The findings, led by Sarah E. I. Bosman of Leiden University and detailed in a paper posted to the arXiv preprint server on Aug. 18, force astronomers to rethink how they count quasars and intensely star-forming galaxies in the early universe. The two objects, J1450−0144 and J1429−0104, were originally identified as faint quasars by the Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs) survey. The paper is authored by Daming Yang et al, and published with DOI: 10.48550/arxiv.2608.18212.
Spotting Extreme UV Luminous Galaxies in the Early Universe
There is a luminosity range where ultraviolet-bright galaxies and faint quasars become indistinguishable using standard discovery techniques. Both display blue ultraviolet continua and prominent hydrogen Lyman-alpha emission lines. Low-sensitivity discovery spectra alone cannot tell them apart. 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 SHELLQs survey. Among these is a subclass called Extreme UV Luminous Galaxies (EUVLGs). These galaxies display key features that can rule out quasars, such as P Cygni profiles, certain broad helium lines, the absence of broad Lyman-alpha or Mg II emission lines, and narrow spectral lines from oxygen and neon arising from photoionization by young stars.
To resolve the ambiguity surrounding J1450−0144 and J1429−0104, researchers used JWST/NIRSpec spectroscopy and Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 millimeter-wave imaging to examine the objects in detail. J1450−0144 lies at redshift 6.63, while J1429−0104 is at redshift 6.80. Both were observed when the universe was under 900 million years old.
The JWST spectroscopy revealed features that definitively 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. The data also showed extremely strong, broad helium (He II) emission, while lacking the broad Lyman-alpha or Mg II emission typical of quasars. Narrow spectral lines from oxygen and neon further indicated photoionization by young stars rather than an accreting black hole.
The Role of Very Massive Stars
Models incorporating stellar populations with a maximum birth mass of 100 times that of the sun could not reproduce the strong helium signal observed in these extreme UV luminous galaxies. Only models that account for very massive stars (VMS)—stars exceeding 100 solar masses—could match the data. Some of these stars may be more than 200 times the mass of the sun.
The team estimated that the galaxies are forming stars at a rate of 300 to 540 solar masses per year. Each galaxy contains about 100,000 to 1 million very massive stars. However, researchers note that the exact upper mass limit for these very massive stars depends heavily on the model. 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.
A Puzzling Spatial Offset
ALMA observations showed that both galaxies are extremely luminous in a specific gas-emission line, [C II], and in cold dust. But one of the galaxies, 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 the galaxy’s UV light and its dust and gas emission remains unexplained. Researchers suggest it could be caused by a merger, a dust-obscured central starburst, or feedback-driven dust removal. Pinning down its origin will require future high-resolution imaging.
What Happens Next
Despite the unexplained offset, the team found that “both J1450−0144 and J1429−0104 closely match the EUVLG composite.” Consequently, both objects have been reclassified as Extreme UV Luminous Galaxies instead of quasars. They are the first objects ever spectroscopically confirmed to be this bright this early in cosmic history.
The reclassification implies that 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.
Astronomers will now need to revisit existing quasar surveys to identify other potential EUVLGs hiding in plain sight. Future high-resolution imaging and refined stellar wind models calibrated for early-universe conditions will be crucial for understanding the true nature of these extreme star-forming galaxies and the very massive stars that power them. The discovery opens a new window into the early universe, suggesting that the bright end of the UV luminosity function may be populated by intense starbursts rather than supermassive black holes.
— Priya Nair, science desk, AXO News