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.
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!