A Chain Cluster: Witnessing the Formation of a Rich Galaxy Cluster7 Billion Years Ago (Forwarded)

Subaru Telescope
National Astronomical Observatory of Japan
Hilo, Hawaii

December 22, 2003

A Chain Cluster: Witnessing the Formation of a Rich Galaxy Cluster 7 Billion
Years Ago

Object: Galaxy Cluster RX J0152.7-1357
Distance: Approximately 7 billion light years
Telescope: Subaru Telescope (Effective Diameter 8.2m)
Focus: Prime Focus
Instrument: Suprime-Cam
Filters: V (0.55micron),R (0.65micron), i' (0.77micron)
Color Composition: Blue =V, Green = R, Red = i'
Observation Date: UT, 26 Sep 2003 (V), 27 Sep (R), 26 Sep (i')
Exposure Time: 120min (V), 116min (R), 75min (i')
Field of View: 3arcmin x 3arcmin
Orientation: North up, East left
On Sky Location: Constellation Cetus

Galaxies tend to be clustered in the Universe -- such systems are called a
'galaxy cluster'. A scientific research project called PISCES (PI: T. Kodama
(NAOJ)) is currently being undertaken with Subaru Telescope, which is aimed to
obtain a panoramic view of cluster formation and the galaxy evolution therein.

This image was taken as a part of this project, showing a central region of a
rich cluster located 7 billion light years from us. The field size is about 4.5
million light years on a side on this cluster scale, corresponding to twice the
distance between our Milky Way Galaxy and our next neighbor Andromeda Galaxy. It
is notable that the hundreds of red galaxies are clustered within this limited
volume of the Universe.

Most of the reddish galaxies in the image are supposed to be associated with
this cluster, and they show a chain-like structure composed of three main clumps
and stretching from NE (top left) to SW (bottom right). Such filamentary
structure extends to even outside of this image. This suggests that we are
witnessing the rich cluster in the process of formation by accreting galaxy
groups along the chain-like structure and growing to a larger system. During the
course of this cluster assembly, the individual galaxies therein are also
influenced by the change of their environments with time. Such formation
processes of a galaxy cluster and the related galaxy evolution will eventually
determine galaxy properties, such as morphology and star formation, which are
known to be strongly dependent on the local number density of galaxies.

Also, a large number of blue, arc-shaped galaxies are seen in the image. These
are background galaxies on the other side of the cluster whose light is bent and
stretched due to the gravitational lensing effect by the cluster. By analysing
these lensed objects in detail (number, shapes, 2-D distribution, and so on), we
are able to map out the total mass distribution of the cluster which is
dominated by the invisible dark matter. From this analysis, we can also learn
the effect of dark matter on galaxy formation.

This image shows only a small fraction (~1%) of the entire Subaru image taken
with the large format camera at prime focus. The wide field of view of the
camera is very unique, part of the reason why this distant cluster project on
Subaru is attracting attention from all over the world.

IMAGES:

* Low Res. (72 dpi, 65 KB)
http://www.subaru.naoj.org/Pressrele...RXJ_VRi_72.jpg
* High Res. (300 dpi, 293KB)
http://www.subaru.naoj.org/Pressrele...XJ_VRi_300.jpg