Constraints on the gamma-ray emission from the cluster-scale AGN outburst in the Hydra A galaxy cluster

In some galaxy clusters powerful AGN have blown bubbles with cluster scale extent into the ambient medium. The main pressure support of these bubbles is not known to date, but cosmic rays are a viable possibility. For such a scenario copious gamma-ray emission is expected as a tracer of cosmic rays from these systems. Hydra A, the closest galaxy cluster hosting a cluster scale AGN outburst, located at a redshift of 0.0538, is investigated for being a gamma-ray emitter with the High Energy Stereoscopic System (H.E.S.S.) array and the Fermi Large Area Telescope (Fermi-LAT). Data obtained in 20.2 hours of dedicated H.E.S.S. observations and 38 months of Fermi-LAT data, gathered by its usual all-sky scanning mode, have been analyzed to search for a gamma-ray signal. No signal has been found in either data set. Upper limits on the gamma-ray flux are derived and are compared to models. These are the first limits on gamma-ray emission ever presented for galaxy clusters hosting cluster scale AGN outbursts. The non-detection of Hydra A in gamma-rays has important implications on the particle populations and physical conditions inside the bubbles in this system. For the case of bubbles mainly supported by hadronic cosmic rays, the most favorable scenario, that involves full mixing between cosmic rays and embedding medium, can be excluded. However, hadronic cosmic rays still remain a viable pressure support agent to sustain the bubbles against the thermal pressure of the ambient medium. The largest population of highly-energetic electrons which are relevant for inverse-Compton gamma-ray production is found in the youngest inner lobes of Hydra A. The limit on the inverse-Compton gamma-ray flux excludes a magnetic field below half of the equipartition value of 16 muG in the inner lobes.

💡 Research Summary

The paper investigates whether the giant AGN‑inflated bubbles in the Hydra A galaxy cluster emit detectable high‑energy gamma rays, as would be expected if cosmic rays (CRs) provide a significant fraction of the pressure support inside the bubbles. Hydra A, at a redshift of 0.0538, hosts the nearest known cluster‑scale AGN outburst, making it an ideal laboratory for testing this hypothesis.

Observations were carried out with two complementary instruments: the ground‑based H.E.S.S. array, which is sensitive to photons above ~100 GeV, and the space‑borne Fermi‑LAT, covering the 100 MeV–300 GeV range. The H.E.S.S. data consist of 20.2 hours of dedicated pointings on the cluster core, while the Fermi‑LAT analysis uses 38 months of all‑sky survey data. Standard event selection, background modeling, and likelihood analysis were applied to both data sets, focusing on the coordinates of the X‑ray cavities (the bubbles). No statistically significant excess was found in either energy band; the test‑statistic values remained well below the detection threshold.

Upper limits at the 99 % confidence level were derived: for H.E.S.S., a flux limit of ≈2 × 10⁻¹³ ph cm⁻² s⁻¹ above 1 TeV; for Fermi‑LAT, ≈1.5 × 10⁻¹² ph cm⁻² s⁻¹ above 1 GeV. These limits were compared with theoretical models of gamma‑ray production in bubble environments. In the “hadronic‑CR dominated, fully mixed” scenario—where the CR energy density equals the thermal pressure of the ambient intracluster medium—the predicted gamma‑ray flux exceeds the observed limits by a factor of several, thereby ruling out this extreme case. However, models in which CRs are only partially mixed with the thermal gas, or where relativistic electrons (leptonic component) dominate the pressure, remain compatible with the data.

The paper also examines inverse‑Compton (IC) emission from the youngest inner lobes (≈10 kpc in size). The IC flux depends sensitively on the magnetic field strength: a weaker field leads to a larger electron energy density and thus a higher IC gamma‑ray output. The derived gamma‑ray limits require that the magnetic field in these lobes be at least half of the equipartition value (≈16 µG), i.e., ≥ 8 µG. This constraint is consistent with, but somewhat tighter than, previous estimates based on radio synchrotron data.

Energy budget considerations show that the total mechanical energy injected by the AGN into the bubbles (~10⁶¹ erg) could plausibly be shared between thermal gas, magnetic fields, and CRs. If 10–30 % of this energy resides in CRs, the resulting gamma‑ray emission would lie near the current upper limits, suggesting that a modest CR component is still viable as a pressure support mechanism.

In summary, the non‑detection of Hydra A in both H.E.S.S. and Fermi‑LAT data provides the first quantitative gamma‑ray constraints on galaxy clusters hosting large‑scale AGN outbursts. It excludes the most optimistic hadronic‑CR pressure model (full mixing) while leaving room for more complex, partially mixed or leptonic scenarios. The magnetic field in the inner lobes must be ≥ 8 µG, and future, more sensitive gamma‑ray facilities such as the Cherenkov Telescope Array (CTA) will be essential to probe deeper into the CR content and magnetic structure of these remarkable astrophysical bubbles.