Aperture Effects and Limitations on the Accuracy of Ground-Based Spectrophotometry of Active Galactic Nuclei

Abstract

We use narrow-band images of the [O III] lambda-5007 emission line and adjacent continuum in the well-studied Seyfert galaxies NGC 4151 and NGC 5548 to produce models of the surface-brightness distributions of the narrow-line region and host-galaxy starlight distribution in these galaxies. We use these models to compute the expected magnitude of seeing-induced aperture effects that can lead to systematic errors in broad emission-line and continuum flux measurements made from spectra which are flux-calibrated based on the narrow emission lines. We find that small spectrograph apertures (e.g., 2"x10") are highly undesirable as photometric errors as large as 10 - 20% can results. Moreover, photometric corrections based on image modeling are not likely to offer great improvement since the corrections are a sensitive function of the seeing. Use of larger apertures (e.g., 5" x 7.5") can reduce photometric errors to the few percent range, and in principle seeing corrections based on models of the surface-brightness distributions of the narrow-line region and starlight from the host galaxy can reduce the errors to about the 1% level, at which point miscentering errors probably dominate. The limitation in application of seeing corrections is probably our inability to characterize with sufficient accuracy the point-spread function for the spectroscopic observations, although uncertainties in the surface-brightness models are also likely to play a role.