As new observations reveal the outer edges of IC 1101 for the first time, scientists have confirmed it is the largest known galaxy. The galaxy’s statistics are staggering, containing an estimated 3.4 trillion solar masses of stars and spanning roughly 1.7 million light-years, eclipsing every other known galaxy.
IC 1101 belongs to a class known as brightest cluster galaxies (BCGs), which form as massive galaxies spend billions of years absorbing smaller neighboring galaxies.
The new images are the deepest ever taken of the galaxy, and were revealed in a recent paper accepted for publication in a future issue of Astronomy and Astrophysics, but available now in preprint form on the arXiv server.
Blending into the Cosmic Background
Despite their enormous sizes, BCGs are notoriously difficult for astronomers to study. Because they are built through the gradual merger of many smaller galaxies, their outer regions are extremely faint, containing diffuse stellar halos and tidal tails left behind by ancient collisions. These wispy edges often blend into the free-floating gas and stars that fill galaxy clusters, making it difficult to determine precisely where the galaxy ends and the surrounding cluster begins.
Carlos Marrero-de la Rosa of the Astrophysical Institute of the Canary Islands led the new study, focusing on IC 1101, the dominant galaxy at the center of the Abell 2029 galaxy cluster.
The research relied on ultra-deep images captured with the Wide Field Camera at the Isaac Newton Telescope, allowing astronomers to resolve the galaxy’s diffuse outer boundary in unprecedented detail.
Battling Light Pollution
With the number of satellites orbiting Earth increasing dramatically in recent years, astronomers have raised concerns about artificial light pollution. In this case, however, the researchers faced interference from a far more distant source: foreground stars within our own galaxy.
These stars, located between Earth and IC 1101, scatter light across telescope images, washing out the extremely faint structures astronomers are attempting to detect. Since the galaxy’s outermost regions are already difficult to distinguish, any additional scattered light significantly reduces image contrast.
To overcome this problem, the researchers constructed a detailed model of how the telescope spreads light using dozens of calibration stars. The model allowed the team to accurately subtract scattered light from more than 250 foreground stars, bringing IC 1101’s faintest structures into much sharper contrast and enabling a more precise measurement of its true extent.
The Largest Known Galaxy
IC 1101’s immense scale becomes even more striking when compared with our own Milky Way. The astronomers measured IC 1101 at roughly 520 kiloparsecs across—about 17 times wider than the Milky Way’s approximately 30-kiloparsec diameter. Even after accounting for all forms of mass, including elusive dark matter, scientists estimate that the Milky Way contains only about 1 to 1.5 trillion solar masses of material, compared with the 3.4 trillion solar masses contained in the stars of IC 1101 alone.
Researchers also found evidence that the giant galaxy continues to grow. They identified eight structures beyond its main edge, two of which align with previously observed X-ray disturbances that suggest ongoing accretion. According to the team, IC 1101 likely collided with another galaxy group between two and three billion years ago, producing a merger that continues to shape the galaxy today.
The paper, “How Large Can Galaxies Be? Ultra-Deep Imaging of IC 1101, the Most Extended Known Galaxy,” is available on arXiv, ahead of later publication in Astronomy and Astrophysics.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org.