Observational Constraints on the Feeding of Supermassive Black Holes

Abstract: 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.