The AGN broad line region as a clumpy turbulent outflow: a physical basis for LOC modeling

Abstract: Many studies have considered the roles of clouds, outflows, and turbulence in producing the broad emission and absorption lines in the spectra of active galactic nuclei (AGNs). However, these are often treated as separate or even competing models. Here, we consider the possibility that AGN clouds are condensations formed within the thermally unstable zones of outflows and then compare the typical sizes of such condensations with the injection scale $k_0^{-1} \sim L_0$ of turbulence, where $L_0$ is assumed to be the scale height of a representative global outflow model. We find that for broad line region (BLR) parameters, clouds are many orders of magnitude smaller than $L_0$ and this has the following implication: BLR cloud dynamics can be modeled using a local approximation through the use of multiphase turbulence simulations of X-ray irradiated plasmas. We present the first such 3D local clumpy turbulent outflow simulations. We show that the condensations share the same type of selection effects characterizing the locally optimally emitting cloud (LOC) scenario, thereby offering a physical interpretation for the LOC model and accounting for its almost uncanny successes. The ubiquitous presence of emission line regions in AGNs can be simply explained as the natural outcome of there being a multiphase interval of $k$-space within the inertial range of a turbulent cascade.