Astronomers identified a galaxy nicknamed Misty Moons that existed when the universe was less than 500 million years old. Using gravitational lensing, the team spotted the galaxy in images from the James Webb Space Telescope. This discovery, published on Oct. 19, helps researchers understand how stars formed in the early universe.
Gravitational lensing is one of astronomy's best tricks for seeing the early universe. When a massive galaxy cluster sits in between the observer and a distant object, its gravity bends and magnifies the distant object's light, acting like a giant natural magnifying glass. Galaxies that would otherwise be too faint to spot suddenly come into view because of gravitational lensing.
Using this effect, astronomers have now identified a galaxy nicknamed Misty Moons that existed when the universe was less than 500 million years old. Located at a redshift of roughly 11 to 12, the galaxy is magnified by the gravity of a foreground galaxy cluster, which allowed researchers to reconstruct its internal structure. Their paper was published in The Astrophysical Journal on Oct. 19.
Early galaxies and their star clusters
The James Webb Space Telescope (JWST) has been able to find many galaxies from the early universe, when it was only a few hundred million years old. Some galaxies appear too bright and have a lot of nitrogen yet are very tiny. The brightness, astronomers think, may hold clues about globular clusters and star formation patterns in the early universe.
Star clusters from the early times tend to be low in heavy elements. They are also extremely dense, in some cases about 1,000 times denser than young star clusters nearby. In our local universe, researchers measure a quantity called the cluster formation efficiency (CFE) to understand what fraction of a galaxy's newly formed stars end up in bound star clusters. Some studies have shown that this efficiency correlates with star formation rate. Since early galaxies were forming stars at a high rate, their CFE might have been very high.
But examining individual star clusters in a massive galaxy so far away is tricky as these early galaxies are too small and distant to be resolved into their components. So, when a massive galaxy cluster acts as a natural lens, the distant object appears brighter and magnified. This allows astronomers to pick out individual clumps and examine their properties. That is why astronomers are searching many lensing cluster fields for more.
Magnifying lens
In this new study, a team of astronomers led by Minami Nakane of the University of Tokyo discovered a strongly magnified, clumpy galaxy at an estimated redshift of about 11 to 12. They found it in the JWST's NIRCam images of the field of galaxy cluster MACS0257-2325 photographed in 10 color filters. These were also complemented by older Hubble images taken with three filters. Nicknamed Misty Moons, it appeared as two blobs (ID1 and ID2) of the same galaxy. The images vanish at shorter wavelengths. The redshift range places the galaxy at a cosmic time of around 370 million to 410 million years after the Big Bang.
The team built two independent lens models to reconstruct the galaxy's internal structure. The models predicted five images of the galaxy. Two of its bright lensed images appear roughly 20 to 30 times brighter than they would without magnification. They also found a third potential image. The remaining two are too faint to be detected.
They smoothed the images and subtracted the blur to make the small bright spots stand out. This revealed four clumps in ID1 and three in ID2. Removing the lensing effects revealed that the two images share the same structure.
Clumpy, Misty Moons
Each clump contains about 10 million solar masses of young, dust-poor stars, around 40 to 70 million years old. Their sizes and masses resemble local nuclear star clusters and young massive clusters, and they may be proto-globular clusters. Together, these clumps were estimated to account for at least 80% of the galaxy's stellar mass, suggesting that star formation in the early universe could be concentrated in compact regions.
"The comparisons of the clumpiness among the observed and simulated galaxies across cosmic time suggest that the galaxies at higher redshift tend to be more clumpy, which is consistent with the observations of many clumpy galaxies at z > 6," the team writes in the paper. This is, however, opposite to what is seen in local dwarf galaxies.
The team found that Misty Moons forms stars intensely. Its star formation density is 100 to 10,000 times higher than in local dwarfs. According to the researchers, the high rate likely drives the clump-dominated mode.
But there are some massive local galaxies that have high star formation rates but have far fewer stars ending up in clusters. So, the team concludes that "high-z clumpy galaxies are likely distinct from the local dwarf galaxies not only in their high SFR surface densities but also in their short, bursty star formation during the early formation phase of the galaxy."
The central clumps are bright and compact enough that the team cannot exclude a contribution from an actively feeding supermassive black hole. If it is present, some of the light, and so possibly some of the inferred star formation, could come from black hole accretion rather than stars.