Andrew Yee[_1_]
January 20th 08, 03:00 AM
Observatoire de Paris
Paris, France
Contact:
Yves Revaz
Observatoire de Paris, LERMA, and Observatoire de Geneve
Tel: 33 1 40 51 20 79 Fax: 33 1 40 51 20 02
4 December 2007
Formation of cold filaments in cooling flow clusters
How hot gas in galaxy clusters (with temperature higher than 100 millions
degrees) may cool and flow towards the cluster center in order to feed the
central galaxy? A team of astronomers from Paris Observatory proposes a new
scenario, taking advantage of the jets emitted by the central active
galactic nuclei (AGN). Very high resolution numerical simulations allowed to
understand the formation of the puzzling filaments of cool gas, observed in
the atmosphere of galaxy clusters, like Perseus. Those filaments result from
the cooling of hot gas, trapped in the wake of plasma bubbles formed by the
central AGN. This gas, dragged at higher radius, has time to cool down to
relatively low temperature (below 10 000 degrees) and to fall back, forming
filamentary structures. The mass and the kinematics of the predicted
filaments are in excellent agreement with the observations.
The first X-rays observations in the 70's revealed that galaxy clusters are
dominated by diffused very hot gas (more that 10 millions degrees). At these
temperatures, the hot gas loses a huge amount of energy by thermal emission
and cools. In the most massive clusters, the electronic density is so high,
that the cooling time of the hot gas is much smaller that the age of the
universe. Under these conditions, the gas is not in hydrostatic equilibrium
and flows slowly towards the galactic center. These clusters are named
"cooling flow clusters". If theory predicts large amounts of cooling gas,
observations fail to find as much gas as predicted in the temperature range
between one and 10 millions degrees, leading to a difficult problem. A
solution to this problem could be the quenching or damping of the cooling
due to the heating produced by the central active galactic nuclei (AGN). An
AGN is effectively present at the center of all cooling flow clusters.
Recent observations of H-alpha emission of relatively cold gas (10 000
degrees) and very cold gas (tens of degrees) traced by the emission of the
CO molecule, showed that gas with low temperature is present in the
atmosphere of those clusters. However, the gas mass derived from these
observations is 10 times below the predictions of the simpler model (without
AGN). The spatial distribution of this gas is surprising (see Fig. 1). The
gas forms filamentary structures, spread all around the cluster center.
But the link between the cluster cooling and the presence of the cold gas if
far from obvious. Those filaments are very extended (more that 200 000 light
years for the longest) and cross regions with cooling times differing by
more than an order of magnitude. Moreover, those filaments have a peculiar
velocity, indicating that they are stretching (the gas seems to raise at the
top of the filaments, while it is falling and being accelerated more and
more near the cluster center).
Using numerical simulations (N-body/hydrodynamics) at very high resolution,
it has been possible to propose a coherent scenario at the origin of the
cold filaments. The central AGN generates a supersonic jet, blowing up
plasma bubbles of very high temperature (hotter than 100 millions degrees).
Those hotter but less dense bubbles (compared to the ambient plasma),
migrate upwards, due to the Archimede's force. During the migration, a
fraction of the ambient gas is dragged by the bubbles at higher radius.
During the migration that lasts more than 600 million years, this gas which
cools relatively rapidly (in 400 million years) , has time to cool below one
million degrees. At this temperature it is no longer supported by the
pressure and consequently falls towards the cluster center, forming a
filamentary structure below the bubble (see Fig. 2). The increase of its
density reinforces its cooling and its temperature quickly falls below 10
000 degrees. The observed stretching of the filaments is well reproduced by
the simulations. The top of the filament is still entrained by the bubble
and moves away from the center, while the bottom is nearly in free fall
towards the center.
In summary, if the central AGN provides heating and contributes to the
global quenching of the cluster cooling, it is also responsible for the
production of cold gas in outlying regions.
The team is composed of:
Yves Revaz (1), Francoise Combes (1), Philippe Salome (2)
(1) LERMA, Observatoire de Paris; (2) IRAM, Grenoble
Reference
Formation of cold filaments in cooling flow clusters
Astronomy & Astrophysics, Letters, December 2007
http://arxiv.org/abs/0711.4051
[NOTE: Images and animations supporting this release are available at
http://www.obspm.fr/actual/nouvelle/dec07/perseus.en.shtml ]
Paris, France
Contact:
Yves Revaz
Observatoire de Paris, LERMA, and Observatoire de Geneve
Tel: 33 1 40 51 20 79 Fax: 33 1 40 51 20 02
4 December 2007
Formation of cold filaments in cooling flow clusters
How hot gas in galaxy clusters (with temperature higher than 100 millions
degrees) may cool and flow towards the cluster center in order to feed the
central galaxy? A team of astronomers from Paris Observatory proposes a new
scenario, taking advantage of the jets emitted by the central active
galactic nuclei (AGN). Very high resolution numerical simulations allowed to
understand the formation of the puzzling filaments of cool gas, observed in
the atmosphere of galaxy clusters, like Perseus. Those filaments result from
the cooling of hot gas, trapped in the wake of plasma bubbles formed by the
central AGN. This gas, dragged at higher radius, has time to cool down to
relatively low temperature (below 10 000 degrees) and to fall back, forming
filamentary structures. The mass and the kinematics of the predicted
filaments are in excellent agreement with the observations.
The first X-rays observations in the 70's revealed that galaxy clusters are
dominated by diffused very hot gas (more that 10 millions degrees). At these
temperatures, the hot gas loses a huge amount of energy by thermal emission
and cools. In the most massive clusters, the electronic density is so high,
that the cooling time of the hot gas is much smaller that the age of the
universe. Under these conditions, the gas is not in hydrostatic equilibrium
and flows slowly towards the galactic center. These clusters are named
"cooling flow clusters". If theory predicts large amounts of cooling gas,
observations fail to find as much gas as predicted in the temperature range
between one and 10 millions degrees, leading to a difficult problem. A
solution to this problem could be the quenching or damping of the cooling
due to the heating produced by the central active galactic nuclei (AGN). An
AGN is effectively present at the center of all cooling flow clusters.
Recent observations of H-alpha emission of relatively cold gas (10 000
degrees) and very cold gas (tens of degrees) traced by the emission of the
CO molecule, showed that gas with low temperature is present in the
atmosphere of those clusters. However, the gas mass derived from these
observations is 10 times below the predictions of the simpler model (without
AGN). The spatial distribution of this gas is surprising (see Fig. 1). The
gas forms filamentary structures, spread all around the cluster center.
But the link between the cluster cooling and the presence of the cold gas if
far from obvious. Those filaments are very extended (more that 200 000 light
years for the longest) and cross regions with cooling times differing by
more than an order of magnitude. Moreover, those filaments have a peculiar
velocity, indicating that they are stretching (the gas seems to raise at the
top of the filaments, while it is falling and being accelerated more and
more near the cluster center).
Using numerical simulations (N-body/hydrodynamics) at very high resolution,
it has been possible to propose a coherent scenario at the origin of the
cold filaments. The central AGN generates a supersonic jet, blowing up
plasma bubbles of very high temperature (hotter than 100 millions degrees).
Those hotter but less dense bubbles (compared to the ambient plasma),
migrate upwards, due to the Archimede's force. During the migration, a
fraction of the ambient gas is dragged by the bubbles at higher radius.
During the migration that lasts more than 600 million years, this gas which
cools relatively rapidly (in 400 million years) , has time to cool below one
million degrees. At this temperature it is no longer supported by the
pressure and consequently falls towards the cluster center, forming a
filamentary structure below the bubble (see Fig. 2). The increase of its
density reinforces its cooling and its temperature quickly falls below 10
000 degrees. The observed stretching of the filaments is well reproduced by
the simulations. The top of the filament is still entrained by the bubble
and moves away from the center, while the bottom is nearly in free fall
towards the center.
In summary, if the central AGN provides heating and contributes to the
global quenching of the cluster cooling, it is also responsible for the
production of cold gas in outlying regions.
The team is composed of:
Yves Revaz (1), Francoise Combes (1), Philippe Salome (2)
(1) LERMA, Observatoire de Paris; (2) IRAM, Grenoble
Reference
Formation of cold filaments in cooling flow clusters
Astronomy & Astrophysics, Letters, December 2007
http://arxiv.org/abs/0711.4051
[NOTE: Images and animations supporting this release are available at
http://www.obspm.fr/actual/nouvelle/dec07/perseus.en.shtml ]