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-& micro;m-selected galaxies atz similar to 1-4 with far-infrared luminosities of L-IR similar to 10(11)-10(13 )L(circle dot), to explore how star formation varies with dust mass and redshift. We revisit Atacama Large Millimetre Array (ALMA) 870-& micro;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 similar to 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 similar to 1 mJy. Using this expanded sample, we find that submillimetre galaxies with S-870 similar to 1 mJy at z greater than or similar to 2.5 share many properties with brighter submillimetre galaxies, including high gas fractions and vigorous star formation. In contrast, similarly faint galaxies at z less than or similar to 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 similar to 1 mJy and z similar to 2.5, separating active secularly evolving discs from compact starbursts that may represent the progenitors of massive local spheroids.