Astronomers using the James Webb Space Telescope (JWST) have confirmed a massive, dust-rich galaxy called Leviathan in the early universe. Seen when the universe was roughly 1.5 billion years old, the galaxy has its stars spread out rather than in a concentrated fashion. A paper, describing its strange stellar distribution, was posted to the arXiv preprint server on Oct. 19.
Dusty bright galaxies
Submillimeter galaxies are extremely luminous, dust-obscured galaxies, typically at redshifts around 2 to 3, forming stars at hundreds to thousands of solar masses per year. Dust absorbs their shorter-wavelength optical and ultraviolet light and re-emits it, which is detected at submillimeter wavelengths. They are thought to be possible progenitors of massive quiescent galaxies (MQGs), which are today’s massive elliptical galaxies.
Studying them can test theories of how galaxies form, how many stars they contain and how efficiently they convert gas into stars. Insights from these galaxies can also explain how heavy their dark matter halos are.
Massive quiescent galaxies, which have stopped forming stars, 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, they hold tens of billions of the sun’s mass in stars. This means their ancestors, which are thought to be submillimeter galaxies, must have built dense stellar cores very quickly.
Their dust emission is compact, with radii of about 1 to 2 kiloparsecs, but it shows only where stars are forming now, not where existing stars are. But now, JWST can detect galaxies’ faint glow, which comes from stars.
Leviathan: An ancient giant
The team led by William M. Baker of the University of Copenhagen has now investigated where the stars are located in a massive submillimeter galaxy and measured its mass. “Whether these galaxies are on the way to becoming compact MQGs therefore requires measuring where their stellar mass (not just their light) is located,” the team writes in the paper.
The team measured the mass and redshift of Leviathan, a massive submillimeter galaxy discovered in 2012. In its spectrum, they detected emissions from hydrogen and ionized nitrogen. Using these lines, they measured the redshift of z = 4.0514 for the galaxy as a whole.
To measure its mass, the team fit 31 different bands of light from Leviathan to models of the galaxy’s stars and dust, revealing a stellar mass of about 219 billion suns. The derived mass, researchers say, makes it one of the most massive spectroscopically confirmed galaxies beyond redshift 4.
Next, researchers estimated that Leviathan forms about 410 solar masses of stars per year. This “places Leviathan on the massive end of the star-forming main sequence at z≈4,” the team notes.
Compact or spread out?
Moving on to estimating its size, researchers found that Leviathan is bigger than most submillimeter galaxies seen about 11.5 billion years ago. They measured three types of radii: the radius containing half of the galaxy’s light, half of its stellar mass and half of its dust glow.
The half-light and half-mass radii are 4.3 and 3.1 kiloparsecs, respectively, so the mass is more concentrated than the light. This is typical of “inside-out” growth, where the center formed first and is older and dustier, while the outskirts are younger and brighter. The star formation is spread across the disk. The dust was found to be only slightly more compact than the stars (about 2.2 to 2.5 kiloparsecs).
They calculated that even if all its gas gets converted into stars, it would only shrink to about 2.8 kiloparsecs. Its central density is below that of compact quiescent galaxies. Thus, Leviathan is unlikely to become a compact quiescent galaxy.
“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
Instead, it may become an extended, disk-like quiescent galaxy. The study suggests that dusty galaxies in the early universe can follow different paths as they grow.
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Publication details
William M. Baker et al., Leviathan: a very massive, extended sub-millimetre galaxy spectroscopically confirmed at z = 4.05, arXiv (2026). DOI: 10.48550/arxiv.2610.27101
Journal information:
arXiv
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Leviathan’s sprawling stars challenge idea that dusty giants become compact galaxies (2026, November 3)
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