On the Scattering Contributions to N V lambda 1240 and C IV lambda 1549 in QSOs

The large relative strength of N V λ1240 emission from QSOs has gained new importance since Hamann and Ferland used it to infer nitrogen overabundances and supersolar metallicities. However, Hamann and Ferland did not consider that N V might be selectively enhanced by Lyα photons scattered in the broad absorption line regions (BALRs). Here we calculate the total BALR scattering contributions to N V λ1240 and C IV λ1549 for cases with average and extreme (very favorable) scattering conditions. We use a large sample of observed BALs to show that the average BALR, with mean covering factor q^bar^ = 0.12, scatters <~ 5.6 A equivalent width in N V and <~ 4.0 A in C IV in non-BALQSOs. These upper limits correspond to <~31% and <~13% of the measured averages for the N V and C IV emission lines, respectively. The scattering of Lyα emission nearly doubles the mean-scattered flux in N V. However, wind models of the BALR show that the narrowest mean scattering profiles are nearly twice as broad as the average observed emission lines. Therefore, only a fraction of the scattered flux contributes to the measured emission lines, especially for N V, where line blending severely limits our ability to measure broad-emission components. From our narrowest scattering profiles, we estimate that BALR scattering accounts for no more than 18% of the measured average N V emission. The wind models also show that any nonspherically symmetric BALR geometry produces broader mean profiles in BAL versus non-BALQSO samples, which contradicts recent observations if scattering is assumed to dominate the emission lines. We conclude that scattering in the BALR does not contribute significantly to either N V or C IV. In particular, it does not account for the large observed strength of N V emission. This result is supported by other evidence, including the observed similarity of the N V and C IV emission profiles and the lack of correlation between the N V and Lyα emission strengths. Therefore, overabundant nitrogen remains the most viable interpretation of the strong N V emission line.