apart from imaging was performed using [Casa]{} versions 4.7 and 5.0 [@McMullin2007].

The spectral structure of the \[\] line overlaps with atmospheric absorption features at 481.2 and 481.6 GHz. Consequently, the noise level in the affected channels is raised by a factor of $\sim$2. This issue is particularly severe for ALESS 49.1. Another atmospheric feature is located in a line-free SPW at 496.35 GHz; here, the affected channels were flagged to improve the continuum SNR. As will be outlined in Section \[subsec:imaging\], we use a channel-dependent threshold for the deconvolution process to avoid introducing noise features from the affected channels.

To increase the signal-to-noise ratio, each line was split into frequency bins $\sim$120 km s$^{-1}$ wide. Additionally, we time-averaged the data using a 30-second bin; this corresponds to an average intensity loss of $<$0.5 per cent at 5 arcsec from the phase-tracking centre, which we consider negligible[^4]. The time-bin size was chosen so as to prevent significant time-averaging smearing. The two linear polarizations were combined into the Stokes intensity $I$.

ALMA Band 3 observations
------------------------

ALMA Band 3 observations of the CO(3–2) and rest-frame 1.0-mm continuum in ALESS 49.1 and 57.1 were presented by @CR18. These consisted of Cycle 2 