Gamma rays from fast black-hole winds

Massive black holes at the centers of galaxies can launch powerful wide-angle winds that, if sustained over time, can unbind the gas from the stellar bulges of galaxies. These winds may be responsible for the observed scaling relation between the masses of the central black holes and the velocity dispersion of stars in galactic bulges. Propagating through the galaxy, the wind should interact with the interstellar medium creating a strong shock, similar to those observed in supernovae explosions, which is able to accelerate charged particles to high energies. In this work we use data from the Fermi Large Area Telescope to search for the $\gamma$-ray emission from galaxies with an ultra-fast outflow (UFO): a fast ($v\sim0.1$c), highly ionized outflow, detected in absorption at hard X-rays in several nearby active galactic nuclei (AGN). Adopting a sensitive stacking analysis we are able to detect the average $\gamma$-ray emission from these galaxies and exclude that it is due to processes other than the UFOs. Moreover, our analysis shows that the $\gamma$-ray luminosity scales with the AGN bolometric luminosity and that these outflows transfer $\sim$0.04% of their mechanical power to $\gamma$ rays. Interpreting the observed $\gamma$-ray emission as produced by cosmic rays (CRs) accelerated at the shock front, we find that the $\gamma$-ray emission may attest to the onset of the wind-host interaction and that these outflows can energize charged particles up to the transition region between galactic and extragalactic CRs. A preprint of the full analysis is available on the arXiv: 2105.11469.