The largest known galaxy is wider than 17 Milky Ways

The blue ribbon for largest known galaxy goes to a behemoth that probably grew by cannibalizing its smaller neighbors.

Known as IC 1101, the galaxy’s diameter is estimated to be a whopping 520 kiloparsecs, or nearly 1.7 million light-years, researchers report in a study submitted July 16 to arXiv.org. That’s wider than 17 Milky Ways lined up in a row.

Researchers analyzed the deepest images — or those with the longest exposure time — ever taken of IC 1101, captured in 2022 by the Isaac Newton Telescope at the Roque de los Muchachos Observatory, located on La Palma in the Canary Islands. That allowed them to identify the galaxy’s faint edge and determine that its girth contains roughly 3.4 trillion solar masses of stellar material.

The galaxy is still growing. Taillike features extend from the galaxy’s edges, which researchers believe are the products of IC 1101 merging with other galaxies. “It’s still accreting things … It has not finished,” says astrophysicist Carlos Marrero de la Rosa, of Instituto de Astrofísica de Canarias, located in Tenerife, Spain.

IC 1101 is located over a billion light-years away in the heart of the Abell 2029 galaxy cluster. It As the most luminous galaxy in its cluster, it belongs to a category of galaxies known as the brightest cluster galaxies. These brilliant systems include among their ranks the largest galaxies in the universe, which grow by repeatedly merging with smaller galaxies nearby. This process is known as galactic cannibalism.

Pinning down the extents of such vast and voracious galaxies is no meager task, Marrero de la Rosa says. Earlier studies had indicated that IC 1101 must be massive, but they did not precisely identify where the galaxy’s edge transitions into the diffuse material of the cluster lying beyond.

IC 1101 demonstrates what can happen when this sort of galactic evolution, involving repeated merging and accreting, reaches extreme levels over time, Marrero de la Rosa says. “This is a kind of galaxy of the future.”