Astronomers have confirmed a powerful radio galaxy from nearly 12.5 billion years ago. The source, TXS 2354+015, was originally identified as a high-redshift candidate from its characteristic drop in optical light, and follow-up spectroscopy placed it at a redshift of 4.946. Their paper was posted to the arXiv preprint server on Sept. 23.
Radio-loud AGNs
When material falls onto a galaxy’s central supermassive black hole, the resulting accretion activity can power an active galactic nucleus (AGN) and launch powerful relativistic jets of plasma, producing large amounts of radio waves. Such a galaxy is called a radio-loud active galactic nucleus (RLAGN). The energy that the jets carry can influence star formation in the galaxy and heat its surrounding gas.
This “AGN feedback” is incorporated in simulations to correctly predict the number of galaxies in the universe. Such powerful galaxies in the early universe—called high-z RLAGNs—tend to mark the locations of the most massive, earliest-forming galaxies and galaxy clusters.
However, the central black hole and surrounding regions of most powerful early-universe RLAGNs are hidden by a bubble of dusty gas, with only the radio jet able to escape and be seen directly. In particular, a 2024 study suggests as many as 90% of these sources at redshifts greater than 3.5 might be hidden in the ultraviolet and optical bands. Because they are obscured, they are difficult to find and confirm spectroscopically.
Missing radio galaxies
The dominant approach to hunting radio sources, which identifies unusually steep spectra, appears to miss a large portion of the true population. To work around this, astronomers instead combined deep optical imaging with radio catalogs, searching for galaxies whose light showed the telltale “dropout” signature of extreme distance. This is known as the Lyman-break technique: Intervening clouds of neutral hydrogen absorb ultraviolet light, causing the galaxy’s observed optical spectrum to drop off abruptly at these high redshifts.
In this work, the team led by Barbara Balmaverde of the INAF Astrophysical Observatory of Turin combined deep optical imaging from Subaru’s Hyper Suprime-Cam Subaru Strategic Program survey with radio catalogs such as TGSS at 150 MHz and VLASS at 3 GHz to find radio sources whose optical counterparts show the characteristic “dropout” feature within the redshift range of 4.5 to 5.3, when the universe was 1.1 billion to 1.3 billion years old.
In their search, they found an interesting candidate, TXS 2354+015, whose optical spectrum showed a prominent Lyman-alpha emission line. They verified the redshift using a second, fainter emission line. Based on the emission line position, the team confirmed the redshift at 4.946, when the universe was less than 1.2 billion years old. This makes it the second-most-distant radio galaxy ever found.
They also checked whether this was a chance alignment. The precise positional match between optical and radio data, the extremely rare radio brightness and the expected ratio between the radio and optical emission all supported the conclusion that this is a genuine radio source with an optical counterpart.
The most powerful of all
When researchers calculated its intrinsic radio power, TXS 2354+015 turned out to be more powerful than any other known high-redshift radio galaxy in the existing literature. Its estimated radio power at 500 MHz exceeds the previous record-holder from a 2008 census. “TXS 2354+015, thus, appears to be the most powerful radio galaxy known to date,” the team writes in the paper.
Researchers also estimated a possible host galaxy mass of around 2 trillion solar masses. This number makes it one of the brightest galaxies known. “This estimate is clearly plagued by several large uncertainties, in particular by the age of the stellar population,” the team notes.
Researchers found that TXS 2354+015 does not meet the usual ultra-steep-spectrum selection criteria. Its discovery through the optical dropout method supports the idea that the standard radio selection technique is incomplete, likely missing a meaningful fraction of the true obscured radio galaxy population in the early universe.
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Publication details
B. Balmaverde et al, The quest for high-redshift radio galaxies II. Discovery of the most powerful known radio galaxy at z=4.946, arXiv (2026). DOI: 10.48550/arxiv.2609.28632
Journal information:
arXiv
Key concepts
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Radio galaxy from 12.5 billion years ago may be most powerful ever found (2026, October 7)
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