Researchers have obtained the deepest images ever taken of IC 1101, allowing them to identify the galaxy’s outer edge for the first time. The observations confirm it as the largest known galaxy, spanning roughly 520 kiloparsecs, or about 1.7 million light-years, and containing an estimated 3.4 trillion solar masses in stars. The paper was posted to the arXiv preprint server on July 16.
Cosmic giants
The largest galaxies, called brightest cluster galaxies (BCGs), are products of hierarchical galaxy formation. They grow by absorbing smaller galaxies over billions of years. Their faint stellar halos blend smoothly into the intracluster medium—the diffuse gas and stars filling the space between galaxies in a cluster.
That blending is exactly what makes BCGs so hard to measure. Their outskirts hold clues to past mergers, in the form of tidal tails and stellar halos that overlap with the surrounding intracluster light, making it difficult to tell where the galaxy ends and the rest of the cluster begins.
The team led by Carlos Marrero-de la Rosa of the Instituto de Astrofísica de Canarias chose one of the most extended stellar systems ever observed: a BCG at the center of the galaxy cluster Abell 2029. Its stellar extent had previously been measured at 607 kiloparsecs. Using ultra-deep images from the Wide Field Camera at the Isaac Newton Telescope (INT), astronomers set out to detect the galaxy’s edge.
Light pollution
Measuring a galaxy’s true edge means detecting very faint starlight and separating that light from an unwanted source of contamination: scattered light from foreground stars, which can spread a faint glow across an image and wash out exactly the faint regions astronomers are chasing.
The team built a detailed model of the telescope’s point spread function using dozens of calibration stars of varying brightness. They then used that model to subtract the scattered light from more than 250 stars from the final image, one by one, revealing faint structures.
The radial profiles—how the starlight changes with the radius from the center—reveal this edge at 260 kiloparsecs, with the galaxy’s confirmed diameter reaching about 520 kiloparsecs, making it the largest galaxy known to date.
“The edge radius measured for IC 1101 remains among the largest values reported to date for an individual galaxy, corresponding to a projected diameter of ∼520 kpc,” the team writes in the paper. The total enclosed stellar mass is estimated at roughly 3.4 trillion solar masses, depending on exactly where the boundary is drawn.
For scale, the Milky Way stretches only about 30 kiloparsecs across. IC 1101 spans roughly 17 times that. Its stellar mass alone also outweighs the total mass of the Milky Way, which includes its dark matter, estimated at somewhere between 1 and 1.5 trillion solar masses.
Still growing
Beyond the main edge, the team detected eight extensive, faint, asymmetric stellar structures. Two of them line up with known X-ray “sloshing” disturbances in the surrounding hot cluster gas, evidence that the galaxy is still growing through ongoing accretion rather than sitting as a finished, settled system. The team writes that IC 1101’s outskirts “show clear signatures of ongoing mass assembly,” indicating its size is still increasing.
This lines up with an earlier X-ray study, which identified similar disturbances as the likely signature of a past collision with another galaxy group roughly 2–3 billion years ago.
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Publication details
Carlos Marrero-de la Rosa et al, How large can galaxies be? Ultra-deep imaging of IC 1101, the most extended known galaxy, arXiv (2026). DOI: 10.48550/arxiv.2607.15340
Journal information:
arXiv
Key concepts
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Astronomers finally measure the full size of the largest known galaxy (2026, July 31)
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