Astronomers have determined just how large the Universe’s largest galaxy is

IC 1101 lies at the centre of the Abell 2029 galaxy cluster. Such giant galaxies are constantly growing by absorbing their smaller neighbours.

The main problem was that the outer regions of the galaxy blend seamlessly into the faint glow of stars filling the space between the galaxies in the cluster. Because of this, scientists were unable to determine for a long time where the galaxy itself ended.

Details

For the new study, astronomers used the wide-angle camera on the Isaac Newton Telescope.

To visualise the faintest outer regions of IC 1101, the researchers removed the scattered light from more than 250 foreground stars from the images. This enabled them to identify the galaxy’s true outer boundary.

According to the scientists’ calculations, its diameter is around 520 kiloparsecs, or approximately 1.7 million light-years.

17 times larger than the Milky Way

By way of comparison, the diameter of our galaxy — the Milky Way — is around 30 kiloparsecs.

Thus, IC 1101 is approximately 17 times larger in diameter than our galaxy.

The researchers also estimated the star mass of this giant to be approximately 3.4 trillion solar masses.

The galaxy continues to grow

Even after determining its main boundary, astronomers discovered eight very faint, extended stellar structures around IC 1101.

According to the authors, these are traces of the ongoing absorption of other galaxies. In other words, the largest known galaxy in the Universe is still growing in size.

Some of these structures coincide with previously detected disturbances in hot gas, which likely arose following a collision between galaxy clusters several billion years ago.

The authors note that the paper has been published on the arXiv preprint server and has not yet undergone independent peer review.

Nevertheless, the results are based on the deepest observations of IC 1101 carried out to date and allow for a much more accurate assessment of the size of this cosmic giant.

Source

Study: Carlos Marrero-de la Rosa et al.

How large can galaxies be? Ultra-deep imaging of IC 1101, the most extended known galaxy.

Platform: arXiv (preprint), 2026.