Photoionization calculations of the radiation force due to spectral lines in AGNs

Abstract

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.