Updated

Updated · ScienceAlert · Aug 21

Astronomers Confirm IC 1101 Spans 1.7 Million Light-Years as Largest Known Galaxy

Updated

Updated · ScienceAlert · Aug 21

Astronomers Confirm IC 1101 Spans 1.7 Million Light-Years as Largest Known Galaxy

Q: If earlier size estimates of the universe's largest galaxy were actually inflated, where does a galaxy truly end and empty space begin?

A galaxy does not end at a crisp physical border; it ends, in practice, where its stars stop behaving as one coherent system and blend into the surrounding environment. That is why earlier estimates for IC 1101 were likely too large. Its outer starlight is so faint that it can be confused with the diffuse intracluster light of Abell 2029—the glow from stars no longer cleanly bound to any one galaxy. In the new study, astronomers used the deepest images yet, removed scattered light from foreground stars and the galaxy’s bright core, and then looked for a common transition in several independent properties: surface brightness, color, stellar mass density, shape and orientation. Those changes all appeared at about 260 kiloparsecs from the center, which they identify as the edge of IC 1101’s main stellar body. So the dividing line between “galaxy” and “empty space” is not really empty space at all. Beyond the main body, there can still be stray stars, tidal plumes, merger debris, hot gas and an even larger dark matter halo. What changes is that the material is no longer part of the galaxy’s main organized stellar structure, and may instead belong to the wider cluster. In short, astronomers define a galaxy’s edge observationally and physically, not by where matter vanishes, but by where the galaxy’s own coherent structure gives way to the diffuse background around it. IC 1101 is now the clearest example of that boundary being measured directly.

3 articles · Updated · ScienceAlert · Aug 21

Deep imaging fixed IC 1101’s main stellar boundary at 260 kiloparsecs from its center, putting the galaxy’s diameter at about 1.7 million light-years.
The team reached that measurement by stripping away scattered foreground and central light, then finding matching shifts in brightness, color, stellar mass, and shape at the same radius.
Faint filamentary and plume-like structures beyond that edge suggest IC 1101 is still accreting stars and debris from smaller galaxies even as its main body remains coherent.
X-ray disturbances around the galaxy line up with some outer features, pointing to an off-axis merger with a smaller galaxy cluster a few billion years ago.
Researchers say IC 1101 is the strongest current constraint on galaxy size, but plan to apply the same methods to test whether other similarly huge systems remain undiscovered.