X-Ray and Microwave Cosmology: synergy and competition - What we expect from SRG/eRosita spacecraft and ground based microwave telescopes?
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.