H.E.S.S. observations of composite Seyfert-starburst galaxies

TL;DR

This study used H.E.S.S. observations to search for very high-energy gamma-ray emission from composite Seyfert-starburst galaxies, aiming to understand cosmic-ray populations, but found no significant signals.

Contribution

First H.E.S.S. observations of these galaxies provide constraints on VHE gamma-ray emission and cosmic-ray models in composite Seyfert-starburst systems.

Findings

No significant VHE gamma-ray signals detected.

Upper limits set on gamma-ray fluxes to constrain models.

Results inform cosmic-ray population theories.

Abstract

Context: Composite galaxies that contain both Seyfert and starburst components may produce very high-energy (VHE; >100 GeV) gamma-ray emission at a wide range of spatial scales, from a few Schwarzschild radii of a supermassive black hole to dimensions of kiloparsec-size jet-driven outflows. In addition to supernova remnants, various sources have been suggested to explain data collected on composite galaxies, including multi-messenger neutrino and ultra-high-energy cosmic-ray data. Aims: The closest composite Seyfert-starburst galaxies (NGC 1068, the Circinus galaxy, and NGC 4945) are observed with the High Energy Stereoscopic System (H.E.S.S.) to provide constraints on cosmic-ray populations in these systems. Methods: Data obtained in H.E.S.S. observations have been analyzed to search for VHE gamma-ray counterparts to the GeV gamma-ray signals detected with Fermi-LAT and for potential spectral components in the VHE range. Results: No significant signals have been found in these H.E.S.S. data. Upper limits on the VHE gamma-ray fluxes were applied to constrain theoretical models involving different spectral components.