What astronomers initially thought was a single distant radio galaxy has turned out to be something far more interesting: a dense cluster of at least six galaxies in the process of merging into one. The discovery, made possible by the James Webb Space Telescope (JWST), offers an unprecedented direct view of how giant galaxies assembled in the early universe.
The system, designated TGSS J1530+1049, was first identified in 2018 as a candidate high-redshift radio galaxy based on its unusually steep radio spectrum. Earlier radio observations detected an active supermassive black hole at its center, but the true nature of the system remained hidden until JWST turned its instruments toward it.
“We didn’t find a single galaxy, but an entire complex of at least six galaxies,” said Aayush Saxena of the University of Oxford, lead author of the study published in The Open Journal of Astrophysics.
A Cosmic Construction Site
At a redshift of 4.0, TGSS J1530+1049 is seen as it existed roughly 1.5 billion years after the Big Bang, more than 12 billion light-years away. JWST’s NIRCam imaging and NIRSpec integral field unit (IFU) spectroscopy revealed not one galaxy but at least six distinct continuum sources packed into a region spanning only a few tens of thousands of light-years across.
Four of the six galaxies are already surprisingly massive for such an early cosmic epoch, each containing hundreds of billions of solar masses in stars. All six combined total hundreds of billions of solar masses, with star formation rates ranging from 70 to 163 solar masses per year.
The IFU spectroscopy also detected four additional sources emitting strong hydrogen-alpha (H-alpha) lines with weaker or no underlying continuum, bringing the total number of distinct objects in the compact field to as many as 10.
“This is one of the densest-known structures of continuum and line-emitting objects at these redshifts,” the team wrote in their paper.
Tandem Growth of Galaxy and Black Hole
What makes this system particularly valuable to astronomers is that it offers a simultaneous view of two processes: the assembly of a giant galaxy and the growth of its central supermassive black hole.
The radio-active galactic nucleus (AGN) host galaxy has been clearly identified as continuum source C2. The H-alpha emission is distributed remarkably linearly along the radio axis, suggesting a biconical illumination zone from a central obscured black hole. This provides a rare direct view of AGN feedback, where the black hole’s energy output affects the surrounding gas and star formation.
A companion study published in Astronomy & Astrophysics, led by Krisztina Gabanyi of Eotvos Lorand University in Budapest, used the European VLBI Network (EVN) and e-MERLIN radio telescopes to produce extremely sharp radio images pinpointing the active black hole. The black hole appears to be relatively young and actively accreting, suggesting the AGN activity is still in its early stages.
“What makes this special is that we can follow both the build-up of a giant galaxy and the growth of the black hole at its centre,” said Huub Rottgering of Leiden University, a co-author on the study.
A Rare Glimpse of Protocluster Assembly
The system qualifies as a protocluster, the precursor of today’s massive galaxy clusters, and is expected to merge into a single giant galaxy within a few billion years. Based on stellar mass analysis, the total dark matter halo mass is predicted to be around 10^13 solar masses, indicating a substantial cosmic structure was already taking shape at this early epoch.
The system “qualitatively resembles the forming brightest cluster galaxies in cosmological simulations that form early through a rapid succession of mergers,” the researchers noted.
This discovery is distinct from another recent JWST finding, “JWST’s Quintet” at redshift 6.71 (seen only 800 million years after the Big Bang), which involves five galaxies. The TGSS J1530+1049 system is slightly later in cosmic time but involves more galaxies in a confirmed protocluster, making it one of the richest structures yet found at this epoch.
The combined use of JWST’s infrared imaging and spectroscopy with high-resolution radio interferometry marks a powerful approach for studying the co-evolution of galaxies and black holes in the early universe. As Saxena put it, “We didn’t find a single galaxy, but an entire complex of at least six galaxies.”