Our deepest view of the X-ray sky

The eROSITA telescope has provided a new, sharp view of hot and energetic processes across the Universe – a view that the Sloan Digital Sky Survey (SDSS) and similar projects will use to enhance our knowledge of the Universe.

“With eROSITA finding more than one million new X-ray sources, the time is ripe for SDSS to follow up the hot and energetic sky, hunting for distant black holes and the mysteries of Dark Energy,” says Andrea Merloni of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, eROSITA’s Project Scientist and a longtime member of the SDSS collaboration.

A map of the entire sky in x-ray false color, with a bright blue-green band across the middle corresponding to the plane of the Milky Way

The energetic universe as seen with the eROSITA X-ray telescope (see below for a longer caption)

Image credit: Jeremy Sanders, Hermann Brunner and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI)

After six months of uninterrupted scanning of the whole sky, eROSITA, the new powerful X-ray telescope onboard the Russian-German Spektrum-Roentgen-Gamma (SRG) space mission, has delivered a new map of the hot and energetic sky.

The map reveals spectacular details on the structure of the hot gas in and around the Milky Way, while detecting more than one million (mostly new) X-ray sources, about as many have been detected over the 60 years history of X-ray astronomy by all its predecessors.

“This all-sky image completely changes the way we look at the energetic universe,” says Peter Predehl, the Principal Investigator of eROSITA at the Max Planck Institute for Extraterrestrial Physics (MPE). “We see such a wealth of detail – the beauty of the images is really stunning.”

Peter Predehl

“This all-sky image completely changes the way we look at the energetic universe. We see such a wealth of detail – the beauty of the images is really stunning.”

Most of these objects seen in the new image are Active Galactic Nuclei (AGN), growing supermassive black holes at cosmological distances, interspersed with clusters of galaxies, which appear in the map as extended X-ray nebulae shining thanks to the hot gas confined by their large potential wells.

Within our own Galaxy, eROSITA reveals a plethora of coronally-active stars, X-ray binary stars containing neutron stars, black holes or white dwarfs, and spectacular supernova remnants.

“The clusters of galaxies that mark out the large-scale structure of the Universe are filled with gas at temperatures of a million degrees or more,” Merloni says. “To see that gas directly, you have to use an X-ray telescope. With eROSITA covering the whole sky, we can see enough clusters to reconstruct their growth history extremely accurately, which in turns tells us the amount, and perhaps the nature, of dark matter and dark energy.”

A picture of the Vela Nebula from eROSITA, a reddish cloud

The Vela Supernova Remnant, about 800 light-years from Earth, as seen by eROSITA (longer caption below)

Image credit: Jeremy Sanders, Hermann Brunner and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI)

One key step in the process that will lead astronomers to extract scientific information from these spectacular maps is the measure of the distance (or redshift) and physical properties of the hundreds of thousands of stars, AGN and clusters that eROSITA will discover. The SDSS, with its capability of taking large number of spectra over very wide areas of the sky in a short time is the perfect complement of eROSITA, and the two teams have partnered to explore the science reward of such X-ray/optical synergies for many years now.

The eROSITA telescope consists of seven identical telescopes, focusing X-rays in the energy range 0.1-10 keV onto seven pnCCD cameras, which not only image the sky, but are capable of measuring the energy of every incoming photon with per-cent accuracy. Launched in July 2019 into a large halo orbit around the second Lagrange equilibrium point of the sun-earth system (L2), SRG/eROSITA has scanned the whole celestial sphere in six months, from December 2019 to June 2020, and is planning to continue doing so for 3.5 more years. By the end of 2023, eROSITA will have reached a sensitivity more than 30 times higher than the only all-sky X-ray survey to date, completed by ROSAT 30 years ago. By that time, the fifth phase of the SDSS (SDSS-V) will be in full swing, providing a rich dataset for comparison with upcoming eROSITA results.

“The MPE has been a member of SDSS since 2012, and we have continued to be part of the program ever since” continues Andrea Merloni. “The long term objective has always been to deploy the multi—object spectroscopic capabilities of SDSS to exploit the full potential of eROSITA as a tracer of large scale structure. We have finally reach the point where this plan can be made concrete. The all-sky nature of SDSS-V is a match made in heaven for eROSITA.”


A map of the entire sky in x-ray false color, with a bright blue-green band across the middle corresponding to the plane of the Milky Way

The first eROSITA all-sky survey was conducted over a period of six months by letting the telescope rotate continuously, thus providing a uniform exposure of about 150-200 seconds over most of the sky, with the ecliptic poles being visited more deeply. As eROSITA scans the sky, the energy of the collected photons is measured with an accuracy ranging from 2% – 6%.

To generate this image, in which the whole sky is projected onto an ellipse (so-called Aitoff projection) with the centre of the Milky Way in the middle and the body of the Galaxy running horizontally, photons have been colour-coded according to their energy (red for energies 0.3-0.6 keV, green for 0.6-1 keV, blue for 1-2.3 keV).

The original image, with a resolution of about 10”, and a corresponding dynamic range of more than one billion, is then smoothed (with a 10’ FWHM Gaussian) in order to generate the above picture. The red diffuse glow away from the galactic plane is the emission of the hot gas in the vicinity of the solar system (the Local Bubble). Along the plane itself, dust and gas absorb the lowest energy X-ray photons, so that only high-energy emitting sources can be seen, and their colour appears blue in the image. The hotter gas close to the galactic centre, shown in green and yellow, carries imprinted the history of the most energetic processes in the life of the Milky Way, such as supernova explosions, driving fountains of gas out of the plane, and, possibly, past outburst from the now dormant supermassive black hole in the centre of the galaxy.

Piercing through this turbulent, hot diffuse medium, are hundreds of thousands of X-ray sources, which appear mostly white in the image, and uniformly distributed over the sky. Among them, distant active galactic nuclei (including a few emitting at a time when the Universe was less than one tenth of its current age) are visible as point sources, while clusters of galaxies reveal themselves as extended X-ray nebulosities.

In total, about one million X-ray sources have been detected in the eROSITA all-sky image, a treasure trove that will keep the teams busy for the coming years.

Image credit: Jeremy Sanders, Hermann Brunner and the eSASS team (MPE); Eugene Churazov, Marat Gilfanov (on behalf of IKI)

A picture of the Vela Nebula from eROSITA, a reddish cloud

Due to its size and close distance to Earth, the “Vela supernova remnant” which is shown in this picture is one of the most prominent objects in the X-ray sky.

The Vela supernova exploded about 12,000 years ago at a distance of 800 light-years and overlaps with at least two other supernova remnants, Vela Junior (in the picture seen as bluish ring at the bottom left) and Puppis-A (top right). Vela Junior was discovered just 20 years ago, although this object is so close to Earth that remains of this explosion were found in polar ice cores.

All three supernova explosions produced both the X-ray-bright supernova remnants and neutron stars, which shine as intense X-ray point sources near the centres of the remnants. The quality of the new eROSITA data of this “stellar cemetery” will give astronomers many exciting new insights into the physical processes operating in the hot supernova plasma as well as for exploring the exotic neutron stars.

Image credit: Peter Predehl, Werner Becker (MPE), Davide Mella


  • Andrea Merloni, Max Planck Institute for Extraterrestrial Physics
    am@mpe.mpg.de, +49 (0)89 30000-3893
  • Peter Predehl, Max Planck Institute for Extraterrestrial Physics
    predehl@mpe.mpg.de, +49 (0)89 30000-3505
  • Hannelore Hämmerle, Press Officer, Max Planck Institute for Extraterrestrial Physics
    hanneh@mpe.mpg.de, +49 (0)89 30000-3980
  • Karen Masters, SDSS Scientific Spokesperson, Haverford College
    klmasters@haverford.edu, +1-610-795-6066
    Twitter: @KarenLMasters / @SDSSurveys
  • Jordan Raddick, SDSS Public Information Officer, Johns Hopkins University
    raddick@jhu.edu, +1-410-516-8889
    Twitter: @raddick

Further Information

On 11 June 2020, the eROSITA telescope completed its first survey of the entire X-ray sky. Launched on 13 July 2019 on-board the SRG spacecraft and now orbiting the second Lagrange point of the Earth-Sun-system, the telescope is in continuous scanning mode. During the first all-sky survey, each point in the sky was exposed to the eROSITA telescope for an average duration of 150-200 seconds. The regions close to the ecliptic poles, where the great circles traced by the telescope on the sky intersect, were revisited many times, accumulating exposures of up to a few hours. SRG will continue scanning the sky for three and half years more, with eROSITA performing seven more all-sky surveys in the process.

eROSITA is the primary instrument aboard SRG, a joint Russian-German science mission supported by the Russian Space Agency (Roskosmos), in the interests of the Russian Academy of Sciences represented by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und Raumfahrt (DLR). The SRG spacecraft was built by Lavochkin Association (NPOL) and its subcontractors, and is operated by NPOL with support from the Max-Planck Institute for Extraterrestrial Physics (MPE).

The development and construction of the eROSITA X-ray instrument was led by the Max Planck Institute for Extraterrestrial Physics (MPE), with contributions from the Dr. Karl Remeis Observatory Bamberg, the University of Hamburg Observatory, the Leibniz Institute for Astrophysics Potsdam (AIP), and the Institute for Astronomy and Astrophysics of the University of Tübingen, with the support of DLR and the Max Planck Society. The Argelander Institute for Astronomy of the University of Bonn and the Ludwig-Maximilians-Universität Munich also participated in the science preparation for eROSITA.

The eROSITA data shown here were processed using the eSASS software system developed by the German eROSITA consortium.

About the Sloan Digital Sky Survey

Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org.

SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatório Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.