Looking at the Extragalactic Sky With the eFeds Survey (Cosmology)

Media Inaf interviewed Marcella Brusa, associate at INAF in Bologna, the first author of one of the two articles first signed by Inaf published in the A&A special dedicated to the eRosita mission. The article presents the eFeds survey, designed to uniformly detect a portion of about 140 square degrees of the sky and to provide researchers with a glimpse of what the X-ray map of the extragalactic sky will look like when eRosita completes its program.

For the second time in a few months ( the first time in March 2021 ) it is the mission of the Max Planck Institute for Extraterrestrial Physics (Mpe) and  Rosita (extended Roentgen Survey with an Imaging Telescope Array) to gain a prominent place among the publications of the Astronomy & Astrophysics magazine with a special of 35 articles published today, after the announcement yesterday afternoon during the meeting of the European Astronomical Society. The data that the consortium of the eRosita collaboration has made public are those of the “calibration and quality verification” phase, dating back to the autumn of 2019, that is to say just after the arrival of the Russian-German satellite Spektr-Rg(on board of which the German mission is located) at the Lagrange point L2 in constant opposition to the Sun with respect to the Earth. Among the articles published, many see the participation of the National Institute of Astrophysics (Inaf).

Marcella Brusa , from the University of Bologna and associated with the INAF of Bologna, led one of the articles of the special: it is an important publication in today’s special, because it presents to the public one of the first scientific results from one of the  surveys  key to the eRosita mission. eFeds (eRosita Final Equatorial Depth Survey) was designed to uniformly detect a portion of approximately 140 square degrees of the sky and to provide researchers with a glimpse of what the X-ray extragalactic sky map will look like when eRosita completes its program at all sky in 2023. The surveyeFeds is surprising: in just four days of observations, eRosita has detected almost 28,000 sources.

Brusa’s research field is mainly dedicated to the study in X-rays of winds from supermassive black holes. We interviewed the researcher.

Among the published datasets, there is one in particular that occupies a special place: the survey called “eFeds”. What is it about?

“In four years, eRosita will have observed the whole sky at a depth never reached before. In order to prepare ourselves to reduce, analyze and interpret the data and classify the population of objects that will reveal the German consortium of the eRosita collaboration has decided to observe, during this initial verification phase, a region corresponding to 1/300 of the sky at the depth foreseen for the final survey along the equatorial plane. It is a kind of “preview” of what awaits us and, to date, it is the largest contiguous area (140 deg2) ever observed in X-rays so thoroughly. The program did not disappoint expectations: we have revealed almost 28 thousand sources, just multiply by 300 to get an idea of ​​the mind-boggling numbers we expect at the end of the survey. We are now ready to publicly release an incredible set of catalogs presenting separately the X and multifrequency properties of active galactic nuclei (Agn), clusters and groups of galaxies, stars and compact objects, all revealed in this portion of the sky, with homogeneous information and documented. The entire astronomical community is therefore invited to peek, and we are sure that this enormous amount of data will stimulate new ideas and new studies, both on single objects and on classes of sources ».

Marcella Brusa, associate researcher at the INAF of Bologna © INAF

What did your group discover after downloading the survey data and Feds ?

«We focused on a subsample of Agn in eFeds, those revealed in the“ hard ”band of eRosita (2.3-5 keV). There are only 246, therefore less than 1% of the total, but we believe that among them are hidden the most interesting supermassive black holes to study the consequences of the impact of Agn activity in the evolution of galaxies. Among these 246 we found one, which we called Xid439, and which had already been present in catalogs from optical, infrared and radio surveys for about twenty years, but it had never come to the attention of scholars and had always been lost in among hundreds or thousands of other objects. Only when it was observed for the first time in the X band, by eRosita, was it recognized as particularly interesting and worthy of further study. In particular, in Xid439 we revealed the presence of material that moves at very high speed, a kind of galactic wind composed of ionized gas that blows at about 6 million km / h! The presence of this wind together with other properties revealed through the analysis of the eRosita and multifrequency data (the growth rate, the presence of X-band and optical obscuration, the presence of a possible companion galaxy in interaction) make Xid439 the “prototype ”Of Agn in the feedback phase as predicted by galaxy formation and evolution models, is an excellent candidate for future follow-up at higher resolution. It’s just an item, but we have confirmed that hard X band selection is very efficient in selecting them, and that the depth that will be reached at the end of the survey will be sufficient to reveal several hundreds of them. These objects are very bright, albeit very rare. It is necessary to map the whole sky and eRosita is therefore the perfect tool for this type of study ».

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

«The articles that accompany the Early Data Release of eRosita and that present the data collected in eFeds are 13. Most of them are mainly” technical “articles, but they are invaluable in providing the tools to investigate the evolution of brighter supermassive black holes. in the universe, the properties of the gas that permeates clusters of galaxies, the spatial distribution of Agn, groups and clusters that can give us information on the large-scale structure of the universe and the properties of dark matter, and much more. In the coming months we will complete the data analysis with these main objectives as a guide. As for the Agn in the feedback phase, we will extend the work to the entire Agn sample in eFeds, with the aim of finding efficient selection diagnostics to then be applied to surveys across the sky, also using innovative methods of statistical analysis. This project will be coordinated by Blessing Musiimenta, also a member of the German consortium of eRosita and associated with the INAF of Bologna, which started a few months ago her PhD at the University of Bologna, as part of the “BiD4Best “(Big Data applications for Black Hole evolution studies”) funded by the Horizon 2020 program of the European community “.

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

“The observations were designed specifically to calibrate the telescope, targeting carefully chosen regions of the sky to control the performance of various aspects such as spatial, spectral, and temporal resolution. Since the announcement of the first images it was immediately clear that eRosita would live up to the expectations and the results that will be presented in the A&A specialthey confirmed it in full. All members of the consortium have worked hard over the past year and a half to validate all products and understand how the tool works, and at the same time they have focused on harvesting the most fruit from this crop. So let’s say that the amount of data was clear right from the start (it had been planned by the members of the consortium who control the operations of the instrument), but we definitely didn’t expect so many articles to accompany the date release, especially if you think that it is already the second time in a few months that A&A dedicates a special to the first results of eRosita. I congratulate all the colleagues of the consortium for this result: it is a great satisfaction to be able to accompany the first data release with a large number of scientific articles that show how revolutionary eRosita is in our understanding of the most diverse phenomena observable at high energies, from gas. heat present in the clusters of galaxies to the irradiation of the atmospheres of exoplanets! ».

eRosita was designed to observe the sky in X-rays. How does it work?

«ERosita observes the sky in the portion of the electromagnetic spectrum beyond the Ultraviolet band, which corresponds to energies between 0.2 and 5 keV. It is a “classic” X telescope, which uses Wolter optics, already used in the past on leading instruments such as Rosat, Chandra and Xmm-Newton. Instead of aiming at a certain region of the sky every time, from December 2019 eRosita began to scan the entire sky, making a complete circle every 4 hours (an “ero-day”), and recording everything that is revealed in the X band. in a thin strip of sky. The sky strips are all contiguous and partly overlapping, so it takes 6 months to map the whole sky. And then it starts again, in a kind of perpetual motion that will last 4 years in all ».

What changes compared to other similar and past missions? 

«We can see eRosita as an instrument similar to Xmm-Newton in terms of sensitivity (and even better in terms of spectral resolution), but with a much larger field of view, similar to that of the predecessor Rosat. This combination of “features” allows to obtain images over the whole sky of higher quality than those of Rosat already after only 6 months, at least a factor of ten deeper and far more detailed . ERosita bubbles are an excellent example of how powerful eRosita is in the discovery and characterization of diffuse emissionpresent on large areas of the sky. The other fundamental feature of eRosita is that it also introduces the temporal dimension in the study of high-energy phenomena, since in fact it will produce not just one, but 8 different images over the whole sky in 4 years. More: as a consequence of the sky mapping strategy (“scanning strategy”) each of the 8 images is actually made up of different images (from a minimum of 6 to over 200, depending on the portion of the sky observed), thus allowing research of phenomena of variability in bright sources on a time scale ranging from a few hours to years. The observation strategy, unique to eRosita for such large areas of the sky, has made it possible to reveal – for example – the sudden awakening of two supermassive black holes. Finally, the great strength of eRosita are the large numbers: so far everything we have deduced about the origin and evolution of Agn has been based on samples of the order of about 10-100 thousand objects collected in more than 30 years of observations with different telescopes. With eRosita we will reach a few million, of the same order of magnitude as the samples of galaxies selected in optical band from surveys such as Sdss. This will allow us to understand much better the evolutionary history of the universe’s growth phenomena and to have a complete census of the hottest and most energetic phenomena ».

Featured image: Some data from the eFeds survey by eRosita. Credits: eRosita Mpe team / Inaf

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Provided by INAF

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