Detection of virial shocks in stacked Fermi-LAT clusters

Abstract

In the hierarchical paradigm of structure formation, galaxy clusters are the largest objects ever to virialize. They are thought to grow by accreting mass through large scale, strong virial shocks. Such a collisionless shock is expected to accelerate relativistic electrons, thus generating a spectrally flat leptonic virial ring. However attempts to detect virial rings have all failed, leaving the shock paradigm unconfirmed. Here we identify a virial γ-ray signal by stacking Fermi-LAT data for 112 clusters, enhancing the ring sensitivity by rescaling clusters to their virial radii and utilizing the anticipated spectrum. In addition to a central unresolved, hard signal (detected at the nominal 5.8σ confidence level), probably dominated by active galactic nuclei, we identify (5.9σ ) a bright, spectrally flat γ-ray ring at the expected shock position. It corresponds to ∼ 0.6% (with an uncertainty factor ∼ 2) thermal energy deposition in relativistic electrons over a Hubble time. This result validates the shock paradigm, calibrates its parameters, and indicates that the cumulative emission from such shocks significantly contributes to the diffuse extragalactic γ-ray and radio backgrounds.

Detection of gamma-ray emission from the Coma cluster with Fermi Large Area Telescope and tentative evidence for an extended spatial structure

“…Very recently, Ref. [29] claimed a ≥ 4.5σ detection of a ring-like structure at the outskirts of the clusters in the Fermi-LAT stacking analysis of galaxy clusters. Nondetection of gamma-rays from the central galaxy clusters thus poses a challenge for the theoretical picture.…”

Galaxy cluster virial-shock sources in eROSITA catalogs

“…In recent years, the long-awaited virial-shock (VS) signals around galaxy clusters and groups (for brevity, henceforth "clusters") were finally detected in inverse-Compton (Keshet et al 2017;Reiss et al 2017;, synchrotron (Keshet et al 2017;Hou et al 2023), and thermal Sunyaev-Zeldovich (Keshet et al 2017;Hurier et al 2019;Keshet et al 2020;Pratt et al 2021;Anbajagane et al 2022) signatures, both in stacking analyses and in individual clusters. The stacked leptonic signals indicate highly localized emission at normalized radii of 2.2 τ ≡ r/R 500 2.5, where the subscript 500 refers to the radius around a cluster enclosing 500 times the critical mass density of the Universe at the respective redshift.…”

First detection of a virial shock with SZ data: implication for the mass accretion rate of Abell 2319

“…Preliminary detection of accretion shocks in the outskirts of galaxy clusters have been achieved recently in the γ-ray in the case of the Coma cluster and for a stacked sample of galaxy clusters (Reiss et al 2017). However, the direct detection of the pressure drop in the intra-cluster medium (ICM) around the virial radius is particularly challenging.…”