IC 1101

IC 1101 is the largest galaxy known. It sits at the centre of the galaxy cluster Abell 2029, about 1.15 billion light-years away, a smooth golden oval of old stars with no spiral arms. For more than thirty years its size was quoted as about 6 million light-years. In 2026 astronomers finally measured where it ends, and the answer was about 1.7 million.

Where does a galaxy end?

A galaxy has no surface. Its starlight simply grows fainter with distance from the centre, and in a crowded cluster it blends into the faint glow of stars that drift between the galaxies. In 1990 Uson, Boughn and Kuhn reported in Science that the central galaxy of Abell 2029 was indistinguishable from this diffuse light, and news reports described it as a galaxy 6 million light-years wide, 60 times the width of the Milky Way. Guinness World Records still lists 5.6 million.

In 2026 a team led by Marrero-de la Rosa published ultra-deep images of IC 1101 taken with the Isaac Newton Telescope on La Palma. They defined its edge as the distance at which its structure changes sharply, where the galaxy itself gives way to the cluster’s free-floating stars. That edge lies 260 kiloparsecs from the centre, so the whole galaxy is about 520 kiloparsecs across: nearly 1.7 million light-years, or about 17 Milky Ways end to end. Inside that edge are about 3.4 trillion Suns’ worth of stars. It is far smaller than the legend, but the team still calls it the largest galaxy known.

Still growing

The new images also show faint, uneven features on the galaxy’s flanks, which line up with a disturbance in the cluster’s hot gas. IC 1101 is still pulling in material from smaller galaxies. “It has not finished,” Marrero-de la Rosa told Science News.

A core too big to explain

Galaxies this large usually have a central region thinned of stars, scoured out when two supermassive black holes spiral together after a merger. In IC 1101 that hollow core is about 4.2 kiloparsecs across, where typical cores measure tens to hundreds of parsecs. In 2017 Dullo, Graham and Knapen worked out that mergers alone would need an unrealistically large number of them, at least 76. They suggested another cause: a black hole kicked off-centre by gravitational waves, swinging back and forth through the middle of the galaxy. Its central black hole is estimated at 40 to 100 billion times the Sun’s mass.