This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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.
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.
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