Observational Constraints on the Feeding of Supermassive Black Holes
  Supermassive Black Holes grow at the center of galaxies in consonance with
them. In this review we discuss the mass feeding mechanisms that lead to this
growth in Active Galactic Nuclei (AGN), focusing on constraints derived from
observations of their environment, from extragalactic down to galactic and
nuclear scales. At high AGN luminosities, galaxy mergers and interactions play
an important role in AGN triggering and feeding. However, gas chaotic cold
accretion (CCA) in galaxy clusters can trigger radiatively inefficient AGNs in
brightest cluster galaxies (BCGs). At lower luminosities, minor mergers feed
AGN in early-type, gas-starving galaxies, while secular processes dominate in
later-type, gas-rich galaxies. While bars do not appear to directly feed AGNs,
AGN flickering leads to the dissociation of small and large scales, hence
affecting the interpretation of cause and effect. At ~ 100 pc scales, recent
observations have revealed compact disks and inflows along nuclear gaseous
spirals and bars, while CCA continues to feed BCGs at these scales. Estimated
mass inflow rates - of 0.01 to a few Msun/yr - are in many cases thousand times
higher than the mass accretion rate to the supermassive black hole. As a
result, 10^6 - 10^9 Msun gas reservoirs can be built on 10^{7-8} yr, that in
turn may lead to the formation of new stars and/or be ejected via the onset of
AGN feedback.
