James Webb Telescope reveals structure of ancient star-forming galaxy in detail

The galaxy, GN20, is over 12 billion light-years away, making it one of the oldest galaxies ever seen with such detail and active star formation.
John Loeffler
An ancient galaxy
An ancient galaxy


The James Webb Space Telescope just spotted the stellar structure of one of the oldest-known galaxies in the universe in such detail that it could even see the spin of its proto-spiral disk.

The galaxy in question, GN20, is one of the most luminous early galaxies we know of, forming only about 1.5 billion years after the Big Bang, and until now it has been shrouded in hot dust and mystery. That was until Luis Colina and colleagues of the Spanish Astrobiology Center in Madrid, Spain, turned to the James Webb Space Telescope (JWST) for help peeling back the veil that surrounded it.

Ancient galaxies like GN20 are special for a lot of reasons but primarily for their opacity owing to a greater volume of dust and gas contained in a relatively confined space, since the density of such material in the early universe was much greater than it is now after 13-plus billion years of intergalactic expansion everything out.

But dust is very hard for light to penetrate under the best of circumstances, with dust clouds blocking out visible and ultraviolet light with ease, while infrared light has a much easier time breaking through. In the case of dusty star-forming galaxies (DSFG) from the early universe, like GN20, the problem is even worse, since the highly active star formation behind the dust heats the dust up, which then emits infrared light of its own, making it harder to differentiate details in infrared light.

The JWST, however, was able to differentiate enough of the infrared spectrum using its mid-infrared instrument (MIRI), that the stellar structure of GN20 was able to be discerned for the first time, and what the researchers found — the details of which have been published to the arXive preprint server — was remarkable.

First, they were able to put the rate of star formation for the galaxy at a robust 1,860 solar masses per year, compared with just six to seven new solar masses a year for the Milky Way. The 45,600-light-year-wide galaxy also has an observable motion consistent with a fast, rotating disk.

What this tells us about galaxy formation in the early universe

James Webb Telescope reveals structure of ancient star-forming galaxy in detail
A composite image of galaxy GN20

While the new findings are fascinating in themselves, the observations are also important to understanding how massive quiescent galaxies at intermediate redshift might have formed. These galaxies are very common in the universe and are characterized by a slowly rotating spheroid structure with little to no new star formation.

It is thought that these galaxies are older than more active spiral galaxies with active star formation and are the product of galaxy mergers and interaction of DSFGs in the early universe that strip away molecular gases needed for star formation while slowing their rotations to produce more "blob-like" shapes.

The observations of GN20 give evidence of this in its discernible galactic nucleus and the stellar envelope around it. The galaxy's center of mass, which under normal circumstances would be roughly within the nucleus, is offset from the nucleus by about 3,300 light-years. Given the location of GN20 in a galaxy-dense proto-cluster, this shift of its centroid to such an extent is likely the product of a recent galaxy merger or interaction that has shifted its mass off-center.

Accordingly, the researchers conclude that GN20 has all of the prerequisites needed to evolve into a massive quiescent galaxy. Given its extreme age, it probably already has.

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