XMM-Newton digs into the secrets of fossil galaxy clusters (Forwarded)
- From: Andrew Yee <ayee@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 15 May 2006 21:07:48 -0400
ESA News
http://www.esa.int
27 April 2006
XMM-Newton digs into the secrets of fossil galaxy clusters
Taking advantage of the high sensitivity of ESA's XMM-Newton and the sharp
vision of NASA's Chandra X-Ray space observatories, astronomers have studied
the behaviour of massive fossil galaxy clusters, trying to find out how they
find the time to form.
Many galaxies reside in galaxy groups, where they experience close
encounters with their neighbours and interact gravitationally with the dark
matter -- mass which permeates the whole intergalactic space but is not
directly visible because it doesn't emit radiation.
These interactions cause large galaxies to spiral slowly towards the centre
of the group, where they can merge to form a single giant central galaxy,
which progressively swallows all its neighbours.
If this process runs to completion, and no new galaxies fall into the group,
then the result is an object dubbed a 'fossil group', in which almost all
the stars are collected into a single giant galaxy, which sits at the centre
of a group-sized dark matter halo. The presence of this halo can be inferred
from the presence of extensive hot gas, which fills the gravitational
potential wells of many groups and emits X-rays.
A group of international astronomers studied in detail the physical features
of the most massive and hot known fossil group, with the main aim to solve a
puzzle and understand the formation of massive fossils. In fact, according
to simple theoretical models, they simply could not have formed in the time
available to them!
The fossil group investigated, called 'RX J1416.4+2315', is dominated by a
single elliptical galaxy located one and a half thousand million light years
away from us, and it is 500 thousand million times more luminous than the
Sun.
The XMM-Newton and Chandra X-ray observations, combined with optical and
infrared analyses, revealed that group sits within a hot gas halo extending
over three million light years and heated to a temperature of 50 million
degrees, mainly due to shock heating as a result of gravitational collapse.
Such a high temperature, about twice as the previously estimated values, is
usually characteristic of galaxy clusters. Another interesting feature of
the whole cluster system is its large mass, reaching over 300 trillion solar
masses. Only about two percent of it in the form of stars in galaxies, and
15 percent in the form of hot gas emitting X-rays. The major contributor to
the mass of the system is the invisible dark matter, which gravitationally
binds the other components.
According to calculations, a fossil cluster as massive as RX J1416.4+2315
would have not had the time to form during the whole age of the universe.
The key process in the formation of such fossil groups is the process known
as 'dynamical friction', whereby a large galaxy loses its orbital energy to
the surrounding dark matter. This process is less effective when galaxies
are moving more quickly, which they do in massive 'clusters' of galaxies.
This, in principle, sets an upper limit to the size and mass of fossil
groups. The exact limits are, however, still unknown since the geometry and
mass distribution of groups may differ from that assumed in simple
theoretical models.
"Simple models to describe the dynamical friction assume that the merging
galaxies move along circular orbits around the centre of the cluster mass,"
says Habib Khosroshahi from the University of Birmingham (UK), first author
of the results. "Instead, if we assume that galaxies fall towards the centre
of the developing cluster in an asymmetric way, such as along a filament,
the dynamic friction and so the cluster formation process may occur in a
shorter time scale," he continues. Such a hypothesis is supported by the
highly elongated X-ray emission we observed in RX J1416.4+2315, to sustain
the idea of a collapse along a dominant filament."
The optical brightness of the central dominant galaxy in this fossil is
similar to that of brightest galaxies in large clusters (called 'BCGs').
According to the astronomers, this implies that such galaxies could have
originated in fossil groups around which the cluster builds up later. This
offers an alternative mechanism for the formation of BCGs compared to the
existing scenarios in which BCGs form within clusters during or after the
cluster collapse.
"The study of massive fossil groups such as RX J1416.4+2315 is important to
test our understanding of the formation of structure in the universe," adds
Khosroshahi. "Cosmological simulations are underway which attempt to
reproduce the properties we observe, in order to understand how these
extreme systems develop," he concludes.
Notes to editors:
The XMM-Newton observations of the fossil galaxy cluster RX J1416.4+2315
were performed in July 2003. Chandra's observations of the same object where
made in September 2001.
The findings will appear in the Monthly Notices of the Royal Astronomical
Society (astro-ph/0603606). The article, titled "A fossil galaxy cluster; an
X-ray and optical study of RX J1416.4+2315", is by Habib G. Khosroshahi,
Trevor J. Ponman and Laurence R. Jones (School of Physics and Astronomy, The
University of Birmingham, UK), Ben J. Maughan (Harvard-Smithsonian Center
for Astrophysics, Cambridge, MA, USA).
For more information:
Habib Khosroshahi
School of Physics and Astronomy, University of Birmingham, UK
Email: habib @ star.sr.bham.ac.uk
Norbert Schartel
ESA XMM-Newton Project Scientist
Email: norbert.schartel @ sciops.esa.int
More about ...
* XMM-Newton overview
http://www.esa.int/esaSC/120385_index_0_m.html
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IMAGE CAPTIONS:
[Image 1:
http://www.esa.int/esaCP/SEMCFFOFGLE_index_1.html]
XMM-Newton observations of the fossil galaxy cluster RX J1416.5+2315, show a
cloud of hot gas emitting X-rays (in blue). The cloud, reaching temperatures
of about 50 million degrees, extend over 3.5 million light years and
surround a giant elliptical galaxy believed to have grown to its present
size by cannibalising its neighbours.
Credits: Khosroshahi, Maughan, Ponman, Jones, ESA, ING
[Image 2:
http://www.esa.int/esaCP/SEMCFFOFGLE_index_1.html#subhead2]
The XMM-Newton spacecraft is the biggest science satellite ever built in
Europe. Its telescope mirrors are the most powerful developed so far and,
with its sensitive detectors, it sees much more than any previous X-ray
satellite.
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