Astronomers confirm second-most-distant radio galaxy, most powerful of its kind

Astronomers have confirmed a radio galaxy whose light comes from nearly 12.5 billion years ago, according to a paper posted to the arXiv preprint server on Sept. 23. The source, TXS 2354+015, sits at a redshift of 4.946, a measure of distance based on how much the expansion of the universe has stretched its light. At that redshift, the universe was less than 1.2 billion years old. The team was led by Barbara Balmaverde of the INAF Astrophysical Observatory of Turin.

Radio galaxies like this one are powered when material falls onto a galaxy's central supermassive black hole. According to the paper, the resulting accretion activity can power an active galactic nucleus and launch relativistic jets of plasma — streams of charged particles moving near the speed of light — that produce large amounts of radio waves. Astronomers call such objects radio-loud active galactic nuclei, or RLAGNs.

The team found the galaxy by combining deep optical imaging from the Subaru Hyper Suprime-Cam Subaru Strategic Program survey with radio catalogs including TGSS at 150 MHz and VLASS at 3 GHz. They searched for galaxies showing a "dropout" signature, in which intervening clouds of neutral hydrogen absorb ultraviolet light and make the observed optical spectrum drop off abruptly. The search covered redshifts of 4.5 to 5.3, when the universe was 1.1 billion to 1.3 billion years old.

The researchers confirmed the redshift using a prominent Lyman-alpha emission line and a second, fainter emission line. They also checked whether the optical and radio signals were a chance alignment; the paper states that the precise positional match, the rare radio brightness and the expected ratio between radio and optical emission all supported a genuine radio source with an optical counterpart. The paper reports that TXS 2354+015 is the second-most-distant radio galaxy ever found.

When the researchers calculated its intrinsic radio power, the paper states, TXS 2354+015 turned out to be more powerful than any other known high-redshift radio galaxy in the existing literature. According to the paper, most powerful early-universe radio galaxies are hidden by a bubble of dusty gas, with only the radio jet able to escape and be seen directly. The paper notes that 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, which makes them 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, the paper adds.