A SEARCH FOR DWARF SEYFERT NUCLEI .4. NUCLEI WITH BROAD H-ALPHA EMISSION

We present the results of an optical spectroscopic survey designed to search for low-luminosity ''dwarf'' Seyfert nuclei in a magnitude-limi ted (B-T less than or equal to 12.5 mag) sample of 486 bright, norther n galaxies. Moderate-resolution spectra of exceptionally high quality were obtained, in part to detect broad Ha emission similar in characte r to, but much weaker than, the broad permitted lines that define type 1 Seyfert nuclei. One of the primary gears 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 de finite or probable evidence of broad Her emission. We outline the procedures fo r determining the presence of this elusive spectral feature, steps for its quantitative measurement, and the associated systematic errors. O f the 211 emission-line galaxies classified as having Seyfert or LINER (low-ionization nuclear emission-line region) nuclei in our survey, t he broad Ha line was detected with confidence in 34 objects and with l ess certainty in another 12. Most of the detections are reported for t he 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 gal axies spectroscopically classified as ''active'' and in more than 10% of all luminous galaxies at the current epoch. Excluding nine well-kno wn Seyfert 1 nuclei, the broad Ha lines of the remaining ''dwarf'' nuc lei have median luminosity of similar to 1 x 10(39) ergs s(-1) and med ian FWHM of similar to 2200 km s(-1). The broad lines constitute appro ximately 30% of the total emission of the H alpha + [N II] lambda lamb da 6548, 6583 blend. Several object shave broad Ha luminosities as low as (1-3) x 10(38) ergs s(-1).