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

TL;DR

This study extends the ALMA survey of submillimeter galaxies at z~1-4, revealing a shift in galaxy properties around S870~1mJy and z~2, indicating different star formation modes and galaxy evolution pathways.

Contribution

It enlarges the sample of submm galaxies by lowering detection thresholds and analyzes how galaxy properties vary with dust mass and redshift, highlighting a division in star formation processes.

Findings

Galaxies at z>~2.5 with S870~1mJy are similar to brighter, active populations.

Galaxies at z<~2.5 with S870~1mJy have lower gas fractions and less structured dust.

A natural division exists around S870~1mJy and z~2, indicating different star formation regimes.

Abstract

We investigate the properties of 870-um selected galaxies at z~1-4 with FIR luminosities of LIR~1e11-1e13Lo, encompassing systems that dominate obscured activity at the peak of cosmic star formation, to identify variations in star-formation processes as a function of dust mass and redshift. We revisit ALMA 870-um continuum maps from the ALMA/SCUBA-2 UDS (AS2UDS) survey, lowering the source selection threshold from 4.3 sigma to 3.1 sigma to enlarge the sample with S870~1mJy. To reduce contamination from noise peaks, we match submm sources to a K-selected galaxy sample and apply cuts on photometric redshift and near-infrared (H-K) colour. This yields 84 sources in our extended AS2UDS survey, AS2UDSx, with S870=0.3-2.2mJy, doubling the sample at S870~1mJy relative to the original study. Using this expanded sample, we find that submm galaxies with S870~1mJy at z>~2.5 share properties with brighter, more active populations, while those at z<~2.5 are distinct, with lower gas fractions, shorter depletion times, and stellar morphologies from JWST imaging that show less structured dust obscuration, resembling less-active field galaxies. This indicates a shift in the characteristics of 870-um-selected galaxies at S870~1mJy and z~2, likely driven by the stability of their gas discs. Brighter and higher-z galaxies can sustain dense, globally unstable discs through efficient gas accretion, powering compact obscured starbursts. In contrast, fainter systems at z<~2.5 lack this accretion, leading to more stable discs and more extended dust continuum emission. This suggests a natural division around S870~1mJy and z~2: lower-z, fainter sources represent the most active secularly-driven extended star-forming discs, while similar and brighter, higher-z submm galaxies form a distinct population of compact starbursts within massive, unstable, gas-rich discs, consistent with progenitors of massive spheroids.