A Search for Low-Level Seyfert Activity in the 500 Brightest Northern Galaxies

A sensitive search for intrinsically faint Seyfert nuclei in the 500 brightest northern galaxies is being conducted with the Hale 5.08 m telescope. The primary aim is to extend the luminosity function of active galactic nuclei (AGNs) to lower luminosities. Thus far, we have detected Hα emission having full-width near zero-intensity (FWZI) ≳ 4000 km s −1 in a significant fraction (≳10%) of nearby galaxies. Moreover, the narrow emission lines in a given nucleus often have markedly different widths, as shown in detail for M81. Those associated with high critical densities are generally the broadest, indicating that a very wide range of densities is present in the narrow-line region. The relative intensities of the lines can readily be explained with models in which clouds of gas having n e ≈ 10 2 to 10 7 cm −3 are photoionized by nonstellar radiation such as that produced by QSOs and luminous Seyfert 1 galaxies.