An appetizer to the all-sky banquet

As announced during the 2021 meeting of the European Astronomical Society, the German eROSITA collaboration will release the first set of data taken with the eROSITA X-ray telescope onboard the SRG observatory. For the first time, astronomers throughout the world will have the chance to download and analyse data from this new powerful telescope. The Early Data Release will be accompanied by the publication of 35 eROSITA science papers by the German eROSITA Consortium on the arXiv preprint server, with these and more to be published in a forthcoming special issue of the journal Astronomy and Astrophysics. Researchers of the ORIGINS Cluster were contributing authors and lead authors to several publications.

The Early Data Release (EDR) contains almost 100 individual observations of 29 distinct fields taken before the start of the all-sky scans. They cover a wide range of different astronomical objects, from galactic neutron stars to clusters of galaxies and showcase the potential and versatility of the eROSITA telescope for imaging, spectroscopy and time domain analysis.

This is an important moment also for the ORIGINS cluster, because eROSITA studies of cosmology and large scale structure using eROSITA selected clusters and AGN together with weak gravitational lensing and optical imaging data from the Dark Energy Survey, Euclid and Rubin were a major focus in our research plans. This collaborative effort moves forward now in multiple ongoing projects supported within the ORIGINS cluster.

One of these early results involved the discovery of an extremely long cosmic gas filament connecting the Abell 3391/95 pair of galaxy clusters. Further investigation of that patch of the Universe unveiled other clusters, including the so-called Northern Clump cluster, in the path along this filament. Using their Magneticum Simulations, LMU and ORIGINS Cluster scientists Dr. Veronica Biffi and PD Dr. Klaus Dolag identified a similar galaxy cluster system embedded in the Cosmic Web and surrounded by infalling galaxy group. Remarkably, both the observations and the simulations show signs that the infalling galaxy cluster is speeding along the gas filaments and will finally merge with the A3391/95 cluster pair. Identifying similar structures in eROSITA observations and in Magenticum Simulations allows new insights into the assembly of cosmic structures.

Among the datasets released to the public, there is one in particular that holds a special place: a mini-survey called “eFEDS” (the eROSITA Final Equatorial Depth Survey). Designed as a preview to the final all-sky survey, eFEDS covers uniformly a patch of about 140 square degrees of the sky (about 1/300th of the all-sky survey), providing a glimpse of what the whole extra-galactic sky will look like in X-rays after eROSITA completes its all-sky survey program in 2023. In just four days of eFEDS observations, eROSITA detected the astonishing number of almost 30,000 sources – more than can be found in any given contiguous X-ray survey field to date. The release includes not only the processed data, but also several catalogues of eROSITA sources describing their X-ray and multi-wavelength properties.

Dr. Matthias Klein led the work to link the X-ray cluster candidates from eROSITA to the associated galaxies in these systems, enabling the removal of contaminating non-cluster sources from the X-ray candidates and providing redshifts or distance measurements for the real systems.  Working together with other LMU and ORIGINS Cluster colleagues Dr. Sebastian Grandis and Prof. Dr. Joseph Mohr, Klein produced a final catalog of 477 clusters, ranging from nearby galaxy clusters to the most distant, which exist at a time when the Universe was only a quarter of its current age.

In an associated work, former LMU and UNIVERSE Cluster PhD student Dr. I-Non Chiu worked with Dr. Sebastian Grandis, Dr. Matthias Klein and others from LMU to present the first results from a weak gravitational lensing study of this cluster sample assembled by Klein and team.  Dr. Chiu’s study presents the most extensive weak lensing study of X-ray selected galaxy clusters to date, and provides new insights into the structure and evolution of this population of most massive collapsed objects in the Universe.

The associated suite of 35 papers led by the German eROSITA Consortium mostly focus on the EDR observations, but also include a few exciting highlights from the ongoing all-sky survey. The objects under study range from stars and diffuse emission in our own Milky Way or the neighbouring Large Magellanic Cloud to Active Galactic Nuclei hosting supermassive black holes, and huge clusters of galaxies. “Apart from the ground-breaking science, another thing that makes me really proud is the contribution of female scientists to the effort, with 40% of the papers that accompany the data release led by women,” says Mara Salvato, eROSITA spokesperson and chair of the eROSITA follow-up working group.

Since acquiring these EDR data early in the mission, the eROSITA X-ray telescope has been continuously surveying the X-ray sky and is current completing its fourth complete survey.  These data over much larger area contain millions of X-ray sources whose properties and data will be presented in upcoming publications and data releases by the German eROSITA Consortium