A Search for “Dwarf” Seyfert Nuclei. IV. Nuclei with Broad Hα Emission

We present the results of an optical spectroscopic survey designed to search for low-luminosity “dwarf” Seyfert nuclei in a magnitude-limited (BT ≤ 12.5 mag) sample of 486 bright, northern galaxies. Moderate-resolution spectra of exceptionally high quality were obtained, in part to detect broad Hα emission similar in character to, but much weaker than, the broad permitted lines that define type 1 Seyfert nuclei. One of the primary goals of the survey is to better quantify the faint end of the luminosity function of active galactic nuclei. This paper describes the subset of nuclei showing definite or probable evidence of broad Hα emission. We outline the procedures for determining the presence of this elusive spectral feature, steps for its quantitative measurement, and the associated systematic errors. Of the 211 emission-line galaxies classified as having Seyfert or LINER (low-ionization nuclear emission-line region) nuclei in our survey, the broad Hα line was detected with confidence in 34 objects and with less certainty in another 12. Most of the detections are reported for the first time, and the detection rate represents a lower limit to the true incidence of active nuclei harboring a broad emission-line region. These statistics imply that broad-lined active nuclei are much more common than previously believed: they exist in at least 20% of all galaxies spectroscopically classified as “active” and in more than 10% of all luminous galaxies at the current epoch. Excluding nine well-known Seyfert 1 nuclei, the broad Hα lines of the remaining “dwarf” nuclei have median luminosity of ~1 × 1039 ergs s-1 and median FWHM of ~2200 km s-1. The broad lines constitute approximately 30% of the total emission of the Hα + [N II] λλ6548, 6583 blend. Several objects have broad Hα luminosities as low as (1-3) × 1038 ergs s-1.