Astronomers using the James Webb Space Telescope (JWST) have studied a massive galaxy nicknamed "Leviathan". It is seen as it was when the universe was about 1.5 billion years old. It turned out that its stars are widely distributed rather than gathered in a dense core. A paper on this was posted on the arXiv preprint server on October 19.
"Leviathan" is a submillimeter galaxy - a very bright object hidden by dust. Dust absorbs the ultraviolet and visible light of young stars and re-emits it in the submillimeter range, which telescopes detect. Such galaxies are thought to be possible precursors of massive quiescent galaxies - today's giant elliptical galaxies, where star formation has already ceased.
It was previously known that massive quiescent galaxies observed when the universe was 11.5-12.5 billion years old look very compact: half of their light comes from a region only 0.2-1 kiloparsec in radius. This means their ancestors must have rapidly formed a dense stellar core. However, dust emission shows only regions of current star formation, not the distribution of already existing stars. JWST made it possible for the first time to see the faint glow of old stars.
A team led by William M. Baker of the University of Copenhagen measured the mass and redshift of "Leviathan". Redshift is a measure of how much light has been stretched by the expansion of the universe; for this galaxy it was z = 4.0514. The mass was estimated by comparing data in 31 wavelength bands with models of stars and dust. It turned out to be about 219 billion solar masses, making "Leviathan" one of the most massive spectroscopically confirmed galaxies at redshift greater than 4.
About 410 solar masses of new stars form in "Leviathan" each year. According to the authors, this places it at the upper edge of the main sequence of star formation for z around 4.
The scientists measured three radii: those containing half the light, half the stellar mass and half the dust emission. The half-light radius was 4.3 kiloparsecs, and the half-mass radius was 3.1 kiloparsecs. This means the mass is concentrated more compactly than the light: this is typical of "inside-out" growth, in which the center forms first and looks older and dustier, while the outskirts are younger and brighter. Star formation is distributed across the disk, and the dust is only slightly more compact than the stars (about 2.2-2.5 kiloparsecs).
Calculations showed that even if all the gas turned into stars, the galaxy would shrink only to a radius of about 2.8 kiloparsecs. The density of its center is lower than that of compact quiescent galaxies. Therefore "Leviathan" most likely will not become a compact quiescent galaxy. If star formation continues with its current spatial distribution, it is unlikely to acquire the compact structure characteristic of massive quiescent galaxies at 3 < z < 5, the researchers write. Instead, it may turn into an extended disk-like quiescent galaxy. The work shows that dusty galaxies in the early universe can follow different evolutionary paths.