Astronomy

Most of the matter and energy in the Universe is of unknown nature, so they are called “dark matter” and “dark energy”. Dark energy accounts for 72% of the total energy in the Universe, while some 23% of the total amount of matter/energy is made of this so-called “dark matter”, which is composed of heavy particles still waiting to be discovered by particle physicists. The remaining 5% of the Universe is made of ordinary matter, the one we know on Earth that constitutes stars and planets. It consists of protons and neutrons – called baryons – and of electrons, all the building blocks of the atoms. But part of this 5% of “baryonic” matter is also missing. Stars, galaxies, and gas that astronomers observed in the Universe account for less than half of the baryonic matter.

Studies of the Big Bang and the fluctuations of cosmic background radiation yield quite exact figures on the existence of baryons in the Universe. So from the calculations astronomers knew for a long time that the vanished pieces of the cosmological puzzle must be hidden somewhere. To trace and apprehend this solely observable component in the Universe is the precondition to learn more about the web of Dark Material and to test the quality of different cosmological models.

Scientists from SRON Netherlands Institute for Space Research using the X-ray space observatory XMM-Newton to observe the two clusters of galaxies Abell 222 and Abell 223 which are connected by a filament. The newly-detected bridge would be part of this missing baryonic matter.

A bridge of hot gas connects two clusters of galaxies (Image credits: ESA/XMM-Newton/ EPIC/ ESO [J. Dietrich]/ SRON [N. Werner]/ MPE [A. Finoguenov]).

The Universe is constructed like an oversized spider web. All visible material is arranged along the filamentary structure of the dark matter. On its threads and knots, this web holds gigantic chunks of baryonic matter, which are made of quarks and leptons. For 10 years, astronomers suspected that the missing baryonic matter is hot gas at very low density permeating the filamentary structure of the cosmic web. Because of its low density, detecting this hot gas was a very challenging task. The hot gas they found there is probably the hottest and densest part of the diffuse gas which constitutes half of the missing baryons in the Universe. The properties of the gas, for example its density and temperature, are consistent with the results of simulations. This is the first time that scientists see the bridge of gas connecting two clusters of galaxies in X-rays.

This discovery was made possible because of the very fortunate geometry of the two clusters. As seen from the Earth, the filament connecting the two clusters is aligned along our line-of-sight, so that the entire emission from the filament is concentrated in a small region of the sky, thereby making its detection possible. Previous observations, at a lower sensitivity level, only allowed astronomers to detect the clusters and some groups of galaxies, the dense knots of the web. The high-sensitivity level now achieved with in-depth XMM-Newton observations makes it possible to observe the connecting wires of the cosmic web.

The study, which has been published on May 2008 issue of Astronomy & Astrophysics Letters, is considered as a step toward understanding the distribution of the matter within the large-scale structure of the Universe. The distribution and composition of the baryonic matter gives information about what happened after the Big Bang and which forces are dominating the Universe today and in the future.

Source: MPE Press Release

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