The galaxy 3C438 and its cluster of galaxies as seen in the optical (left) and in X-rays by the Chandra X-ray Observatory (right). Astronomers have concluded that the hot gas is the result of a collision between two clusters of galaxies. Credit: X-ray: NASA/CXC/CfA/R.P.Kraft; Optical: Pal.Obs. DSS
Galaxy
clusters contain a few to thousands of galaxies and are the largest
bound structures in the Universe. Most galaxies are members of a
cluster. Our Milky Way, for example, is a member of the "Local Group," a
set of about fifty galaxies whose other large member is the Andromeda
galaxy. The closest large cluster of galaxies to us, about fifty
million light-years away, is the Virgo Cluster, with about 2000 members.
Clusters are believed to grow as the result of mergers between
smaller galaxy groups and from the accretion of gas and dark matter. The
energy released in these mergers is largely dissipated in the hot gas
within the cluster, where X-ray observations can spot evidence for
shocks and high temperatures. Mergers between two equally massive
galaxy clusters provide particularly important diagnostics since these
energetic collisions have the most dramatic and long-lasting effects.
These major mergers are relatively rare events, however. The Bullet
Cluster is one recently analyzed example, and because it also happens to
act as a gravitational lens for background galaxies, it became famous
for showing the distribution of its dark matter.
CfA astronomers Deanna Emery, Akos Bogdan, Ralph Kraft, Filipe
Andrade-Santos, Bill Forman, and Christine Jones, and another colleague
studied another major merger in the cluster around the galaxy 3C438.
Members of the team used the Chandra X-ray Observatory to examine the
hot cluster gas. Previous observations had concluded that activity was
due either to a supermassive black hole or from a massive merger, but
the two could not be distinguished. Using additional Chandra
observations and new calibration procedures, the scientists re-reduced
all the data. They found that the hot cluster gas extends over a
distance of about 2.5 million light-years and has brightness features
apparently caused by a merger bow shock. They are even able to
calculate the estimated relative velocity of the merger as about 2600
kilometers per second. Since few observations of bow shocks in clusters
have been made, this detection makes an important contribution to the
study of the dynamics of cluster mergers and how massive clusters may
have formed.
Reference(s):
"A
Spectacular Bow Shock in the 11 keV Galaxy Cluster Around 3C 438,"
Deanna L. Emery, Akos Bogdan, Ralph P. Kraft, Felipe Andrade-Santos,
William R. Forman, Martin J. Hardcastle, and Christine Jones, ApJ, 2016 (in press).
