Astronomers Confirm the Most Powerful Radio Galaxy Ever Found, Seen 12.5 Billion Years in the Past

Astronomers have confirmed a radio galaxy from nearly 12.5 billion years ago that may be the most powerful ever found. The source, TXS 2354+015, was first flagged as a high-redshift candidate because of a characteristic drop in its optical light. Follow-up spectroscopy placed it at a redshift of 4.946, which means we see it as it was when the universe was less than 1.2 billion years old. That makes it the second-most-distant radio galaxy known. The team posted its paper to the arXiv preprint server on Sept. 23.

Radio galaxies like this one are powered by a supermassive black hole. When material falls onto the black hole, the resulting accretion can light up an active galactic nucleus and launch jets of plasma moving close to the speed of light, producing huge amounts of radio waves. The energy those jets carry can influence star formation in the host galaxy and heat the surrounding gas. Simulations include this "AGN feedback" to correctly predict how many galaxies the universe contains.

Powerful radio galaxies in the early universe tend to mark the locations of the most massive, earliest-forming galaxies and galaxy clusters. They are also hard to find. The black hole and its surroundings in most of them are hidden by a bubble of dusty gas, so that only the radio jet escapes and can be seen directly. A 2024 study suggested that as many as 90% of these sources at redshifts above 3.5 may be hidden in ultraviolet and optical light.

The standard hunting method looks for unusually steep radio spectra, and it appears to miss a large share of the true population. The team, led by Barbara Balmaverde of the INAF Astrophysical Observatory of Turin, took a different route. They combined deep optical images from the Subaru telescope's Hyper Suprime-Cam survey with radio catalogs, including TGSS at 150 MHz and VLASS at 3 GHz, and looked for radio sources whose optical counterparts showed the telltale "dropout" of extreme distance. Known as the Lyman-break technique, it relies on clouds of neutral hydrogen absorbing ultraviolet light, which makes a galaxy's observed spectrum fall off abruptly at these redshifts. They searched the range from redshift 4.5 to 5.3, when the universe was 1.1 billion to 1.3 billion years old.

TXS 2354+015 stood out, with a prominent Lyman-alpha emission line. The team verified the redshift using a second, fainter emission line. They also checked for a chance alignment. The precise match between the optical and radio positions, the extremely rare radio brightness and the expected ratio between radio and optical emission all pointed to a genuine radio source with an optical counterpart.

Then came the headline number. Calculating the source's intrinsic radio power, the researchers found it exceeds that of every other known high-redshift radio galaxy in the literature, and its power at 500 MHz tops 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.

They also estimated a possible host-galaxy mass of around 2 trillion solar masses, which would make it one of the brightest galaxies known. The authors caution that the estimate is "clearly plagued by several large uncertainties, in particular by the age of the stellar population."

The object does not meet the usual ultra-steep-spectrum selection criteria, and the authors say its discovery through the optical dropout method supports the idea that the standard radio technique is incomplete. It likely misses a meaningful fraction of the obscured radio galaxies of the early universe. The result is a preprint and has not yet been through peer review.