Webb Confirms 'Leviathan,' a Galaxy of 219 Billion Suns That Defies a Key Growth Theory
Seen 1.5 billion years after the Big Bang, the dusty giant has its stars spread wide instead of packed tight, and is unlikely to become a compact elliptical.
Astronomers using the James Webb Space Telescope have confirmed a massive, dust-rich galaxy called Leviathan in the early universe, and its structure does not fit a popular picture of how giant galaxies grow. Seen when the universe was roughly 1.5 billion years old, Leviathan has its stars spread out rather than concentrated in a dense core. A paper describing the result was posted to the arXiv preprint server on Oct. 19.
Leviathan is a submillimeter galaxy, a class of extremely luminous, dust-obscured systems that form stars at hundreds to thousands of solar masses per year. Dust absorbs their ultraviolet and optical light and re-emits it at submillimeter wavelengths. They are thought to be ancestors of massive quiescent galaxies, today's giant ellipticals that have stopped forming stars. Those quiescent galaxies are very compact when seen 11.5 to 12.5 billion years ago, with half-light radii of only about 0.2 to 1 kiloparsec, yet they hold tens of billions of solar masses of stars. That implies their ancestors built dense stellar cores very quickly.
The catch is that dust emission only shows where stars are forming now, not where existing stars sit. Webb can detect the faint glow from the stars themselves. A team led by William M. Baker of the University of Copenhagen used it to measure where the stellar mass lies in Leviathan, a galaxy first discovered in 2012. "Whether these galaxies are on the way to becoming compact [quiescent galaxies] therefore requires measuring where their stellar mass (not just their light) is located," the team writes.
Using emission from hydrogen and ionized nitrogen, they measured a redshift of 4.0514. Fitting 31 bands of light to models of stars and dust gave a stellar mass of about 219 billion suns, making Leviathan one of the most massive spectroscopically confirmed galaxies beyond redshift 4. It forms about 410 solar masses of stars per year, at the massive end of the star-forming main sequence at that time.
It is also big. The radius containing half its light is 4.3 kiloparsecs and the radius containing half its stellar mass is 3.1, so the mass is more centrally concentrated than the light, a pattern consistent with inside-out growth in which the older, dustier center formed first. Star formation is spread across the disk, and the dust is only slightly more compact than the stars, at about 2.2 to 2.5 kiloparsecs.
The team calculated that even if all of Leviathan's gas turned into stars, it would only shrink to about 2.8 kiloparsecs, and its central density would still fall below that of compact quiescent galaxies. "If its star formation continues with its present spatial distribution, Leviathan is therefore unlikely to develop the compact structure of the massive quiescent galaxies at 3 < z < 5," the researchers say. It may instead become an extended, disk-like quiescent galaxy. The study suggests dusty galaxies in the early universe can follow different paths as they grow, so compact ellipticals may not have a single origin story.
