Using the most detailed images ever captured, scientists have now delineated the outer boundary of IC 1101, the largest galaxy discovered to date. This colossal galaxy measures about 520 kiloparsecs in diameter, equivalent to nearly 1.7 million light-years, and harbors an estimated 3.4 trillion solar masses in stars, establishing its position as the most massive galaxy on record.
The research, spearheaded by Carlos Marrero-de la Rosa from the Instituto de Astrofísica de Canarias, was shared on the arXiv platform on July 16. By pairing ultra-sensitive imaging techniques with sophisticated light-scattering corrections, the team successfully distinguished the galaxy’s dim peripheral regions from the diffuse glow emanating from its cluster.
Located at the heart of the Abell 2029 galaxy cluster, IC 1101 is classified among the brightest cluster galaxies (BCGs). These galaxies grow over billions of years by gradually merging with smaller neighbors. As they develop, their extended stellar halos become intertwined with the cluster’s intracluster light, making the exact boundaries challenging to determine.
This blending has historically made it difficult to set precise limits on IC 1101. Previous data hinted at this galaxy’s vast size, but a definitive physical edge remained elusive until now. The new observations were specifically designed to trace the faintest visible light to reveal that border.
Pinpointing IC 1101’s Outer Limits with Ultra-Deep Imaging
The study details how researchers utilized ultra-deep observations from the Wide Field Camera mounted on the Isaac Newton Telescope to explore the galaxy’s faint outskirts. Detecting these subtle features required removing foreground star interference, as scattered light from these stars can obscure extremely low-intensity galactic emissions.
To address this challenge, a detailed model of the telescope’s point spread function was developed using a broad sample of calibration stars with varying brightness. This allowed the team to systematically subtract the scattered light from over 250 foreground stars, revealing a much clearer view of IC 1101’s extended halo.
The cleaned radial brightness profiles unveiled a definitive edge around 260 kiloparsecs from the galaxy’s core, corresponding to a span of about 520 kiloparsecs across. The authors note:
“The edge radius measured for IC 1101 remains among the largest values reported to date for an individual galaxy, corresponding to a projected diameter of ∼520 kpc.”
Enormous Scale Compared to the Milky Way
The findings emphasize the immense proportions of IC 1101, which possibly contains around 3.4 trillion solar masses worth of stars, though the exact figure depends on where the galaxy’s boundary is drawn.
By way of comparison, the Milky Way stretches just about 30 kiloparsecs in diameter. In other words, IC 1101’s span is roughly 17 times larger than that of our home galaxy. Notably, the stellar mass of IC 1101 alone surpasses existing estimates of the Milky Way's entire mass, including dark matter, which is believed to range from 1 to 1.5 trillion solar masses.
The preprint confirms that IC 1101 is the largest individual galaxy whose size has been firmly established through direct observation.
Delicate Faint Features Shed Light on Galaxy Growth
The high-sensitivity images revealed even more than just the boundary of IC 1101. The team identified eight large, faint, and uneven stellar streams extending beyond the main edge, offering clues about its dynamic growth.
The study indicates that two of these features coincide with previously detected X-ray “sloshing” patterns in the hot intracluster gas of the Abell 2029 cluster. These X-ray irregularities were linked to a merger event with another galaxy group that took place around 2 to 3 billion years ago.
“IC 1101 stands as the largest galaxy known to date, yet its outskirts reveal signs of ongoing mass assembly,” the authors wrote.
These faint stellar structures suggest that IC 1101 is still actively growing by absorbing surrounding material and has yet to reach a stable, settled state.
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