Extending the ALMA survey of the SCUBA-2 CLS UDS field: tracing the obscured formation of spheroids across <i>z</i>  ∼ 1–4

ABSTRACT We investigate the properties of 870-$\mu$m-selected galaxies at z $\sim$ 1–4 with far-infrared luminosities of $L_{\rm IR}$ $\sim$ 10$^{11}$–10$^{13}$ L$_\odot$, to explore how star formation varies with dust mass and redshift. We revisit Atacama Large Millimetre Array (ALMA) 870-$\mu$m continuum maps from the ALMA/SCUBA-2 UDS (AS2UDS) survey, lowering the detection threshold from 4.3 $\sigma$ to 3.1 $\sigma$ to increase sensitivity to sources with $S_{870}$ $\sim$ 1 mJy. To minimize contamination from noise peaks, we match detections to K-selected galaxies and apply photometric-redshift and near-infrared colour criteria. This yields 84 sources in the extended AS2UDS survey (AS2UDSx) with $S_{870}$ $=$ 0.3–2.2 mJy, doubling the sample at $S_{870}$ $\sim$ 1 mJy. Using this expanded sample, we find that submillimetre galaxies with $S_{870}$ $\sim$ 1 mJy at z $\gtrsim$ 2.5 share many properties with brighter submillimetre galaxies, including high gas fractions and vigorous star formation. In contrast, similarly faint galaxies at z $\lesssim$ 2.5 exhibit lower gas fractions, shorter depletion times, and morphologies from James Webb Space Telescope imaging that show less structured dust obscuration, resembling less-active field galaxies. We suggest that these differences reflect the stability of their gas discs. Higher-redshift and brighter systems can maintain dense, globally unstable gas reservoirs through sustained accretion, fuelling compact obscured starbursts. Lower-redshift, fainter galaxies may instead host more stable discs with more extended dust emission. This points to a division around $S_{870}$ $\sim$ 1 mJy and z $\sim$ 2.5, separating active secularly evolving discs from compact starbursts that may represent the progenitors of massive local spheroids.