ROSCOSMOS & DLR - Spectrum-RG (Spektr-RG) Mission patch.
Jan. 25, 2021
Clusters of galaxies are dynamic systems that continually grow due to the accretion of large and small portions of matter. Such a process should lead to a complex structure in the distribution of dark matter within the clusters, as well as to shock waves and "cold fronts" in the hot gas. Very detailed X-ray images of the Coma galaxy cluster were obtained by telescopes of the Russian orbital observatory Spektr-RG, which has been operating near the L2 point for more than a year. Thanks to them, it was possible to study in detail the process of merging clusters, incredibly stormy and long.
Image above: Electron temperature map (weighted with gas density) obtained from the ratio of the microwave, image obtained by the Planck satellite (ESA) based on the Sunyaev-Zeldovich effect to the X-ray.
The Coma Cluster of Galaxies (also known as Coma) is special. It is very massive - it contains thousands of galaxies, and the close one is located at a distance of less than 100 Mpc. This is the first object in which the presence of "dark matter" (hidden mass) has been established. This was done by astrophysicist Fritz Zwicky in 1933, and in the 1950s it became the first cluster to contain a diffuse radio halo.
In the late 1960s, the idea arose that "dark matter" could be hot intergalactic gas. Indeed, the hot gas in Coma was soon discovered by the first X-ray satellite, Uhuru. Moreover, it turned out that it is hot gas that makes up almost 80% of all normal "baryonic" matter, while the stars and galaxies of the Coma cluster contain no more than 20% of the cluster's baryons (baryons are a family of elementary particles, which include nuclear particles protons and neutrons). However, the hot gas was not enough to explain the phenomenon of "dark matter" - the latter still had to be much more. The total mass of baryons in hot gas and in stars of a galaxy cluster does not exceed 15% of the total mass of the cluster.
Image above: X-ray image, in which the brightness of the central part is artificially suppressed, with schematic designations of the most significant structures associated with the merger of the cluster with the NGC 4839 group. The blue dashed line shows the supposed trajectory of the group, which began to move towards the center of the Coma cluster from the northwest and is currently close to the apocenter X-ray image, in which the brightness of the central part is artificially suppressed, with schematic designations of the most significant structures associated with the merger of the cluster with the NGC 4839 group. The blue dashed line shows the supposed trajectory of the group, which began to move towards the center the Coma cluster from the northwest and is currently close to the apocenter
X-ray observations have not yet completely solved the problem of "dark matter", but they have significantly enriched the knowledge of astrophysicists about what is happening in galaxy clusters. Thanks to X-ray astronomy, it is possible to determine the density, temperature, and other properties of the hot gas filling the cluster, "see" how it is distributed in space. Observations of the hot gas itself have become the most important source of information about the parameters of the invisible "dark" matter. It is it that determines the gravitational potential of the cluster (to put it simply, how strongly the cluster "attracts" matter to itself) and how the hot gas itself is distributed in it. The closeness of the Coma makes it attractive for research in all energy ranges, although the cluster's huge angular dimensions often complicate the task: telescopes with a large field of view usually cannot "see" all the details of the cluster, and more "sensitive" telescopes cannot view the entire cluster.
Russian X-ray observatory "Spektr-RG" with telescopes ART-XC them. MN Pavlinsky and eROSITA on board was specially designed to meet these challenges. In scan mode, she managed to build a complete map of the entire cluster. The X-ray image obtained by eROSITA as a result of two sessions of raster observations shows a region of the sky with a size of ~ 10 Mpc (at the distance of the cluster), which is at least twice the virial radius of the cluster. In addition to many sources, two bright diffuse spots stand out, which correspond to the main cluster and the group of galaxies NGC 4839 (bottom right of the center). The congestion and group are in the process of being merged. In fact, NGC 4839 has already passed through the core of the main cluster once and is about to "fall" back to the center again.
Image above: Microwave image of the Coma cluster from the Planck satellite.
Computer simulations predict some of the phenomena associated with this particular stage of the merger that can be observed. The bow shock created by NGC 4839 during its first pass should now be located at the edge of the cluster, and the gas displaced from the core of the main cluster should fall back to form a "secondary" shock. New data suggest that a structure several megaparsec long, observed to the right of the nucleus, is precisely a "secondary" shock wave. The figure shows the correspondence between numerical hydrodynamic calculations and observations of the eROSITA telescope.
Another interesting consequence of the fusion scenario is that the radio halo, limited by the secondary shock wave, actually passed through two shock waves - the first time through the bow shock wave caused by the first passage of NGC 4839 through the Coma core at a speed of about 3500 kilometers per second, and completely recently through a secondary shock wave. This process, accompanied by particle acceleration and gas compression, is capable of slowing down the rapid "aging" of relativistic particles in the radio halo, which lose energy due to synchrotron losses in the magnetic field to radio emission and Compton backscattering from relic radiation photons.
Image above: Electron temperature map (weighted with gas density) obtained from the ratio of the microwave image obtained by the Planck satellite (ESA) based on the Sunyaev-Zeldovich effect to the X-ray image of the Coma cluster Electron temperature map (weighted with gas density) obtained from the ratio of the microwave image obtained by the Planck satellite (ESA) based on the Sunyaev-Zeldovich effect to the X-ray image of the Coma cluster.
“It is possible that a similar mechanism works in other clusters with radio halos,” says Academician Yevgeny Churazov, lead author of the article. "And our next task is to explore the outermost regions of the cluster, where the gas falling on Komu is decelerated by the shock wave and becomes part of the cluster."
Astronomers are well aware of the remarkable image of Coma in the microwave range, taken by the Planck Observatory. Due to the Sunyaev-Zeldovich effect, the brightness of the CMB is lowered towards the hot gas cluster. The microwave image of Coma is very similar to the one taken by the eROSITA telescope. However, the X-ray flux of the cluster and the amplitude of the Sunyaev-Zeldovich effect depend differently on the density and temperature of the gas. This opens up the possibility of evaluating the temperature of the hot gas in relation to the brightness ratio in two different wavelength ranges.
The relationship between the images taken by the eROSITA and Planck telescopes gives an idea of the gas temperature map. Such temperature measurements do not require any spectral information in the X-ray range. At first glance, this is a rather unexpected method. It uses only the surface "negative" brightness of the cluster in microwave rays and the surface brightness of X-rays in the range of 0.4–2 keV, where the eROSITA telescope has a high sensitivity, and photons have energies well below the measured temperature. To obtain a temperature distribution map, it is necessary to know (or assume) the gas density distribution in the cluster. As expected under the discussed merger scenario, the core of the main cluster is hotter (temperatures close to 100 million degrees), while the less massive group NGC 4839 is able to hold some of its more than 3 times cold gas.
Spektr-RG (or Spectrum-RG) x-ray telescope
“The first article on long-term observations of the Coma galaxy cluster has already been sent to the journal and published as an astro-preprint,” says Academician Rashid Sunyaev, scientific director of the Spektr-RG observatory. - Work on the data of these observations continues and promises many new interesting results on the physics of the cluster and the behavior of dark matter in it.
The Coma Cluster is the deepest field studied by the Russian consortium of the Spektr-RG observatory during its flight from Earth to L2. The depth of this field makes it possible to study in detail not only the most interesting Coma cluster, but also to search in X-rays for manifestations of other astronomical objects included in the surrounding Coma supercluster of galaxies. And this supercluster contains more than 3,000 galaxies.
ROSCOSMOS Press Release: https://www.roscosmos.ru/29844/
Спектр-РГ (Spektr-RG): https://www.roscosmos.ru/srg/
Images, Text, Credits: ROSCOSMOS/Orbiter.ch Aerospace/Roland Berga.
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