Extending the ALMA survey of the SCUBA-2 CLS UDS field: Tracing the obscured formation of spheroids across z ~ 1–4

We investigate the properties of 870-μm-selected galaxies at z#x00A0;~ 1–4 with far-infrared luminosities of LIR ~ 1011–1013 L⊙, to explore how star formation varies with dust mass and redshift. We revisit ALMA 870-μm continuum maps from the ALMA/SCUBA-2 UDS (AS2UDS) survey, lowering the detection threshold from 4.3 σ to 3.1 σ to increase sensitivity to sources with S870 ~ 1 mJy. To minimise 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 S870 = 0.3–2.2 mJy, doubling the sample at S870 ~ 1 mJy. Using this expanded sample, we find that submillimetre galaxies with S870 ~ 1 mJy at z#x00A0;≳ 2.5 share many properties with brighter submillimetre galaxies, including high gas fractions and vigorous star formation. In contrast, similarly faint galaxies at z#x00A0;≲ 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 S870 ~ 1 mJy and z#x00A0;~ 2.5, separating active secularly evolving discs from compact starbursts that may represent the progenitors of massive local spheroids.