megavideolinks
Joined: 18 Nov 2011
Posts: 63
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 Posted: Mon Feb 27, 2012 4:06 am Post subject: PARADOX BETWEEN |
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The paradox remained: there was absolutely no doubt that the hot gas existed, and according to all the rules of
physics it also had to cool down. But
apparently it does not. Why? All previous observations suffered from the fact
that the X-ray telescopes did not have
a high enough angular resolution, and
so did not show any details in the centre of the galaxy cluster. There were also no sensitive spectrographs. However, an instrument like this is needed to
measure gas temperatures, for example. The European X-ray Observatory
XMM-Newton, launched at the end of
1999, has both of these capabilities. In
2000, Hans Böhringer and colleagues
from the Max Planck Institute for Extraterrestrial Physics in Garching used
XMM-Newton to observe the central
galaxies of a total of three galaxy
clusters, looking for their cooling
flows. The researchers obtained their
best data from the Virgo galaxy cluster
and its central galaxy Messier 87 (M
87 in short), approximately 50 million
light years away. It is one of the largest
known galaxies. The peculiarity of this
galaxy is a collimated gas jet, emerging from the centre and shooting out
into space over a distance of approximately 5,000 light years.
The astronomers had to direct the telescope onto the galaxy for over eleven
hours, until they had an excellent
spectrum, in which they identified the
signatures of various chemical elements in the form of emission lines.
These occur when matter is located
within hot and radiation-rich surroundings and they themselves are excited to light up. The emission line of
Black holes, with a mass millions to billions times greater than that of the
sun, probably exist at the centres of most galaxies. Over the years, astronomers
have collected a wealth of scientific data on these cosmic phenomena. But
it is only in recent years that it has become obvious that black holes played
an important part in the development of galaxies. With the help of the
European X-ray Observatory XMM-NEWTON, Max Planck scientists have
been able to shed more light on this dark side of the universe.
Black Holes as
Intergalactic
Gas Heaters
XMM image of the environment around the galaxy M 87.
The surrounding gas has an essentially smooth distribution
and is significantly hotter than ten million degrees.
PHOTO: MPE/ESA1 / 2 0 0 2 M A X P L A N C K R E S E A R C H 31
Invisible UNIVERSE
30 M A X P L A N C K R E S E A R C H 1 / 2 0 0 2
FOCUS
solutely certain, the Max Planck scientists also examined very special models which could explain these observations in the framework of the cooling
flow model, for example, strong absorption of the X-ray light, which only
occurred very selectively in the cool
areas. âBut in our opinion, we can
even rule out this possibilityâ, says
Hans Böhringer.
The Garching astronomers are supported by their colleagues in the United
States. With the Chandra X-ray telescope, they have obtained pictures of
very high resolution from the M 87
iron was particularly clear and to a
certain extent served the astronomers
as a thermometer for the gas in the
centre of the Virgo galaxy cluster. If
the model of the cooling flows was
correct, there would have to be regions
of greatly intermingled gases with very
different temperatures in the environment of M 87. However, nothing of
this kind could be identified in the
spectra. They could be most easily explained with an essentially uniform
high temperature of more than ten
million degrees, clearly in conflict with
the cooling flow model. Just to be abfrom the jets into the hot gas. However, just some simple estimates already
show that the scientists from Garching
are on the right track. For example, the
jet from M 87 supplies approximately
twice as much energy as that radiated
by all the stars in our Milky Way. In
fact, the jets in the central galaxies in
the Perseus and Hydra A galaxy clusters are ten times more powerful in energy. A comparison with the theoretical cooling rate of the gas shows that
the jets supply energy in precisely the
right strength to keep the gas from
cooling - a surprising degree of finetuning.
A self-regulating mechanism could explain how this works. Let us assume
that the intergalactic gas streams into
the centre of gravity, the black hole.
The dark giant will consume one part
of this gas; the other part will be shot
galaxy environment and likewise find
no indications at all of large cool
clouds. Just in the central areas of
some clusters, they were able to identify small areas containing somewhat
cooler gas. However, they are ten to
one hundred times lower in mass than
the advocates of the cooling flow model would expect. However, if the hot
gas - which clearly exists - does not
cool down, it must be constantly heated. This âheatâ must moreover be very
finely tuned to its task: if it were too
strong, it would drive the intergalactic
gas away from the centres of the
galaxy clusters - something which has
not been observed. If it were too weak,
the huge masses of gas would still cool
down over a protracted period of time.
In the view of the Garching scientists,
there is only one possibility: black
holes in the centres of active galaxies.
Nowadays the astronomers are fairly
certain that black holes lurk at the
centre of almost every galaxy. These
objects can be several million to billion times more massive than the sun.
They are surrounded by hot gas discs,
from which matter slowly swirls into
the central black hole and disappears
for eternity. Presumably these cosmic
particle streams rotate and twist magnetic field lines which wind along their
rotational axes - perpendicular to the
disc plane - into space. According to
the theory, electrically charged particles, presumably electrons and their
antiparticles (positrons) are shot into
space along these field lines. This scenario is supported by astronomers
from Tübingen, who have used XMMNewton to study the area in the immediate environment of a black hole.
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heritage mauritius
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