New astronomical observations have confirmed the size of IC 1101, the largest galaxy known to date. The detailed study, conducted by researchers and published on July 16, 2026, revealed that the giant system spans about 1.7 million light years. This impressive dimension reaffirms the galaxy as one of the most colossal structures in the universe.
Before these analyses, the outermost regions of IC 1101 were a mystery to scientists. Previous observations pointed to an exceptional extent, but identifying a clear physical boundary was a challenge. The new images, the deepest ever taken of the galaxy, have allowed astronomers to distinguish its faint edges from the surrounding glow of the galaxy cluster where it resides.
Unraveling gigantism: astronomers confirm dimensions of IC 1101
Recent surveys have provided an accurate measure of IC 1101’s proportions, estimating its stellar mass at approximately 3.4 trillion solar masses. This new delimitation was crucial to understanding the true magnitude of the galaxy. The work was led by Carlos Marrero-de la Rosa, from the Instituto de Astrofisica de Canarias, and the results were shared on a preprint server.
The team combined ultra-deep images with a rigorous correction for scattered light, making it possible to view the most distant parts of the galaxy. This innovative approach allowed the previously undefined edges of IC 1101 to finally be demarcated, providing concrete data about its vast extent.
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Ultra-deep observations reveal the edges of the galaxy
To determine IC 1101’s full range, the team used detailed images captured by the Isaac Newton Telescope’s Wide Field Camera. Detecting such faint structures required a sophisticated method to eliminate interference from foreground stars, whose scattered light could obscure the galaxy’s faint stellar emission.
The researchers developed an accurate model of the telescope’s point scatter function using dozens of calibration stars with different brightness levels. They then subtracted the scattered light from more than 250 foreground stars one by one, resulting in a much sharper view of the outer regions of the colossal system. The resulting radial brightness profiles indicated a distinct edge about 260 kiloparsecs from the center of the galaxy.
“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 study authors stated.
The scale of IC 1101 compared to the Milky Way
The new measurements of IC 1101 highlight its extraordinary scale, which positions it as an unparalleled cosmic giant. To put its vastness into context, just compare it to our own galaxy, the Milky Way, which appears tiny in contrast.
- IC 1101 Diameter:about 1.7 million light years (520 kiloparsecs).
- Diameter of the Milky Way:approximately 100,000 light years (about 30 kiloparsecs).
- Stellar mass of IC 1101:estimated at 3.4 trillion solar masses.
- Total mass of the Milky Way:between 1 and 1.5 trillion solar masses, including dark matter.
This means that IC 1101 has a diameter approximately 17 times larger than that of the Milky Way. Furthermore, the stellar mass of IC 1101 alone exceeds the estimated total mass of the Milky Way, which comprises both visible and dark matter. These comparisons solidify IC 1101’s position as the largest known individual galaxy whose size has been directly established by observations.
Signs of growth and merger underway in largest galaxy
IC 1101 is located at the center of the Abell 2029 galaxy cluster and is classified as one of the brightest cluster galaxies (BCGs). These immense systems form over billions of years, growing by the gradual absorption of smaller galaxies. As they evolve, their diffuse stellar halos mix with intracluster light, which has historically made it difficult to define their exact boundaries.
In addition to its outer border, the detailed images revealed other important information. Researchers have identified eight large, faint asymmetric stellar structures that extend beyond the galaxy’s leading edge. These discoveries offer new perspectives on the evolution of the system. Two of these structures coincide with previously identified X-ray “wobble” perturbations in the hot gas surrounding the Abell 2029 cluster. Previous X-ray observations have linked these perturbations to a collision with another group of galaxies that occurred approximately 2 to 3 billion years ago.
“IC 1101 stands out as the largest galaxy known to date, however, its outer regions reveal evidence of ongoing mass assembly,” the authors wrote. This evidence suggests that the giant galaxy is still in the process of evolving, continuing to accumulate mass, rather than being a fully stable and consolidated system.