In the distant universe, when it was just a couple of billion years old, many galaxies resembled boiling cauldrons: gas clumped so quickly that stars ignited one after another, and dust heated up and shone brightly in the submillimeter range. Such objects are called galaxies with active star formation, and they are thought to be the ancestors of today's massive elliptical galaxies. But astronomers long wondered whether all such galaxies are the same or whether there are different growth paths among them.
New work, based on a reanalysis of images from the ALMA telescope, shows that it all depends on distance from us: that is, on the age of the universe. The researchers took maps of a patch of sky where bright dusty galaxies had previously been found and lowered the detection threshold — now fainter objects also entered the sample. The sample doubled, and this revealed an important pattern. Galaxies with the same observed brightness but located farther away (i.e., existing earlier) are compact, dense systems where star formation is intense and concentrated in the center. Meanwhile, equally bright but closer galaxies (existing later) are extended disks where stars form quietly, spread across the entire area.
The reason for this division, according to the authors, lies in the stability of the gas disk. A distant galaxy receives a lot of gas from its surroundings — accretion is powerful, the disk becomes massive but unstable, and dense clumps easily form and collapse into stars. Such a galaxy flares brightly but lives a turbulent life briefly. A closer galaxy no longer receives as much fresh gas, its disk stabilizes, and star formation stretches over billions of years, becoming calm and uniform. The boundary lies roughly at a redshift of z~2, when the universe was about three billion years old.
The outcome of the work is a natural boundary between two types of galaxies. Faint and nearby ones are the most active of the 'quiet' disk galaxies, which grow gradually. Bright and distant ones are compact starbursts in massive unstable disks, which likely evolve over time into dense spheroidal galaxies populated mainly by old stars.
as written by the authors arXiv:2604.18700
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.
The ALMA telescope sits high in Chile's Atacama Desert, where the air is so dry that it barely absorbs submillimeter radiation — which is why it can see dust in distant galaxies.
In Isaac Asimov's novel 'The Gods Themselves', matter flows between universes, feeding stars — what a metaphor for gas accretion that makes distant galaxies flare up!