The largest known galaxy is still growing: ultra-deep imaging maps IC 1101’s true stellar envelope

For years, the supergiant elliptical IC 1101 has held the informal title of «largest known galaxy» in astronomy textbooks — but with an uncomfortable footnote. Nobody had actually measured where it ended. Its stellar envelope faded so gradually into the surrounding intracluster gas that its outer boundary lay below the sensitivity of every available survey, and the diameter cited in the literature drifted from paper to paper depending on how deep the underlying imaging happened to reach.

That footnote can now come off. A team led by Carlos Marrero-de la Rosa has just published the first defensible measurement of IC 1101’s true extent, using ultra-deep imaging that finally reaches the faint tidal surface brightness where the galaxy’s stars actually stop being detectable. The result, posted to arXiv on July 16, 2026, settles the number: about 260 kiloparsecs from the center, or roughly 1.7 million light-years across, with an enclosed stellar mass near 3.4 trillion solar masses.

Why the edge was so elusive

Brightest Cluster Galaxies (BCGs) sit at the deepest gravitational wells of their host clusters and grow through prolonged hierarchical accretion — successive mergers with smaller galaxies that get shredded and add their stars to an extended stellar halo. That halo does not stop cleanly. It fades logarithmically, dimming by orders of magnitude before dropping below the sky background of typical imaging surveys. In practical terms, «the edge of a BCG» has almost always meant «wherever your telescope quits picking up light.»

For IC 1101, at the heart of the massive cluster Abell 2029, that quitting point has moved outward every time a deeper survey came along. The physics of the galaxy was never really in doubt; the photometry was.

The Isaac Newton Telescope’s deep dive

Marrero-de la Rosa’s team used the Isaac Newton Telescope’s Wide Field Camera (INT/WFC), pushing to surface-brightness levels well below anything achieved by SDSS DR16 or by the DESI Legacy Imaging Surveys DR9 based on DECaLS. The INT/WFC frames recover a coherent outer stellar envelope that neither previous survey could resolve, and the isophotal edge criterion the team applies converges on a well-defined radius of approximately 260 kpc — implying a stellar diameter of about 520 kpc, some seventeen times the size of the Milky Way’s disk. The enclosed stellar mass comes out to roughly 3.4 × 10¹² solar masses.

Still accreting

The real scientific news is what sits beyond the edge. The imaging also picks up faint, asymmetric plumes and tidal debris in the outermost regions, and these features align spatially with previously mapped X-ray disturbances in the intracluster gas. The straightforward interpretation is that IC 1101 has not finished. Smaller galaxies are still being torn apart and deposited in its outer halo; the record-holder is still assembling itself in real time.

That distinction matters beyond IC 1101 itself. Broader claims about «the largest galaxy ever discovered» often cite the giant radio galaxy Alcyoneus, which spans some sixteen million light-years — but that structure is defined by radio lobes launched far beyond its central host. IC 1101 remains the largest known galaxy in the strict stellar sense of the term, and now the largest one whose extent astronomers can defend with a specific, reproducible number.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: Carlos Marrero-de la Rosa et al., arXiv:2607.15340 (2026). DOI: 10.48550/arXiv.2607.15340