A Note on the Viability of Gaseous Ionization in Active Galaxies by Fast   Shocks
  Observational evidence suggest that shocks may affect the spatial and velocity distributions of gas in the NLR/ENLR of some active galaxies. It thus seemed plausible that shocks may also energize the NLR. The observed emission line ratios strongly favor photoionization as the heating source, but it is not clear whether the ionizing radiation is generated in the NLR by "photoionizing shocks" or whether it originates at the central continuum source. Here I point out that shocks are highly inefficient in producing line emission. Shocks in the NLR can convert at most 10^{-6} of the rest mass to ionizing radiation, compared with a maximum conversion efficiency of ~0.1 for the central continuum source. The required mass flow rate through shocks in the NLR is thus a few orders of magnitude higher than the mass accretion rate required to power the NLR by the central continuum source. Since gravity appears to dominate the NLR cloud dynamics, shocks must lead to an inflow, and the implied high inflow rates can be ruled out in most active galaxies. NLR dynamics driven by a thermal wind or by some jet configurations may produce the mass flux through shocks required for photoionizing shocks to be viable, but the mass flux inward from the NLR must be kept ~100-1000 times smaller. Photoionizing shocks are a viable mechanism in very low luminosity active galaxies if they are highly sub-Eddington (<~10^{-4}) and if they convert mass to radiation with a very low efficiency (<~10^{-4}). 
Ari Laor (Technion)