Researchers see the light 

New scientific data from the Euclid Space Telescope reveals the mystery of the faint glow in the Perseus galaxy cluster

The Perseus galaxy cluster was one of the first targets of ESA's Euclid space telescope. It contains thousands of brilliant galaxies, as heavy as 650 trillion suns, held together by their own gravity. For a long time, however, it was not clear where the faint and uniform glow that flows through the galaxy cluster originated. With the publication of new scientific data and analyses, this mystery now seems to have been solved.

Before the Euclid space telescope began its main task of observing the large-scale structures of the cosmos by mapping billions of galaxies across more than a third of the sky on 14 February 2024, the telescope observed more than a dozen astronomical objects very precisely and in full depth. With these measurements, the team behind Euclid not only demonstrated what the new telescope is capable of, but also provided data of such quality that astronomers are still analysing it today. And they are using the images to answer open questions in astrophysics: where does the diffuse light that streams through galaxy clusters come from and for which no origin has yet been identified? In the case of the Perseus galaxy cluster, this question now seems to have been answered.

The Perseus galaxy cluster is a worthwhile observation target in our cosmic neighbourhood, as it is one of the largest structures in the universe at a distance of "only" 240 million light years. For comparison: Our Milky Way and the neighbouring Andromeda galaxy are quite close to each other at 2.5 million light years as part of the local group. With its high total mass of 650 trillion solar masses, the gravity of the Perseus cluster binds thousands of galaxies together. 

Shining travelers

For the first time, a team led by the Max Planck Institute for Extraterrestrial Physics has now succeeded in analysing the diffuse light that shines through the Perseus cluster of galaxies right to the edges. “The high sensitivity at optical and near-infrared wavelengths over a huge field of view allows us to capture the extended faint light in the Perseus cluster,” says Matthias Kluge, lead author of the study, which has now been published together with 14 other papers. “This light is more than 100,000 times fainter in the infrared than the darkest night sky on Earth. Nevertheless, due to its large volume, it accounts for about 20% of the luminosity of the entire cluster.”

In addition, the team also used Euclid's excellent visible light imaging capabilities - comparable to the Hubble Space Telescope - to detect 50,000 free-flying globular clusters, which are very densely packed and spherical collections of tens of thousands to millions of stars. The diffuse light from the spaces between the Perseus galaxy cluster is distributed similarly to the globular star clusters inside the galaxy cluster. The stars in these globular clusters therefore appear to be the origin of the light. A lot is also known about the stars in the star clusters: They are old (and therefore poor in metals). The globular clusters in which these stars reside originate from the metal-poor outer regions of massive cluster galaxies and were probably drifted away by the tidal forces of the galaxies.

With increasing distance from the cluster centre, the proportion of dwarf galaxies also increases. “Euclid is located at the second Lagrange point far outside the Earth's atmosphere. Thanks to the dark image background, the excellent image resolution and the large field of view, we were able to detect 1100 dwarf galaxies, including hundreds with much fainter luminosity than ever before in the Perseus galaxy cluster,” says Raphael Zöller from the Max Planck Institute for Extraterrestrial Physics and the Ludwig-Maximilians-Universität München. Low-metallicity globular clusters are a dominant component of such dwarf galaxies, so there could be additional diffuse light coming from such globular clusters, whose home dwarf galaxies have been completely torn apart by tidal forces and thus flung into intergalactic space. 

Five new pictures and lots of data

In addition to this data, the Euclid consortium also published five new images. These are more than just nice snapshots of galaxy clusters or a star-forming region. Thanks to Euclid's novel observational capabilities, they reveal new physical properties of the Universe, which are explained in more detail in a series of publications by the Euclid collaboration (see ESA press release). The new images accompany the first scientific data from the mission, which have now also been published, as well as several scientific papers. Part of the data package is a catalogue of more than eleven million astronomical objects that Euclid observed in visible light in just one day. A further five million objects were added in infrared light. In addition, five other papers describe important aspects of the Euclid mission in more detail.


Background information

Euclid is a space mission of the European Space Agency (ESA) with contributions from the National Aeronautics and Space Administration (NASA). It is the second M-class mission in ESA's Cosmic Vision programme. Euclid explores how our universe has evolved over the course of cosmic history and examines its fundamental components: Dark Energy and Dark Matter.

VIS and NISP were developed and built by a consortium of scientists and engineers from 17 countries, many from Europe, but also from the USA, Canada and Japan. From Germany, the Max Planck Institute for Astronomy (MPIA) in Heidelberg, the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, the Ludwig Maximilian University (LMU) in Munich, the University of Bonn (UB), the Ruhr University Bochum (RUB) and the German Space Agency at the German Aerospace Centre (DLR) in Bonn are participating. As partner of the Euclid project, MPE is responsible for the optical components of the NISP instrument as well as for the optical design and modelling of the image quality and is hosting one of Euclid's nine Science Data Centers.

The German Space Agency at DLR coordinates the German ESA contributions and also provides funding of 60 million euros from the National Space Programme for the participating German research institutes.

With around 21%, Germany is the largest contributor to the ESA science programme.


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