I/CO abundance ratios based on our radiative transfer analysis. A C I-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.