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XMM-Newton
digs into the secrets of fossil galaxy clusters
27 April 2006
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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
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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 as 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.
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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.
Credit:
ESA
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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.
Source
/ Credit: ESA
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