Context. Composite galaxies, containing both a starburst and Seyfert component, may produce very-high-energy (VHE; > 100 GeV) γ-ray emission at vastly different spatial scales ranging from several Schwarzschild radii of a supermassive black hole (SMBH) to a dozen kiloparsecs. Some cosmic-ray sources, including cores of active galaxies, heads of kiloparsec-scale jets, and galactic superwinds, additional to core-collapse supernova remnants were suggested to explain multi-wavelength and/or muti-messenger data collected on composite galaxies. For a variety of scenarios VHE γ-ray observations are expected to provide stringent constraints on cosmic-ray populations in these systems.
Aims. The closest composite Seyfert–starburst galaxies, NGC 1068, the Circinus galaxy, and NGC 4945, are investigated for being γ-ray emitters with the High Energy Stereoscopic System (H.E.S.S.). Most of the H.E.S.S. NGC 1068 observations were taken prior to the observations by the Major Atmospheric Gamma Imaging Cherenkov telescopes in order to provide coverage of NGC 1068 observations in time and tighten the previously reported H.E.S.S. flux upper limit by factor of 10. The H.E.S.S. observations of the Circinus galaxy and NGC 4945 were taken with the purpose to provide the first constraints on their VHE γ-ray signals.
Methods. Data obtained in dedicated H.E.S.S. observations have been analyzed to search for VHE γ-ray counterparts to the detected Fermi-LAT GeV γ-ray signals and for potential spectral sub-components substantially emitting in the VHE range.
Results. No signals have been found in these H.E.S.S. data. Upper limits on the VHE γ-ray fluxes are derived and are compared to models, involving starburst activities in NGC 1068 and NGC 4945, kiloparsec-scale bubbles in NGC 1068 and the Circinus galaxy, possible multiple components in NGC 4945 previously suggested from Fermi-LAT data, propagation of VHE γ rays for a SMBH surrounded by gas or photons in NGC 1068, and at last hypothetical sources of ultra-high-energy cosmic rays in NGC 4945.
Conclusions. The H.E.S.S. observations of the nearby composite Seyfert-starburst galaxies probe a broad range of energetic astrophysical phenomena. The non-detection of NGC 1068, the Circinus galaxy, and NGC 4945 in the H.E.S.S. data has implications on the cosmic-ray populations existing under different physical conditions.