Intermittent AGN episodes drive outflows with a large spread of
  observable loading factors
  The properties of large-scale galactic outflows, such as their kinetic energy
and momentum rates, correlate with the luminosity of the active galactic
nucleus (AGN). This is well explained by the wind-driven outflow model, where a
fraction of the AGN luminosity drives the outflow. However, significant
departures from these correlations have been observed in a number of galaxies.
This may happen because AGN luminosity varies on a much shorter timescale
($\sim 10^4-10^5$~yr) than outflow properties do ($\sim 10^6$~yr). We
investigate the effect of AGN luminosity variations on outflow properties using
1D numerical simulations. This effect can explain the very weak outflow in PDS
456: if its nucleus is currently much brighter than the long-term average
luminosity, the outflow has not had time to react to this luminosity change.
Conversely, the outflow in Mrk 231 is consistent with being driven by an almost
continuous AGN, while IRAS F11119+3257 represents an intermediate case between
the two. Considering a population of AGN, we find that very low momentum
loading factors $\dot{p}_{\rm out} < L_{\rm AGN}/c$ should be seen in a
significant fraction of objects - up to $15\%$ depending on the properties of
AGN variability and galaxy gas fraction. The predicted distribution of loading
factors is consistent with the available observational data. We discuss how
this model might help constrain the duty cycles of AGN during the period of
outflow inflation, implications for multiphase and spatially distinct outflows,
and suggest ways of improving AGN prescriptions in numerical simulations.
