How big is Abell 2029?

Abell 2029 is a large cluster of galaxies about 1 billion light-years from Earth, in the constellation Virgo. It is about 12.5 million light-years across, measured to the boundary inside which the cluster has settled under its own gravity. A light-year is the distance light travels in one year, about 9.46 trillion kilometers.

Size

A galaxy cluster has no edge. Its galaxies, hot gas and dark matter thin out gradually, and more matter keeps falling in from outside. So astronomers measure a cluster to a standard boundary called the virial radius. Inside it, the cluster's gas has settled into balance under gravity, while outside it matter is still falling in.

In 2012, Stephen Walker and colleagues used the Suzaku X-ray satellite to map the cluster's hot gas out to that boundary. They put the radius at about 1.9 megaparsecs, where a megaparsec is about 3.26 million light-years. A 2019 survey of the cluster's galaxies by Jubee Sohn and colleagues uses nearly the same radius. That makes the cluster about 12.5 million light-years across, uncertain by less than 1 million light-years either way.

Matter reaches well beyond this boundary. In the surrounding region, where galaxies are still falling toward the cluster, Sohn's team found member galaxies as far as about 29 million light-years from the center, measured across the sky rather than along the line of sight.

Size comparisons

Abell 2029 is about 125 times as wide as the disk of the Milky Way, which is about 100,000 light-years across.
If the Milky Way were shrunk to the size of a basketball, Abell 2029 would be about 30 meters (100 feet) across.
Its width is about five times the distance from the Milky Way to the Andromeda Galaxy, about 2.5 million light-years.
The faint halo of IC 1101, the giant galaxy at the cluster's center, spans about 4 million light-years, about a third of the cluster's width.

What the cluster is made of

Abell 2029 holds more than a thousand galaxies: Sohn's survey counted 1,215 member galaxies in the cluster and the groups falling into it. The brightest is IC 1101, at the center. Along with Abell 2033, Abell 2028 and Abell 2066, Abell 2029 forms a small supercluster.

Between the galaxies is a vast cloud of gas heated to roughly 90 million degrees Celsius, hot enough to glow in X-rays. Walker's team estimates the total mass inside the virial radius at about 800 trillion times the mass of the Sun. Hot gas makes up about a sixth of that mass near the boundary. Most of the rest is dark matter, an invisible form of matter detected by its gravity.

A calm cluster with a stormy past

Abell 2029 is sometimes described as "the most relaxed cluster in the Universe", because its hot gas looks smooth and undisturbed. In 2026, the deepest X-ray observation yet made of it, with NASA's Chandra X-ray Observatory, showed that it has not always been calm.

The Chandra images show a spiral of gas reaching about 2 million light-years from the center. The researchers think it formed when a smaller cluster, about one-tenth as massive, collided with Abell 2029 about 4 billion years ago. The collision made the cluster's gas slosh to one side, like wine swirled in a glass.

The cluster is still growing. Two smaller groups of galaxies, Abell 2033 and a group to the south, are falling toward it and are expected to merge with it within a few billion years.