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Astronomers Measure the Full Size of IC 1101, Confirming It as the Universe's Largest Known Galaxy

Ultra-deep imaging from the Isaac Newton Telescope resolves IC 1101's faint outer edge at 520 kiloparsecs across, with 3.4 trillion solar masses in stars.

Science & Research Astronomy
astronomy galaxies space IC 1101 Isaac Newton Telescope
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Overview

A team of astronomers has measured the full physical extent of IC 1101, the brightest cluster galaxy at the center of the Abell 2029 cluster, finding that its faint outer halo stretches to roughly 520 kiloparsecs, or about 1.7 million light-years, across, according to phys.org and The Debrief. The finding confirms IC 1101’s long-held status as the largest known galaxy by directly resolving a boundary that had never been fully imaged before.

What We Know

  • The measurement comes from a study led by Carlos Marrero-de la Rosa of the Instituto de Astrofísica de Canarias, titled “How large can galaxies be? Ultra-deep imaging of IC 1101, the most extended known galaxy,” posted to arXiv on July 16, 2026.
  • The team used ultra-deep g- and r-band imaging from the Wide Field Camera on the Isaac Newton Telescope (INT) in La Palma, reaching a surface-brightness detection limit of 30 magnitudes per square arcsecond, according to the arXiv preprint.
  • The galaxy’s edge sits at a radius of 260 kiloparsecs along its semi-major axis, giving a total diameter of about 520 kiloparsecs, as reported by phys.org and confirmed in the arXiv preprint.
  • IC 1101 spans roughly 17 times the diameter of the Milky Way, which stretches about 30 kiloparsecs across, according to phys.org and The Debrief.
  • The galaxy contains an estimated 3.4 trillion solar masses in stars, which on its own exceeds the Milky Way’s total mass — including dark matter — estimated at 1 to 1.5 trillion solar masses, according to phys.org and The Debrief.
  • To reach that depth, researchers built a detailed model of the telescope’s point-spread function using calibration stars, then individually subtracted scattered light from more than 250 stars to reveal structures that had previously been buried in noise, according to phys.org and The Debrief.
  • Beyond the main body of the galaxy, the team detected eight faint, asymmetric stellar structures, according to phys.org and The Debrief. Two of those structures align with known X-ray disturbances in the surrounding cluster gas, which The Debrief reports point to an ongoing accretion event from a collision roughly 2-3 billion years ago.
  • The paper is expected to appear in the journal Astronomy & Astrophysics, according to The Debrief.

Why It Was Hard to Measure

Brightest cluster galaxies like IC 1101 are notoriously difficult to measure at their true edges. Their outermost regions consist of extremely faint, diffuse stellar halos and tidal tails left behind by ancient galaxy collisions, while foreground stars within the Milky Way scatter light across the same images, according to The Debrief. Earlier estimates of IC 1101’s size varied because previous imaging could not reliably separate the galaxy’s genuine light from this scattered-light contamination. The new approach — modeling the telescope’s point-spread function star by star — let the team push the detection threshold faint enough to trace the galaxy’s true boundary for the first time.

What We Don’t Know

  • Neither the arXiv preprint nor the news coverage of it specifies IC 1101’s distance from Earth in the reporting reviewed for this article, so no distance figure is included here.
  • The exact timeline for the paper’s formal publication in Astronomy & Astrophysics has not been announced beyond the current preprint stage.
  • It remains unclear how many other brightest cluster galaxies might rival IC 1101 in extent once measured with similarly deep imaging, since surveys of this depth are still rare.

Analysis

The result underscores a broader pattern in observational astronomy: many of the universe’s largest structures are not fully understood not because they are rare, but because they are too faint to see with standard imaging depth. IC 1101 has been cited as a candidate for the largest known galaxy for years, but this study is the first to trace its boundary with a systematic, star-by-star correction for scattered light — turning a rough estimate into a directly measured figure. The eight outer structures detected beyond the main body, some tied to X-ray disturbances in the cluster gas, also suggest IC 1101 is not a static, fully formed object but one still assembling mass by absorbing material from its surroundings, consistent with the hierarchical growth long predicted for brightest cluster galaxies at the centers of massive clusters like Abell 2029.