Photoionization calculations of the radiation force due to spectral
  lines in AGNs
  One of the main mechanisms that could drive mass outflows in AGNs is
radiation pressure due to spectral lines. Although straightforward to
understand, the actual magnitude of the radiation force is challenging to
compute because the force depends on the physical conditions in the gas, and
the strength, spectral energy distribution (SED), and geometry of the radiation
field. We present results from our photoionization and radiation transfer
calculations of the force multiplier, $M(\xi,t)$, using the same radiation
field to compute the gas photoionization and thermal balance. We assume low gas
density ($n = 10^4~\rm{cm^{-3}}$) and column density ($N \leq 10^{17}~\rm{
cm^{-2}}$), a Boltzmann distribution for the level populations, and the Sobolev
approximation. Here, we describe results for two SEDs corresponding to an
unobscured and obscured AGN in NGC 5548. Our main results are the following: 1)
although $M(\xi,t)$ starts to decrease with $\xi$ for $\xi \gtrsim 1$ as shown
by others, this decrease in our calculations is relatively gradual and could be
non-monotonic as $M(\xi,t)$ can increase by a factor of few for $\xi \approx
10-1000$; 2) at these same $\xi$ for which the multiplier is higher than in
previous calculations, the gas is thermally unstable by the isobaric criterion;
3) non-LTE effects reduce $M(t,\xi)$ by over two orders of magnitude for $\xi
\gtrsim 100$. The dynamical consequence of result (1) is that line driving can
be important for $\xi$ as high as $1000$ when the LTE approximation holds,
while result (2) provides a natural cloud formation mechanism that may account
for the existence of narrow line regions. Result (3) suggests that line driving
may not be important for $\xi\gtrsim100$ in tenuous plasma.
