Scientific observed the process of galaxy mergers that gave rise to the most massive galaxies in the Universe

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An international team of astronomers has observed what could be one of the earliest stages in the formation of the most massive galaxies in the Universe. Thanks to images obtained by the James Webb Space Telescope (JWST), the researchers discovered that the object known as TGSS J1530+1049, located at such a distance that we see it as it was in the past, when the Universe was less than 2 billion years old—barely 10 percent of its current age of 14 billion years—is composed of several enormous galaxies that interact with each other and will likely eventually merge into a single gigantic structure.

On the left, in green, are the detected galaxies. On the right, the gas structures are in blue. Number 2 is the identified radio galaxy. Image credit: JWST.

The finding, detailed in two scientific papers published in the journals Open Journal of Astrophysics and Astronomy & Astrophysics, respectively, included the participation of CONICET researcher Victoria Reynaldi, who works at the Institute of Astrophysics of La Plata (IALP, CONICET-UNLP).

TGSS J1530+1049 was originally identified as a possible radio galaxy, a special type of galaxy that, unlike normal galaxies —which shine with the light of the stars, gas, and dust that compose them—emits unusually intense radio radiation of a different origin. These emissions are usually associated with the presence of a supermassive black hole at its core—hence the name active or active-nuclear galaxies—which accumulates large amounts of matter and releases enormous amounts of energy across the entire electromagnetic spectrum, that is, in all types of radiation (visible light, infrared, radio waves). This sometimes results in jets or emissions with speeds close to that of light. While this object was a candidate for a radio galaxy, its nature had not been precisely confirmed until now.

The images obtained by the James Wwebb Space Telescope confirmed this suspicion, but revealed a much more complex reality than expected: what appeared to be a single radio galaxy turned out to be a cluster of at least ten different objects, grouped into two sets with distinct characteristics, based on the radiation they emit: “One group exhibits properties typical of the interstellar medium; that is, they are structures dominated by radiation from the gas,” explained Reynaldi. “In the other group, there are six galaxies that are dominated by the light from the stars that compose them,” she pointed out. One of these was identified as the active galaxy from which the originally detected radio emission originated, thus confirming the presence of a radio galaxy within the TGSS J1530+1049 complex.

Victoria Reynaldi. Image credit: Victoria Reynaldi

However, the most surprising finding is that several of these galaxies possess extremely high masses and are located very close to one another. Despite being observed at a very early stage of cosmic history, they had already formed an enormous number of stars and continued to do so at an extraordinary rate and more efficiently than those surrounding us today: while our own Milky Way currently produces three to five stars similar to the Sun per year, some of these galaxies generate more than 100 annually. “Four of the most massive are located in an area that, if it were to be divided into a square, would have sides only slightly larger than the distance between the Sun and the center of the Milky Way,” the researcher explained.

The proximity of these systems indicates that they are gravitationally interacting and will eventually merge. This process will give rise to a much larger galaxy, comparable to the giant galaxies that currently occupy the centers of galaxy clusters and are among the most massive and luminous objects in the Universe.

“These are very exciting results that challenge current theories about galaxy formation in the early Universe,” the IALP researchers pointed out. “According to these theories, these colossal structures must have formed from smaller ones, but this process had not been directly observed until now. Since galaxy clusters were not yet formed in those early stages, the conclusions of this work indicate that we are very likely witnessing the spectacular merger process by which the most massive galaxies in the Universe are born,” she concluded.

Citation
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Saxena, A.; Overzier, R.; Aydar, C.; Lyu, J.; Rieke, G.H.; Reynaldi, V.; Villar-Martín, M.; Gabányi, K.; … & Venemans, B. (2026). JWST observes the assembly of a massive galaxy at z~4 . Open Journal of Astrophysics, 9. DOI: https://doi.org/10.33232/001c.159461
Gabányi, K. É., Frey, S., Gurvits, LI, Paragi, Z., Perger, K., Saxena, A., … & Mező, G. (2026). High-resolution radio imaging of TGSS J1530+ 1049, a radio galaxy in a dense environment at z= 4. Astronomy & Astrophysics, 710, A20 . DOI: https://doi.org/10.1051/0004-6361/202558162
The article Observan cómo es el proceso de fusión de galaxias que dio origen a las más masivas del Universo fue publicado en el sitio web del CONICET**