How large can a galaxy become? Astronomers have spent decades trying to answer that question, and a new study of IC 1101, the giant elliptical at the heart of the Abell 2029 cluster, has pushed the known upper limit further than ever before.
Using the deepest images ever taken of this galaxy, obtained with the Isaac Newton Telescope’s Wide Field Camera (INT/WFC) in Chile, a team led by Carlos Marrero-de la Rosa and Ignacio Trujillo has confirmed that IC 1101’s main body extends to an edge radius of 260 kiloparsecs (848,000 light-years) along its semi-major axis. That gives it a confirmed diameter of roughly 520 kiloparsecs, or about 1.7 million light-years. For comparison, the Milky Way’s stellar disk spans approximately 100,000 light-years.
The study, accepted for publication in Astronomy & Astrophysics, reached a surface brightness depth of 30 magnitudes per square arcsecond, deep enough to detect the galaxy’s faintest outer extremities. At that level, the team used advanced point spread function modeling and a hybrid wavelet-based deconvolution technique to subtract scattered light from both stars and the galaxy itself, revealing the true edge.
At the Extreme End of the Mass-Size Relation
IC 1101 encloses an estimated 3.4 trillion solar masses in stars, placing it at the extreme upper end of the mass-size relation for galaxies. The edge radius of 260 kpc is among the largest ever measured for any galaxy, confirming IC 1101 as the most extended galaxy known.
But the galaxy is not a static relic. The ultra-deep images reveal a large number of asymmetrical, very low surface brightness features around the galaxy’s outskirts. These features correlate spatially with disturbances in the intracluster medium of Abell 2029 previously observed in X-ray studies, suggesting that IC 1101 is actively accreting material from its environment.
“The outskirts show clear signatures of ongoing mass assembly, indicating that its spatial extent is still growing,” the authors report. This means that even at its already record-breaking size, IC 1101 has not yet reached its final form.
A cD Galaxy at the Heart of a Cluster
IC 1101 is classified as a cD galaxy, a type of supergiant elliptical found at the centers of rich galaxy clusters. These galaxies grow by cannibalizing smaller galaxies that fall into the cluster and by accreting intracluster gas. The Abell 2029 cluster, located about 1.15 billion light-years away in the constellation Virgo, provides a rich feeding ground.
The galaxy also hosts one of the largest supermassive black holes ever discovered, with an estimated mass between 50 and 70 billion solar masses, and possesses the largest diffuse core of any known galaxy. The core’s properties suggest that the merger of central black holes during the galaxy’s formation may have flung stars out of the core region, creating the observed stellar mass deficit.
The Question of an Upper Limit
The study provides an observational constraint on one of the fundamental open questions in astrophysics: is there a maximum size that galaxies cannot exceed? The existence of IC 1101 at 520 kpc, still actively growing, suggests that the upper limit may be higher than previously thought. Gravitational dynamics, gas supply, and the cluster environment all play roles in setting a galaxy’s ultimate extent, but IC 1101 indicates that nature can build structures far larger than the Milky Way.
Whether even larger galaxies await discovery, perhaps in clusters richer than Abell 2029, remains an open question. For now, IC 1101 holds the record, and it is not done growing.