Detection of pure warm-hot intergalactic medium emission from a 7.2 Mpc long filament in the Shapley supercluster using X-ray spectroscopy

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, the Netherlands
² Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
³ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, NL-2333 CA Leiden, the Netherlands
⁴ Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2, FI-00014 Helsinki, Finland
⁵ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405, Orsay, France

^⋆ Corresponding author: kmigkas@strw.leidenuniv.nl

Received: 1 April 2025
Accepted: 13 May 2025

Abstract

A significant fraction of the local Universe baryonic content still remains undetected. Cosmological simulations indicate that most of the missing baryons reside in cosmic filaments in the form of warm-hot intergalactic medium (WHIM). The latter shows low surface brightness and soft X-ray emission, making it challenging to detect. Until now, X-ray WHIM emission has been detected only in very few individual filaments, whereas in even fewer filaments WHIM was spectroscopically analyzed. The Suzaku X-ray telescope is ideal for studying X-ray WHIM emission from filaments because of its low instrumental background. We used four Suzaku pointings to study the WHIM emission of a filament in the Shapley supercluster, connecting the galaxy cluster pairs A3530/32 and A3528-N/S. We additionally employ XMM-Newton observations to robustly account for point sources in the filament, which Suzaku fails to detect because of its poor angular resolution, and to fully characterize the neighboring clusters and their signal contamination to the filament region. We report the direct imaging and spectroscopic detection of extended thermal WHIM emission from this single filament. Our imaging analysis confirms the existence of (21±3)% additional X-ray emission throughout the filament compared to the sky background at a 6.1σ level. We constrain the filament gas temperature, electron density, and baryon overdensity to be k_BT≈(0.8−1.1) keV, n_e≈10⁻⁵ cm⁻³, and δ_b≈(30−40), respectively, at a >3σ detection level, in agreement with cosmological simulations for the first time for a single filament. Independently of the X-ray analysis, we also identify a spectroscopic galaxy overdensity throughout the filament using the Shapley Supercluster velocity Database and constrain the filament's 3D length to be 7.2 Mpc at a 53° angle with the plane of the sky. Overall, this is the first X-ray spectroscopic detection of pure WHIM emission from an individual, pristine filament without significant contamination from unresolved point sources and gas clumps.