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

Abstract

A significant fraction of the local Universe's baryonic content 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 has been spectroscopically analyzed. In this work, 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 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\pm 3)\% $ excess 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 $k_{\text{B}}T\approx (0.8-1.1)$ keV, $n_{\text{e}}\approx 10^{-5}$ cm$^{-3}$, and $δ_{\text{b}}\approx (30-40)$, respectively, at a $>3σ$ detection level, in agreement with cosmological simulations for the first time for a single filament. Independent 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. 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.