and 4.4 focuses on the central regions ($R\leq2$ kpc) of ALESS 49.1 and 57.1, including the uncertainty from the extended components in further analysis.

![image](uvplot_aless49_cii_fir_10+50_linear.png){height="6.5cm"} ![image](uvplot_aless57_cii_fir_10+50_linear.png){height="6.5cm"}

### How robust are the source sizes determined from the uv-plane fitting? {#subsec:mock_data}

Before analyzing our resolved \[\] and 160-$\mu$m continuum observations, we assess the reliability of the inferred source sizes. In particular, we investigate a possibility that the \[\]/160-$\mu$m emissions follow CO(3–2) surface distribution, and that the source sizes inferred in Section \[subsec:uv-plane\] are an artifact of the limited short-spacing coverage of our Band 8 observations. We estimate this uncertainty using simulated ALMA observations.

The mock observations are created as follows. First, we calculate the mean and rms scatter for the \[\] and 160-$\mu$m continuum visibilities within a given $uv$-bin. We then subtract the mean signal from the data, which gives us a $uv$-plane coverage corresponding exactly to a given observation, along with a realistic noise measurement for each baseline. We choose this approach to account for the different noise levels for each dataset, and the effect of the atmospheric lines on our \[\] data. We then inject an artificial source into the data, generating 1000 datasets with different noise realizations for each 