Clumpy AGN outflows due to thermal instability
  One of the main mechanisms that could drive mass outflows on parsec scales in
AGN is thermal driving. The same X-rays that ionize and heat the plasma are
also expected to make it thermally unstable. Indeed, it has been proposed that
the observed clumpiness in AGN winds is caused by thermal instability (TI).
While many studies employing time-dependent numerical simulations of AGN
outflows have included the necessary physics for TI, none have so far managed
to produce clumpiness. Here we present the first such clumpy wind simulations
in 1-D and 2-D, obtained by simulating parsec scale outflows irradiated by an
AGN. By combining an analysis of our extensive parameter survey with physical
arguments, we show that the lack of clumps in previous numerical models can be
attributed to the following three effects: (i) insufficient radiative heating
or other physical processes that prevent the outflowing gas from entering the
TI zone; (ii) the stabilizing effect of stretching (due to rapid radial
acceleration) in cases where the gas enters the TI zone; and (iii) a flow speed
effect: in circumstances where stretching is inefficient, the flow can still be
so fast that it passes through the TI zone too quickly for perturbations to
grow. Besides these considerations, we also find that a necessary condition to
trigger TI in an outflow is for the pressure ionization parameter to decrease
along a streamline once gas enters a TI zone.
