November 3, 2011

The oldest mature cluster of galaxies

Astronomer working on data from various observatories including XMM-NEWTON discovered the oldest mature cluster of galaxies.
Amas de galaxie évolué le plus ancien
Reconstruction from pictures taken by the Very Large Telescope of ESO and the Subaru telescope of NAOJ. The red objects on the right of the centre are the cluster CL J1449+0856. © ESO/NOAJ/Subaru/R

An international team of astronomers used an armada of ground-based and space-borne telescopes in order to analyse and accurately measure the distance to the most remote mature cluster of galaxies yet discovered. The telescopes used for this purpose were:

  • the VLT of ESO;
  • the Keck and Subaru telescopes in Hawaii;
  • the Spitzer and Chandra satellites of NASA;
  • the ESA's XMM-NEWTON and NASA/ESA's Hubble space-borne telescopes.

Although this cluster is seen when the universe was less than one quarter of its current age, it looks surprisingly similar to galaxy clusters in the current universe.

“We have measured the distance to the most distant mature cluster of galaxies ever found”, Raphael Gobat (lead author of the study) of the Laboratory Astrophysics, instrumentation and modelling (AIM - CEA, CNRS, University Paris Diderot), said. “The surprising thing is that when looking closely at this galaxy cluster, it does not look young - many of the galaxies have settled down and do not resemble the usual star-forming galaxies seen in the early universe.”

The clusters of galaxies are the largest structures of the Universe. The astronomers expect these clusters to grow over time and hence that massive clusters would be rare in the early universe. Although even more distant clusters were been seen, they appear to be young clusters in the process of formation and are not settled mature systems.

This grouping, called CL J1449+0856, had all the hallmarks of being a very remote cluster of galaxies, just like it was when the universe was approximately three billion years old - less than one quarter of its current age.

Once the team knew the distance to this very rare object, they looked carefully at the component galaxies using both the NASA/ESA Hubble Space Telescope and ground-based telescopes, including the VLT.  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 Galaxy.

Further evidence that this is a mature cluster comes from observations of X-rays coming from CL J1449+0856 made with ESA's XMM-NEWTON space observatory. The cluster is giving off X-rays that must be coming from a very hot cloud of tenuous gas filling the space between the galaxies and concentrated towards the centre of the cluster. This is another sign of a mature galaxy cluster, held firmly together by its own gravity, as very young clusters have not had time to trap hot gas in this way.

As Gobat concluded: “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 spot one.“ ”But if further observations find many more then my way mean that our understanding of the early universe needs to be revised.”

Article references

A mature cluster with X-ray emission at z = 2.07, R. Gobat1, E. Daddi1, M. Onodera2, A. Finoguenov3, A. Renzini4, N. Arimoto5,6, R. Bouwens7, M. Brusa3, R.-R. Chary8, A. Cimatti9, M. Dickinson10, X. Kong11 and M. Mignoli12Astronomy & Astrophysics, 2011

1 Laboratoire AIM-Paris-Saclay, CEA/DSM-CNRS–Université Paris Diderot, Irfu/Service d'Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France e-mail:
2 Institute for Astronomy, ETH Zürich, Wolfgang-Pauli-strasse 27, 8093 Zürich, Switzerland
3 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
4 INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
5 National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo, Japan
6 Graduate University for Advanced Studies, Osawa 2-21-1, Mitaka, Tokyo, Japan
7 UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
8 Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
9 Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
10 National Optical Astronomy Observatory, PO Box 26732, Tucson, AZ 85726, USA
11 Center for Astrophysics, University of Science and Technology of China, Hefei 230026, PR China
12 INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, I 40127 Bologna, Italy


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