Two teams of astronomers announced the discovery of a part of the missing baryonic matter of our universe. The researchers managed to register a signal from gas filaments that connect neighboring galaxies. The articles are directed to the journals Monthly Notices of the Royal Astronomical Society and Nature , preprints are available on ArXiv.org ( 1 , 2 ).
According to the modern generally accepted lambda-CDM model, the universe is more than 95 percent composed of dark matter and dark energy and only 4.6 percent of baryonic matter (including baryons (protons, neutrons) and electrons). At large redshifts (z ≧ 2), the bulk of the baryons are found in the Ly α- leus (Lyman-alpha forest): scattered clouds of neutral hydrogen with temperatures of 10 4 -10 5 kelvin, which leave the repeated Lyman-alpha absorption lines in the spectra of distant galaxies . However, at smaller redshifts (z ≦ 2), the baryonic matter found in the stars, the cold interstellar medium, the residual gas Ly α-gas, hot gas in galactic clusters and other objects, is only half of the quantity predicted by the theory.
Hydrodynamic simulations predict that about 40-50 percent of the missing substance can be in the form of a gas heated by shock waves in the intergalactic web. By analogy with filaments, it must interconnect galaxies and congestions. The temperature of the web, also called the hot-hot intergalactic medium ( WHIM ), reaches 10 5 -10 7 kelvins. Its filaments are very difficult to see because of the low density of rarefied gas – some researchers reported the detection of WHIM in the X-ray and far ultraviolet range, but the observational data were unreliable or did not explain the lack of matter completely.
In their observations, both teams of researchers relied on the thermal effect of Syunyaev-Zeldovich. Our Universe is uniformly filled with relic radiation , which has a high degree of isotropy and a spectrum characteristic of an absolutely black body with a temperature of about 2.7 kelvin. In addition, in the universe between galaxies, especially in clusters, there are clouds of very rarefied and hot gas. Photons of relic radiation passing through a gas are scattered by hot electrons and as a result receive additional energy (inverse Compton scattering). As a result, a change in the frequency of background photons is observed, which helps to study baryonic matter at small and medium redshifts.
The first team of astronomers combined two maps – a full sky map with traces of the Syunyaev-Zeldovich effect, compiled from observations of the background of the cosmic observatory “Plank” , and a map with bright red galaxies (LRG’s) obtained during the SDSS review . The second group of scientists combined the Planck map with the CMASS catalog, also compiled as part of the SDSS review.
One team investigated 260 thousand pairs of galaxies, and another – already a million. The researchers were looking for traces of gas threads, which, according to theory, should couple the pairs. In the Planck data, astronomers tried to detect not individual filaments, the signal of which would be too weak, but the signs of the web as a whole. As a result, the second team of researchers, led by Anna de Graaff, managed to register a filament signal with a statistical significance of 3.8 sigma, and the first team of researchers, led by Hideki Tanimura, detected a signal with a significance of 5.3 sigma.
According to the results of the Tanimura group, the density of gas in filaments is three times higher than the average density of baryonic matter in the universe. Anne de Graaff’s group reported that the density of structures of about 15 megaparsec is six times higher. “Differences are quite expected, as we look at the filaments removed at different distances. If we take this factor into account, then our results are consistent with the opening of another group, ” Tanimura comments . In addition, the work of astronomers also correspond to predictions of simulations.
Based on this, the researchers claim that they managed to detect at least 30 percent of the missing baryonic substance. The articles are sent to peer-reviewed journals where they will be tested. In the past, false reports have already been received about the discovery of missing baryonic matter, but experts consider the results to be quite convincing.