A new study finds that two early-universe objects thought to be faint quasars are actually extraordinarily luminous galaxies powered by bursts of star formation. James Webb Space Telescope observations of the galaxies, seen when the universe was less than a billion years old, reveal signatures of extremely massive stars, including some potentially more than 200 times the mass of the sun. The paper outlining this discovery was posted to the arXiv preprint server on Aug. 18.
Shining bright like a diamond
There’s a luminosity range where ultraviolet-bright galaxies and faint quasars become impossible to tell apart using standard discovery techniques. Both can show blue ultraviolet continua and prominent hydrogen Lyman-alpha emission lines, and low-sensitivity discovery spectra alone cannot distinguish which is which.
This ambiguity affects how astronomers count quasars and extremely bright, actively star-forming galaxies in the early universe. During their quasar searches, astronomers have detected a significant number of UV-bright galaxy candidates in the Subaru High-z Exploration of Low-Luminosity Quasars survey (SHELLQs).
Among these UV-bright galaxies is a subclass called Extreme UV Luminous Galaxies (EUVLGs), which displays key features that can rule out quasars. These include:
- P Cygni profiles, distinct patterns in stellar spectra that are the canonical signature of radiatively driven winds from massive stars rather than an accreting black hole;
- certain broad helium lines that are too strong to come from ordinary star-forming populations and require very massive stars;
- the absence of broad Lyman-alpha or Mg II emission lines typical of quasars; and
- narrow spectral lines from oxygen and neon arising from photoionization by young stars rather than an accreting black hole.
In this study, a team led by Sarah E. I. Bosman of Leiden University examines two objects, J1450−0144 at redshift 6.63 and J1429−0104 at redshift 6.80, which were originally found and classified as faint quasars by SHELLQs. They were observed when the universe was under 900 million years old.
Spotting impostors
Researchers used JWST/NIRSpec spectroscopy and Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 millimeter-wave imaging to revisit both objects in detail. The JWST spectroscopy revealed features such as P Cygni wind profiles and extremely strong, broad helium (He II) emission. It also showed no broad Lyman-alpha or Mg II emission. These findings are inconsistent with a quasar.
Models incorporating stellar populations with a maximum birth mass of 100 times that of the sun could not reproduce the strong helium signal. Only models that account for very massive stars (VMS)—stars exceeding 100 times the sun’s mass—could match the data.
The team estimated that the galaxies are forming stars at a rate of 300 to 540 solar masses/year and that each galaxy contains about 100,000 to 1 million very massive stars. Researchers note that the exact upper mass limit for these very massive stars depends heavily on the model and that the physics of their stellar winds has been calibrated only using nearby examples in our galaxy, rather than the primitive galaxies of the early universe.
Puzzling offset
ALMA showed that both galaxies are extremely luminous in a specific gas-emission line ([C II]) and cold dust, and one of them, J1429−0104, has its UV light originating at a different physical location from its dust and gas emission—a separation of 17,600 light-years. This unusual offset between one galaxy’s UV light and its dust and gas emission remains unexplained. It could be caused by a merger, a dust-obscured central starburst or feedback-driven dust removal. Pinning down its origin requires future high-resolution imaging, researchers note.
Nevertheless, “both J1450−0144 and J1429−0104 closely match the EUVLG composite,” the team writes in the paper. Therefore, both objects are reclassified as Extremely UV Luminous Galaxies instead of quasars and are the first objects ever spectroscopically confirmed to be this bright this early in cosmic history. This means some other objects currently classified as faint quasars in existing surveys may actually be galaxies like these.
“Taken together, these results identify J1450−0144 and J1429−0104 as a new class of laboratories at the very bright end of the UVLF at high redshift: compact, gas-rich, intensely star-forming galaxies that simultaneously host substantial very massive star populations,” the team concludes.
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Publication details
Daming Yang et al, Quasar Impostors: Two Extremely UV-Bright (MUV≈−23.5) Reionisation-Epoch Galaxies Powered by Very Massive Stars, arXiv (2026). DOI: 10.48550/arxiv.2608.18212
Journal information:
arXiv
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JWST finds extreme star-forming galaxies masquerading as faint quasars (2026, September 8)
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