Orateur
Prof.
Rashid Sunyaev
(Max Planck Garching)
Description
Our Universe is filled by cosmic microwave background radiation (CMB)
which is extremely isotropic and has excellent black body spectrum with
temperature 2.7 Kelvin and no spectral deviations from black body are
detected in the CMB monopole till now. But 50 years ago it was recognised
that "shadows" in the angular distribution of CMB in the directions where clouds
of very hot T_e~10^6-10^8 K and rarefied plasma exist. Today we know that such objects exist
and they are clusters of galaxies containing thousands of galaxies each,
a lot of dark matter, and a hot gas filling the huge potential well. And "shadows"
with very peculiar frequency spectrum arise due to Thomson interaction of CMB
photons with hot electrons. Today this method permitted to discover several
thousands of before unknown clusters of galaxies at relatively high redshifts
0.25 < z < 2. Behind practically every new discovered rich cluster of galaxies
we see the extremely distant galaxies with shape distorted and brightness
increased due to gravitational lensing by huge gravitational potential of a cluster
connected mainly with invisible "dark matter". Good to mention that amplitude of
the CMB brightness "shadow" corresponds only to a few tens or hundreds of microKelvin.
There is also a usual way to observe the same hot gas, and Russia plans to
launch this June SRG spacecraft with German eRosita X-Ray telescope
having grazing incidence optics. This telescope is planning
to discover more than 100,000 clusters of galaxies (i.e. all rich clusters of
galaxies in the observable Universe) during 4 years of the all sky survey.
At the same time, ground-based millimeter wavelenghts telescopes on
the South Pole of the Earth and Atacama desert at 5 km altitude, equipped by
tens of thousands cryogenic bolometers in their focal planes, promise
to detect all these clusters due to their "shadows" on CMB.
The data will be very complimentary because the X-ray emission due to
free-free emission of the hot gas is proportional to the square of the electron
density and amplitude of shadows depends linearly on the plasma
pressure NeTe. There will be a lot of synergy in these data. At the same
time there is a competition: who will be first to discover the most
interesting clusters of galaxies? Ensemble of 10^5 clusters, their
distribution in space, mass and redshift will provide a unique sample
of data for testing cosmological models. Detailed spectroscopic study of
the shadow with high angular resolution will permit to understand the
internal structure of the hot plasma (including merging shocks, turbulent
motions) and to measure the temperature of the plasma using
the relativistic corrections to the thermal effect.
Interaction of CMB photons with free electrons opens a unique way to
measure a peculiar velocity of a cluster of galaxies relative to the unique
system of coordinates in which CMB is isotropic. Observers can now
measure peculiar velocities and even bulk and turbulent velocities
inside clusters of galaxies at any distance from us because both effects
(thermal and kinematic) do not depend on the redshift of the object.
Auteur principal
Prof.
Rashid Sunyaev
(Max Planck Garching)
