Colossal IC 1101: The Universe’s Largest Known Galaxy Emerges

Ultra-deep imaging confirms IC 1101, at the center of Abell 2029, as the largest known galaxy, spanning about 1.7 million light-years and containing ~3.4 trillion solar masses in stars. Using the Isaac Newton Telescope and a star-subtraction technique, researchers teased out the galaxy’s faint outer halo from foreground Milky Way stars, revealing eight structures beyond its edge and evidence of ongoing growth via accretion and a 2–3 billion-year merger. The work, in arXiv ahead of Astronomy and Astrophysics, highlights how brightest cluster galaxies grow by absorbing neighbors and how their outer regions blend into the cluster.