eRosita Encounters The Giant Cluster Abell 3391/95 (Cosmology)

Media Inaf interviewed Veronica Biffi, of INAF of Trieste, first author of one of the two articles by Inaf first published in the A&A special dedicated to the eRosita mission. In the article, the researchers analyze in detail the behavior of the galaxy cluster Abell 3391/95

It is the great moment of the mission of the Max Planck Institute for Extraterrestrial Physics (Mpe) and Rosita (extended ROentgen Survey with an Imaging Telescope Array), which is back in the spotlight thanks to a special of 35 articles published today in the journal Astronomy & Astrophysics . The German consortium, in which the National Institute of Astrophysics (Inaf) also participates, has released a significant amount of data from the “calibration and quality verification” phase, dating back to the autumn of 2019 (the mission was launched in June of the same year). Among the articles published, many see the participation of INAF. Veronica Biffi , researcher at INAF in Trieste, led one of the articles dedicated to clusters of galaxies.

The cluster of galaxies Abell 3391/95. 
Credits: Reiprich et al.

One of the first results obtained by eRosita was the discovery of an extremely long cosmic gas filament connecting the clusters of galaxies in the Abell 3391/95 double system . In the article published in the special, researchers from the Ludwig Maximilian University of Munich and the INAF of Trieste explain that they have found a similar system of galaxy clusters: surprisingly, both observations and simulations show signs that this cluster of galaxies is accelerating. along these gas filaments to merge with Abell 3391/95. Within the eRosita collaboration, Biffi deals with theoretical studies based on numerical simulations and “mock” observations in the X band. We interviewed her.

After scanning the sky with X-rays, what did you discover about the Abell 3391/95 cluster of galaxies?

«Personally I deal with theoretical predictions from cosmological and hydrodynamic computer simulations. I collaborated with these predictions in the interpretation of the data on the multiple system of galaxy clusters A3391 / 95. In fact, it is a pair of clusters of galaxies quite close in the sky (separated by about 50 arminutes, or 3 Mpc, and to be exact, A3395 is itself a double cluster, in which two clusters are merging). In the work published last December, led by TH Reiprich of Bonn, eRosita observations of the sky region around A3391 / 95 had shown for the first time a long cosmic gaseous filament extending from north to south for about 4 degrees (15 Mpc in the sky), encompassing both clusters A3391 and A3395 and extending beyond, north of A3391 to the Northern Clump cluster (Mcxc J0621.7-5242) and south of A3395 to another cluster of galaxies. This result confirmed the predictions of the theory of the formation of cosmic structures.

In the research we studied in detail the formation and evolution of a pair of galaxy clusters very similar to A3391 / 95. We have selected this system among a few dozen potential candidates, extracted from the “Magneticum Pathfinder” cosmological simulations, precisely on the basis of the similarities with the real system. In the simulations we were able to study the origin of the system over time, that is how the two clusters of the pair formed and where the diffused gas comes from, inside and outside the clusters. In fact, one of the most important questions is to understand if one can use pairs of clusters like this one to look for the “missing” baryon matter, the one in the form of a very dense and not very hot gas that is so difficult to observe. The theory tells us that it should reside in cosmic filaments and “soft” X-band observations, such as those with eRosita, look for it especially near clusters of galaxies and in the midst of clusters that are part of pairs such as A3391 / 95. Indeed, in the large-scale model of structure, matter is thought to be distributed as in a web of filaments that connect to nodes where clusters of galaxies are formed and interspersed with large empty spaces. The observations of A3391 / 95 have identified some “soft” X-band emission from gas in the “bridge” filament between the two clusters, but various works reach different conclusions on the nature of this gas, for example that it may be torn gas to the two clusters because of their interaction rather than cosmic gas like that in a filament ».

Did your simulations confirm the theoretical predictions underlying the study?

“With our simulations we found that the gas in the simulated cluster pair is really what would be found in the filaments, in fact it has similar thermal and chemical properties. Furthermore in the simulations we can also directly explore where it comes from, i.e. where it was in the past. And this confirmed to us that it was not torn from the two clusters, but that it comes from very different directions than those of the intra-cluster gas. It seems to be a cosmic filament that unites the two clusters of the pair.The large-scale structure, which is formed by collapsing on central nodes, such as the one where we find the pair, causes the other clusters and groups of galaxies connected to the pair from filaments move in their direction, eventually collapsing in the same region. This, for example, has been deepened in another article among those of the special of Astronomy and Astrophysics, conducted by A. Veronica: by combining the observations of eRosita with those of Xmm-Newton and Chandra, and in the radio band, indications of his probable motion along the cosmic filament of gas towards A3391, with which it will probably eventually merge. This is also consistent with what we find in the Magneticum simulations, where some groups show similar characteristics and are in fact moving at full speed towards the pair of clusters. One of these groups has already entered the atmosphere of one of the two main clusters, and about 3 billion years ago it showed a distance and characteristics similar to those of the true Northern Clump compared to A3391 / 95. 

After the results published in this paper, what will be the developments of your survey with eRosita?

“With the data obtained on A3391 / 95 by eRosita, we will continue to investigate other aspects of this interesting system, to study in detail the gas in the observed bridge-filament and possibly other less bright filaments than the north-south one, always connected to the two clusters. . The survey, then, will allow us to observe many clusters, finding many more candidates of multiple cluster systems, pairs but also more numerous agglomerates, where to look for cosmic filaments. The enormous number of clusters that eRosita will be able to discover, on the order of a hundred thousand, will also allow us to refine our knowledge of the cosmological model of our Universe, in which clusters are the largest and most recent structures to be formed ».

What are the advantages of eRosita compared to other similar and past missions?

«ERosita is a tool dedicated to observations of the sky in the” soft “X-band, and has the aim of scanning the entire sky 8 times between December 2019 and December 2023. From a certain point of view it is the heir of Rosat , another German mission (also from the Max Planck Institute of extraterrestrial Physics) which in the 1990s carried out the first “soft” X-band survey. The great advantages of eRosita are given by its great sensitivity in the “soft” X-band, comparable to that of Xmm-Newton, combined with a large field of view, about 1 degree in diameter. This means that images of larger portions of the sky can be obtained in much less time than with other tools such as Xmm-Newton and Chandra. With its characteristics, in particular, it will allow us to find many clusters that are not very bright, but in a limited time ».

Did you expect this amount of data and the results obtained?

«There were expectations and certainly the results obtained have fully satisfied them and even surpassed them. The work on the eRosita “ performance verification ” data that has been made public has been intense and massive, and for this reason the collaboration has managed to obtain so many results. Indeed, there are many interesting aspects, and therefore there are also many other works in development, in addition to those that have been published on the special ».

Featured image: Veronica Biffi, researcher at the INAF of Trieste © INAF

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