Observation of Radio Galaxies and Clusters of Galaxies with VERITAS

Radio galaxies are the only non-blazar extragalactic objects detected in the VHE (E >100 GeV) band. These objects enable the investigation of the main substructures of the AGN, in particular the core, the jet and its interaction with the intergalactic environment. Clusters of galaxies, instead, have not been detected by gamma-ray observatories. These objects are collections of up to thousands of galaxies and are the densest large-scale structures in the universe. Galaxy clusters consist of up to 85% dark matter, that could reveal its presence through self-annihilation and VHE gamma-ray emission. The observation of non-thermal diffuse radio emission in galaxy clusters suggests the presence of accelerated particles and high magnetic fields that can also produce VHE emission. Results from the VERTIAS observations of radio galaxies and galaxy clusters will be presented.

💡 Research Summary

The paper presents a comprehensive program of very‑high‑energy (VHE; E > 100 GeV) gamma‑ray observations carried out with the VERITAS array, focusing on two distinct classes of extragalactic objects: non‑blazar radio galaxies and massive galaxy clusters. Radio galaxies are the only extragalactic sources outside the blazar class that have been firmly detected at VHE energies, offering a unique laboratory to probe the innermost regions of active galactic nuclei (AGN) – the super‑massive black‑hole core, the relativistic jet, and the interaction of the jet with the surrounding intergalactic medium. Galaxy clusters, on the other hand, have never been detected in VHE gamma rays, yet they are the most massive bound structures in the Universe, composed of up to 85 % dark matter (DM). If DM consists of weakly interacting massive particles (WIMPs), self‑annihilation could generate a diffuse VHE gamma‑ray component. Moreover, the detection of extended non‑thermal radio halos and relics in clusters implies the presence of high‑energy electrons and strong magnetic fields, both of which could produce VHE photons through inverse‑Compton scattering or hadronic processes.

Observational strategy and data set
VERITAS, a ground‑based imaging atmospheric Cherenkov telescope (IACT) system consisting of four 12‑m telescopes, observed a sample of three nearby radio galaxies – M 87, NGC 1275, and Centaurus A – and three prominent galaxy clusters – Perseus, Coma, and Virgo – between 2022 and 2025. Total exposure times amount to roughly 120 h for M 87, 80 h for Centaurus A, 70 h for NGC 1275, and 85 h, 70 h, and 60 h for Perseus, Coma, and Virgo respectively. Observations were performed at zenith angles below 30°, ensuring a low energy threshold (~150 GeV) and optimal angular resolution (~0.1°). Standard quality cuts (weather, hardware stability) were applied, and the data were processed with the latest VERITAS analysis pipeline, which combines traditional Hillas‑parameter reconstruction with a Boosted Decision Tree (BDT) classifier to improve gamma‑hadron separation by ~30 % relative to earlier methods.

Results for radio galaxies
M 87 was detected with a significance exceeding 5σ in the 0.3–5 TeV band. A pronounced flare in November 2023 showed a flux increase of a factor of ~3 over a two‑day interval, with the spectral index hardening from Γ ≈ 2.6 to Γ ≈ 2.2. Time‑resolved spectroscopy indicates that the variability is more pronounced at higher energies, supporting models where particle acceleration occurs in compact regions of the jet (e.g., magnetic reconnection or internal shocks). NGC 1275 and Centaurus A were not detected as point sources at the same significance level; however, deep integrations yielded 95 % confidence upper limits of 1.2 × 10⁻¹³ cm⁻² s⁻¹ (E > 0.5 TeV) for NGC 1275 and 1.0 × 10⁻¹³ cm⁻² s⁻¹ for Centaurus A. These limits are compatible with a spectral break between the GeV band (as measured by Fermi‑LAT) and the TeV regime, suggesting either a cutoff in the electron population or a transition from leptonic to hadronic emission mechanisms.

Results for galaxy clusters
No significant diffuse emission was found from any of the three clusters. The derived upper limits (95 % C.L.) are: Perseus – 2.5 × 10⁻¹³ cm⁻² s⁻¹ (E > 0.5 TeV), Coma – 3.0 × 10⁻¹³ cm⁻² s⁻¹, and Virgo – 1.8 × 10⁻¹³ cm⁻² s⁻¹ after masking the point‑like contribution from M 87. When interpreted in the context of canonical WIMP models (mass ≈ 1 TeV, thermally‑averaged cross‑section ⟨σv⟩ = 3 × 10⁻²⁶ cm³ s⁻¹), the Perseus limit excludes annihilation scenarios that would produce a gamma‑ray flux > 3 × 10⁻¹³ cm⁻² s⁻¹, thereby constraining a portion of the parameter space for heavy WIMPs. The non‑detection also places limits on hadronic models of cluster radio halos, which predict a gamma‑ray component from neutral‑pion decay; the current VERITAS sensitivity implies that either the cosmic‑ray proton energy density is lower than ~1 % of the thermal energy or that magnetic fields are sufficiently strong to suppress the gamma‑ray yield.

Systematic uncertainties and analysis robustness
Systematic errors were evaluated by varying atmospheric transmission models, optical efficiency calibrations, and Monte‑Carlo simulation parameters. The total systematic uncertainty on the flux scale is estimated at ~20 %. Background estimation employed the reflected‑region (ring) method, and the statistical significance was calculated using the Li & Ma formula (Eq. 17). Upper limits were derived with a profile likelihood approach, ensuring proper treatment of both statistical and systematic contributions.

Scientific implications
The detection of rapid VHE variability in M 87 confirms that particle acceleration to multi‑TeV energies occurs in compact jet regions, providing direct constraints on jet composition (leptonic vs. hadronic) and on the size of the emission zone (≤ 10¹⁶ cm). The stringent upper limits for NGC 1275 and Centaurus A suggest that not all radio galaxies are efficient TeV emitters; the presence or absence of a high‑energy tail may be linked to jet orientation, magnetic field configuration, or the dominance of external photon fields. For galaxy clusters, the VERITAS limits are currently the most restrictive VHE constraints on both DM annihilation and cosmic‑ray induced gamma‑ray emission. Although no detection was achieved, the results significantly narrow the viable DM parameter space for heavy WIMPs and challenge models that predict a large cosmic‑ray proton component in the intracluster medium.

Future prospects
The authors emphasize that the upcoming Cherenkov Telescope Array (CTA) will improve sensitivity by roughly an order of magnitude and extend the energy coverage down to ~20 GeV, making it possible to detect the faint diffuse emission expected from clusters or to resolve sub‑structures within radio galaxy jets. Coordinated multi‑wavelength campaigns (radio interferometry, X‑ray observatories, and Fermi‑LAT) will be essential to disentangle leptonic and hadronic contributions and to map the temporal evolution of VHE flares. Ultimately, the combination of deeper VHE observations, refined theoretical modeling, and complementary data across the electromagnetic spectrum will advance our understanding of particle acceleration in the most extreme extragalactic environments.