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MACS: The Most Massive Cluster of Galaxies in the Universe

by Harald Ebeling

X-ray contours from a Chandra observation overlaid on an optical image of MACSJ0717.5+3745. The very extended X-ray emission traced by the contours marks the distribution of the hot gas in this system. Image by Harald Ebeling.

Clusters of galaxies are the largest objects in the Universe that are held together by gravity. Massive galaxy clusters extend over several megaparsecs (millions of light-years) and contain easily ten thousand times as much mass as our own galaxy (the Milky Way).

Forming these systems takes a long time. Clusters grow to their enormous size by merging with other, smaller clusters and with galaxies over several billion years. Massive systems consist of hundreds to thousands of groups of galaxies (which, however, make up only 5 percent of the overall cluster mass), hot gas (15 percent), and dark matter (80 percent). This mix of constituents makes clusters of galaxies ideal subjects for studies of the interaction and evolution of luminous (visible) matter and dark matter.

dark matter: Matter that cannot be seen because it emits no radiation, but whose presence is inferred from its gravitational effect on the motions of stars and galaxies. Scientists now believe that most of the matter in the Universe is dark matter.

megaparsec (Mpc): One million parsecs. The distance between the Sun and nearby stars is about one parsec. The distance to the Milky Way's nearest neighbor galaxy, Andromeda, is about 1 Mpc. A gigaparsec is one billion parsecs.

The X-ray Advantage Finding massive clusters of galaxies is a challenging task. Images showing galaxies taken in visible light often show spurious clusters that are optical illusions resulting from galaxies at different distances along the same line of sight.

Currently, the most objective way of selecting a sample of real massive clusters is through X-ray observations. This is because the X-ray emission from a galaxy cluster does not originate from the individual galaxies but rather from the diffuse, hot gas trapped by the cluster's gravity. Galaxy clusters thus stand out in X-ray images as huge, fuzzy blobs among the almost pointlike emission from, for instance, stars or black holes. This makes clusters easily detectable from their X-ray emission out to large distances.

MACS: The MAssive Cluster Survey Led by researchers from IfA, an international collaboration of scientists has recently taken statistical cluster studies to a new level by compiling the first large X-ray-selected sample of galaxy clusters that are both massive and distant (until now, it was one or the other). The aptly named MAssive Cluster Survey (MACS) discovered well over a hundred giant clusters at distances of at least 1 gigaparsec, providing a well-defined sample that is more than thirty times larger than the previously best compilation of this kind.

A unique experiment, MACS allows us to study many dozens of massive galaxy clusters in great detail. It thereby provides a snapshot of the evolutionary sequence of cluster growth through mergers and thus unprecedented insight into the processes governing the formation and growth of large-scale structure on timescales far beyond what is directly measurable.

Although finding these elusive systems was perhaps the biggest challenge, it is but the first step toward the MACS project's ambitious science goals. Using the world's best observational facilities, including the optical telescopes on Mauna Kea and NASA's orbiting Chandra X-ray Observatory, the MACS team is now conducting deep follow-up observations of MACS clusters in the radio, optical, and X-ray wavebands.

The Assembly of a Giant Cluster A particularly striking MACS discovery and a superb example of the power of this project's multiwavelength observations is the detection of an enormous galaxy cluster named MACSJ0717.5+3745 (see page 1). The overall elongation of both the X-ray emission and the galaxy distribution suggests an ongoing cluster-cluster collision. The contours in the second image (this page) represent the distribution of the total mass (visible and dark matter) of the cluster. While the peaks of the mass distribution roughly follow those of the galaxy distribution, they are nowhere near the peak of the X-ray emission. We do not yet know the cause of this discrepancy, but it is this type of unexpected results that will eventually advance our understanding of the processes by which clusters form and grow.

This larger field of view shows the distribution of all matter in MACSJ0717.5+3745 (upper right), with the newly discovered cluster (lower left).

The mass map of MACSJ0717.5+3745 shown in the second image also holds another surprise. It shows a second mass concentration associated with an overdensity of red galaxies southeast of MACSJ0717.5+3645. Observations obtained recently with the Keck I telescope on Mauna Kea confirm that this second cluster is at the same distance as MACSJ0717.5+3645 proper. This means that another cluster-cluster collision and merger will cause this already giant system to grow yet more massive in the near future. Well, within another billion years or two.

MACS Researchers at the IfA
  Harald Ebeling
  Nick Kaiser
  Elizabeth Barrett
  David Donovan