Galaxy clusters are the largest known gravitationally-bound objects in the Universe. For the first time a team of astrophysicists including Postdoctoral Fellow Dr. Nobuhiro Okabe and Associate Research Fellow Dr. Keiichi Umetsu from the Institute of Astronomy and Astrophysics at Academia Sinica (ASIAA) and Special Postdoctoral Researcher Dr. Madoka Kawaharada from the Institute of Physical and Chemical Research (RIKEN, Japan), have mapped the growth of these cosmic structures. Their research will be published online in The Astrophysical Journal, a leading astrophysics journal, on April 9, 2010.

    Galaxy clusters are composed of a few hundred to a few thousand galaxies, as well as X-ray emitting hot gas and dark-matter. The research team has revealed a snapshot of the dramatic growth of a galaxy cluster named Abell 1689. They discovered that the cluster grows through heating triggered by matter falls from a filamentary large-scale structure outside the cluster known as a “cosmic web”.

    Using the Suzaku X-ray satellite, the team was able to measure X-ray emission from the hot gas in the outermost regions of Abell 1689. They discovered that the anisotropic temperature distribution of the hot gas is 58 million degrees in one direction and 23 million degrees in the other around the boundary of the cluster. They then compared their X-ray data with a galaxy map made from Sloan Digital Sky Survey (SDSS) data and found that the filamentary large-scale structure of galaxies outside the cluster contacts the hotter realm inside the cluster; in contrast, the other low-density fields, which contain fewer galaxies, contact the cooler gas. The team also compared their X-ray data with gravitational lensing data from the Japanese Subaru telescope and the Hubble Space Telescope, and found that the cooler gas is likely to have sub-sonic motion. Joint analyses of the X-ray, optical and gravitational lensing data have unveiled a clear snapshot of the growth of galaxy clusters being affected by the large-scale structure within which clusters are embedded.

    The Suzaku satellite is Japan's fifth X-ray observatory managed by the Japan Aerospace Exploration Agency (JAXA). It was developed by several Japanese institutions in collaboration with NASA Goddard Space Flight Center (GSFC) in the U.S.

    Dr. Nobuhiro Okabe is a Postdoctoral Fellow at the Institute of Astronomy and Astrophysics (ASIAA). His fields of specialization include galaxy clusters, weak lensing, X-ray and optical analyses and plasma physics. Dr. Keiichi Umetsu is an Associate Research Fellow at ASIAA. He has been a science team member of The Yuan-Tseh Lee Array for Microwave Background Anisotropy (AMiBA) project since 2001. His research primarily focuses on the study of galaxy clusters, in particular, weak and strong gravitational lensing.

    The other researchers in the project are from a number of Japanese institutions including RIKEN (The Institute of Physical and Chemical Research); Astronomical Institute, Tohoku University; Department of Physics, Yamagata University; Department of Physics, Tokyo University of Science; Department of Physical Science, Hiroshima University; National Astronomical Observatory of Japan; Department of Physics, The University of Tokyo; Department of Physics, Tokyo Metropolitan University.

    The research entitled “Suzaku Observation of Abell 1689: Anisotropic Temperature and Entropy Distributions Associated with the Large-scale Structure”, will be published online in The Astrophysical Journal on April 9, 2010. In 2008 The Astrophysical Journal had an impact factor of 6.33.