2010ApJ...710...78W

The N V emission line of active galactic nuclei shows peculiar behavior in the line-continuum correlation, which may be indicative of an extra line component in addition to that from the normal broad emission line region. In this paper, we investigate possible contribution to the N V emission via resonant scattering of both continuum and Lyα in a broad absorption line (BAL) outflow by performing the Sobolev Monte Carlo simulations. The contribution is dependent on the covering factor, optical depth, and velocity profile of the outflow, as well as the equivalent width (EW) of Lyα. Adopting model parameters constrained by observations, we find that the measured N V EW in the spectra of non-BAL quasi-stellar objects (QSOs) could have been enhanced by a factor of 1.82-2.73 on average, while there is only moderate absorption of Lyα along the BAL outflow direction. Our model can produce a relatively narrow scattering line profile. About 80% of the total scattered flux falls within the central ±4500 km/s. We find that the resonant scattering can produce a prominent polarized emission line around N V. Both the broad excess emission and the unusually large polarized flux observed around N V in BAL QSOs are considered as strong evidence for the scattering enhancement. Future spectropolarimetric observations and spectroscopic monitoring of luminous QSOs may offer crucial tests for this interpretation, and provide useful information on the physical and geometrical properties of QSO outflows. We argue that the scattering offers a promising and robust process for producing the peculiar behavior of N V emission compared to the other processes proposed previously.