A Mature Cluster of Galaxies in the Young Universe
March 9, 2011
An international team of astronomers has used a combination of space and ground-based telescopes, including the Subaru Telescope in Hawaii, to discover and study the most distant mature cluster of galaxies yet found. Although this newly found cluster existed when the Universe was less than one quarter of its current age, it looks surprisingly similar to full-grown galaxy clusters in the current Universe.
"We have measured the distance to the most distant mature cluster of galaxies ever found", says the research team's lead, Raphael Gobat (CEA, France). "The surprising thing is that when we look closely at this galaxy cluster it doesn't look young --- many of the galaxies appear to have settled down and don't resemble the usual star-forming galaxies seen in the early Universe."
Clusters of galaxies are the largest structures in the Universe that are held together by gravity. Astronomers expect these clusters to grow through time and hence infer that massive clusters would be rare in the early Universe. Although even more distant clusters have been seen, they appear to be so-called "proto-cluster", clusters in the process of formation, and are not settled mature systems.
CL J1449+0856, the newly-found galaxy cluster, is seen at an era when the Universe was only 3 billion years old --- less than a quarter of its present age. The distance to CL J1449+0856 was measured using the VIMOS and FORS2 instruments on the European Southern Observatory's Very Large Telescope (VLT) by targeting some of the galaxies in a curious patch of very faint red galaxies first identified with NASA's Spitzer Space Telescope.
Once the team knew the distance to this very rare object, they looked carefully at the component galaxies using both ground-based telescopes, including the Subaru Telescope, and space telescopes. They found evidence suggesting that most of the galaxies in the cluster were not forming stars but were composed of stars that were already about one billion years old. This makes the cluster a mature object, similar in mass to the Virgo Cluster, the nearest rich galaxy cluster to the Milky Way. The team made extensive use of the images taken with the Subaru Telescope's Suprime-Cam and MOIRCS (Multi-Object Infrared Camera and Spectrograph) to study the properties of stars in member galaxies of CL J1449+0856. The MOIRCS instrument was particularly crucial in estimating the age of the faint red galaxies in the cluster.
The researchers also used the NASA/ESA Hubble Space Telescope to look at the shape of these old galaxies in the cluster and found that most of them are quite compact, a feature consistent with what is expected for galaxies dominated by old stars like mature elliptical galaxies seen in the current Universe.
Further evidence that this is a mature cluster comes from X-ray observations made with the European Space Agency's XMM (x-ray multi-mirror) - Newton space observatory. The cluster shows diffuse X-ray emissions that are concentrated toward the center of the cluster and must be coming from a very hot cloud of tenuous gas filling the space between the galaxies. This is another sign of the cluster's maturity; it is held firmly together by its own gravity, a state which very young proto-clusters cannot attain.
As Gobat concludes: "These new results support the idea that mature clusters existed when the Universe was less than one quarter of its current age. Such clusters are expected to be very rare according to current theory, and we have been very lucky to serendipitously discover one. But if further observations find many more, then this may mean that our understanding of the early Universe needs to be revised."
A Release by ESO (European Southern Observatory) with more images and videos is available from the ESO website, and an article about the methods used in this study, with particular focus on XMM-Newton's contribution, is on the ESA (European Space Agency) website .
This research was presented in a paper, "A mature cluster with X-ray emission at z=2.07", by R. Gobat et al. (Astronomy & Astrophysics, Volume 526, A133)
Raphael Gobat, Emanuele Daddi (CEA/Saclay, France), Masato Onodera (ETH Zurich, Switzerland), Alexis Finoguenov, Marcella Brusa (Max-Planck-Institut fur extraterrestrische Physik, Germany), Alvio Renzini (Osservatorio Astronomico di Padova, Italy), Nobuo Arimoto (National Astronomical Observatory of Japan, Japan), Rychard Bouwens (Lick Observatory, USA), Ranga-Ram Chary (California Institute of Technology, USA), Andrea Cimatti (Universita di Bologna, Italy), Mark Dickinson (National Optical Astronomy Observatory, USA), Xu Kong (University of Science and Technology of China, China), Marco Mignoli (Osservatorio Astronomico di Bologna, Italy)
Figure 2: X-ray overlaid composite image of CL J1449+0856 in which X-ray data is from XMM-Newton observation.