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

TL;DR

This study searched for gamma-ray emission from Hydra A's AGN-inflated bubbles to understand cosmic ray support, but found no signal, setting new upper limits and constraining models of particle populations in galaxy cluster bubbles.

Contribution

First gamma-ray emission limits for galaxy clusters with AGN outbursts, constraining cosmic ray support models in Hydra A's bubbles.

Findings

No gamma-ray signal detected in H.E.S.S. and Fermi-LAT data.

Hadronic cosmic ray support in bubbles can be excluded under full mixing assumption.

Magnetic field in inner lobes is at least half of the equipartition value.

Abstract

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.