Astronomers have observed a vast, hot filament of intergalactic gas stretching between four galaxy clusters in the Shapley Supercluster – potentially accounting for some of the Universe’s long-theorised, but previously unseen, “missing” matter.
This “missing matter” refers not to dark matter, but to ordinary, baryonic matter – atoms made of protons and neutrons – which has long been predicted by cosmological models but has proven difficult to detect in the local Universe.
The discovery, published in Astronomy & Astrophysics, used X-ray spectroscopy data from ESA’s XMM-Newton and JAXA’s Suzaku satellites, and marks one of the clearest detections yet of the warm-hot intergalactic medium (WHIM) – a diffuse phase of baryonic matter predicted by cosmological models but notoriously difficult to observe.
“For the first time, our results closely match what we see in our leading model of the cosmos – something that’s not happened before,” said lead author Konstantinos Migkas, an astronomer at Leiden Observatory, in a press release. “It seems that the simulations were right all along.”
The filament, measuring 7.2 megaparsecs (approximately 23 million light-years) in length, connects two pairs of galaxy clusters in the local Universe. It emits at X-ray wavelengths due to its extremely high temperature – above 10 million kelvin – and is estimated to contain about ten times the mass of the Milky Way.
To isolate the filament’s emission from background sources, the team combined Suzaku’s broad-field sensitivity with XMM-Newton’s higher resolution. XMM-Newton was used to identify and subtract point sources such as active galactic nuclei. “Thanks to XMM-Newton we could identify and remove these cosmic contaminants, so we knew we were looking at the gas in the filament and nothing else,” said co-author Florian Pacaud of the University of Bonn.
The resulting data revealed a uniform, faint X-ray signal with a temperature and density profile consistent with WHIM predictions. According to the paper, the filament contains a baryon overdensity of roughly 120 times the cosmic mean and a thermal pressure of about 3×10⁻¹⁴ erg cm⁻³ – aligning well with hydrodynamical simulations.
The detection adds weight to the hypothesis that a significant fraction of the Universe’s baryonic matter exists in these extended, ionised filaments. Cosmological models predict that up to 40–50 percent of baryons reside in WHIM structures, but direct observations remain rare due to their low surface brightness.
“This research is a great example of collaboration between telescopes,” said Norbert Schartel, ESA’s XMM-Newton Project Scientist. “It creates a new benchmark for how to spot the light coming from the faint filaments of the cosmic web.”
The work also provides observational support for ESA’s Euclid mission, launched in 2023, which is mapping the large-scale structure of the Universe to investigate the roles of dark matter and dark energy.
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