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~0.1c), 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 ~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 accelerate CRs in the transition region
between galactic and extragalactic CRs.
