AGN must be very efficient at powering outflows
  Galaxy evolution is affected by competing feedback processes. Stellar
feedback dominates in low-mass galaxies, while AGN feedback predominantly
affects massive ones. Recent observational results reveal the dependence of
black hole accretion rate (BHAR) and star formation rate (SFR) on galaxy
stellar mass, and give information on the galaxy mass at which the changeover
between dominant feedback mechanisms occurs. I use this information to derive
an empirical estimate of the coupling efficiency, $f_{\rm AGN}$, between AGN
luminous energy output and AGN-driven galactic outflows, and the momentum
loading factor $f_{\rm p,AGN}$ between the momentum of AGN radiation field and
the outflow. The results are independent of any particular model of AGN
feedback and show that AGN feedback must be very efficient and/or have very
large momentum loading in order to explain current observations. I discuss
possible ways of reaching the required efficiency and loading factor, and the
selection effects that might result in only weak outflows being observed, while
the most powerful ones may be generally obscured. There are significant
uncertainties involved in the derivation of the result; I suggest ways of
reducing them. In the near future, better estimates of coupling efficiency can
help distinguish among AGN feedback models, investigate the redshift evolution
and mass dependence of feedback efficiency.
