Quasar Feedback: More Bang for Your Buck
  We propose a two-stage model for the effects of feedback from a bright quasar
on the cold gas in a galaxy. It is difficult for feedback from near the
accretion disk to directly impact dense molecular clouds at ~kpc. But if such
feedback can drive a weak wind or outflow in the hot, diffuse ISM (a relatively
'easy' task), then in the wake of such an outflow passing over a cold cloud, a
combination of instabilities will drive the cloud material to effectively
expand in the direction perpendicular to the outflow. Such expansion
dramatically increases the effective cross section of the cloud material and
makes it more susceptible to ionization and momentum coupling from absorption
of the incident quasar radiation field. Even a moderate effect of this nature
can dramatically alter the ability of clouds at large radii to be fully ionized
and driven into a secondary outflow by radiation pressure. Since the amount of
momentum and volume which can be ionized by observed quasar radiation field is
more than sufficient to affect the entire cold gas supply once it has been
altered in this manner (and the 'initial' feedback need only initiate a
moderate wind in the low-density hot gas), this reduces by an order of
magnitude the required energy budget for feedback to affect a host galaxy.
Instead of ~5% of the radiated energy (~100% momentum) needed if the initial
feedback must directly heat or blow out the galactic gas, if only ~0.5% of the
luminosity (~10% momentum) can couple to drive the initial hot outflow, this
mechanism could be efficient. This amounts to hot gas outflow rates from near
the accretion disk of only 5-10% of the BH accretion rate.
