New results from H.E.S.S. observations of galaxy clusters

Clusters of galaxies are believed to contain a significant population of cosmic rays. From the radio and probably hard X-ray bands it is known that clusters are the spatially most extended emitters of non-thermal radiation in the Universe. Due to their content of cosmic rays, galaxy clusters are also potential sources of VHE (>100 GeV) gamma rays. Recently, the massive, nearby cluster Abell 85 has been observed with the H.E.S.S. experiment in VHE gamma rays with a very deep exposure as part of an ongoing campaign. No significant gamma-ray signal has been found at the position of the cluster. The non-detection of this object with H.E.S.S. constrains the total energy of cosmic rays in this system. For a hard spectral index of the cosmic rays of -2.1 and if the cosmic-ray energy density follows the large scale gas density profile, the limit on the fraction of energy in these non-thermal particles with respect to the total thermal energy of the intra-cluster medium is 8% for this particular cluster. This value is at the lower bounds of model predictions.

💡 Research Summary

The paper presents deep observations of the massive, nearby galaxy cluster Abell 85 with the H.E.S.S. (High Energy Stereoscopic System) array, aiming to detect very‑high‑energy (VHE, E > 100 GeV) gamma‑ray emission that would signal the presence of a substantial population of cosmic‑ray (CR) protons in the intracluster medium (ICM). Galaxy clusters are expected to host non‑thermal particles accelerated by large‑scale structure formation shocks, supernova remnants, galactic winds, and active galactic nuclei (AGN). Hadronic CRs can produce gamma rays through inelastic collisions with thermal protons, generating neutral pions that decay into photons; leptonic CRs can up‑scatter cosmic‑microwave‑background photons via inverse‑Compton scattering. Despite these theoretical expectations, no galaxy cluster has yet been firmly identified as a VHE gamma‑ray source.

Observations were carried out in October–November 2006 and August 2007, accumulating 32.5 hours of good‑quality live time with a mean zenith angle of 18°, which yields an energy threshold of ≈460 GeV. The analysis considered three spatial integration regions to account for possible emission morphologies: (i) a compact core region of radius 0.1° (≈0.4 Mpc), (ii) an intermediate region of radius 0.49° (≈1.9 Mpc) matching the extent of the X‑ray emitting gas, and (iii) a very extended region of radius 0.91° (≈3.5 Mpc) intended to probe emission from a putative accretion shock. Upper limits were derived using the Feldman‑Cousins method at 95 % confidence, assuming a power‑law photon spectrum with index –2.1.

No statistically significant excess was found in any region. The resulting integral flux upper limits above 460 GeV are:

• Core (0.1°): F < 3.9 × 10⁻¹³ ph cm⁻² s⁻¹,
• Intermediate (0.49°): F < 1.5 × 10⁻¹² ph cm⁻² s⁻¹,
• Extended (0.91°): F < 9.9 × 10⁻¹² ph cm⁻² s⁻¹.

These limits were translated into constraints on the total energy stored in hadronic CRs within Abell 85. The authors adopt two key assumptions: (1) the CR proton spectrum follows a hard power law with index –2.1, and (2) the spatial distribution of CRs follows the large‑scale gas density profile, excluding the very central cooling core. The first assumption reflects the expectation that CRs in clusters experience negligible radiative losses, preserving the injection spectrum; the second is motivated by magneto‑hydrodynamic simulations that disfavor strongly centrally peaked CR profiles.

Under these assumptions, the total CR energy is found to be less than 8 % of the thermal energy of the ICM. This upper bound lies at the lower end of theoretical predictions, which typically range from a few percent up to ~20 % depending on the acceleration efficiency of structure‑formation shocks and the contribution of AGN. Similar constraints have recently been obtained from deep radio observations of other clusters (e.g., Abell 521), reinforcing the notion that current VHE instruments are approaching but have not yet reached the sensitivity required to test the full range of models.

The authors conclude that while the H.E.S.S. data provide the most stringent VHE limits for Abell 85 to date, a definitive detection of gamma‑ray emission from galaxy clusters will likely require the next generation of Cherenkov telescopes, such as the Cherenkov Telescope Array (CTA). CTA’s improved sensitivity and broader energy coverage will enable probing CR energy fractions well below the 5 % level, allowing a decisive test of acceleration scenarios, the role of AGN feedback, and the interplay between non‑thermal and thermal components in the evolution of large‑scale structures.