Starbursts and Seyferts: The Mystery of the Missing Gamma Rays

In this episode, we dive deep into the fascinating world of "composite" galaxies—cosmic beasts that host both an actively feeding supermassive black hole (a Seyfert nucleus) and regions of intense star formation (a starburst component).

We explore recent research from the High Energy Stereoscopic System (H.E.S.S.) observatory, which conducted deep observations of three nearby composite galaxies: NGC 1068, the Circinus galaxy, and NGC 4945. The big question driving the research: Can we detect very high-energy (VHE) gamma rays from the extreme environments at the centers of these galaxies?

Surprisingly, H.E.S.S. detected no significant VHE gamma-ray signals from any of the three targets. Tune in to find out why this lack of detection is actually highly revealing! We discuss how these newly established upper limits on gamma-ray fluxes are helping astrophysicists test and constrain major theories, including:

Jet-Driven Bubbles: How the outflows in these galaxies compare to the giant "Fermi bubbles" found in our own Milky Way.

Cosmic Ray Calorimeters & UHECRs: Whether these galaxies act as traps for cosmic rays, and if they could be the source of mysterious ultra-high-energy cosmic rays (UHECRs) hitting Earth.

The Neutrino Connection: How the absence of gamma rays in NGC 1068 perfectly complements the detection of high-energy neutrinos by the IceCube observatory, suggesting that gamma rays are being heavily absorbed by a dense X-ray photon field right next to the supermassive black hole.

Reference to the Article:

H.E.S.S. Collaboration, Acharyya, A., Aharonian, F., et al. (2026). "H.E.S.S. observations of composite Seyfert–starburst galaxies." Astronomy & Astrophysics (Preprint online version: March 24, 2026).

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: NASA/ESA/A. van der Hoeven