How big is IC 1101?

IC 1101 is a giant galaxy at the center of the galaxy cluster Abell 2029, about 1 billion light-years from Earth. It has no sharp edge, so its size depends on how faint a glow is counted. Its bright main body is about 350,000 light-years across, and a very faint halo of stars around it has been traced across about 4 million light-years. A light-year is the distance light travels in one year, about 9.46 trillion kilometers.

Size

To compare galaxies fairly, astronomers measure each one out to the same faint brightness, a level called D25. At that level, IC 1101 is about 1.2 arcminutes long on the sky. An arcminute is one-sixtieth of a degree. At the galaxy's distance, that angle spans about 350,000 light-years.

Deeper images reach fainter light. The Siena Galaxy Atlas, which traces galaxies to a fainter level in red light, measures IC 1101 at about 4.9 arcminutes, or about 1.4 million light-years. In 1991, Juan Uson and colleagues used many overlapping exposures to trace an even fainter halo, a stretched-out oval about twice as long as it is wide. Averaged between its long and short directions, it reaches about 2 million light-years from the center.

Whether that halo belongs to IC 1101 or to the cluster around it is partly a matter of definition. The motions of stars far out in the halo follow the gravity of the whole cluster rather than of the galaxy alone.

Size comparisons

IC 1101's bright main body is about three and a half times as wide as the disk of the Milky Way, which is about 100,000 light-years across. Its faint halo is about 40 times as wide.
Measured to the same D25 level, IC 1101 is about 1.4 times as wide as NGC 4889, the brightest galaxy in the Coma Cluster.
The halo is about 1.6 times the distance from the Milky Way to the Andromeda Galaxy (about 2.5 million light-years).
If the Milky Way were shrunk to the size of a basketball, IC 1101's main body would be about 85 centimeters (2.8 feet) across, and its halo about 9.5 meters (31 feet) across.

A galaxy built by mergers

IC 1101 is the brightest galaxy in Abell 2029, a rich cluster of galaxies. Galaxies like it, at the centers of rich clusters, are thought to grow by merging with other galaxies. Most of IC 1101's light comes from its outer parts. In a 2017 analysis of Hubble images, the outer halo gave about 57% of the light, a middle layer about 17%, and the central body only about 25%.

An oversized core

That 2017 study, by Bililign Dullo and colleagues, found that IC 1101 has an unusually large core. A core is a central region where stars are spread more thinly than the galaxy's outer parts would suggest. IC 1101's core is about 14,000 light-years in radius. When it was measured, it was the largest core found in any galaxy, about 10 times the size of typical cores.

Astronomers think such cores form after two galaxies merge. Their central black holes sink toward the middle and circle each other, flinging nearby stars away. The size of IC 1101's core suggests that its central black hole could be 40 billion to 100 billion times the mass of the Sun, though that estimate is indirect.