A UNIFORMLY SELECTED SAMPLE OF LOW-MASS BLACK HOLES IN SEYFERT 1 GALAXIES

We have conducted a systematic search of low-mass black holes (BHs) in active galactic nuclei (AGNs) with broad H alpha emission lines, aiming at building a homogeneous sample that is more complete than previous ones for fainter, less highly accreting sources. For this purpose, we developed a set of elaborate, automated selection procedures and applied it uniformly to the Fourth Data Release of the Sloan Digital Sky Survey. Special attention is given to AGN-galaxy spectral decomposition and emission-line deblending. We define a sample of 309 type 1 AGNs with BH masses in the range 8 x 10(4)-2 x 10(6) M-circle dot (with a median of 1.2 x 10(6) M-circle dot), using the virial mass estimator based on the broad H alpha line. About half of our sample of low-mass BHs differs from that of Greene & Ho, with 61 of them discovered here for the first time. Our new sample picks up more AGNs with low accretion rates: the Eddington ratios of the present sample range from less than or similar to 0.01 to similar to 1, with 30% below 0.1. This suggests that a significant fraction of low-mass BHs in the local universe are accreting at low rates. The host galaxies of the low-mass BHs have luminosities similar to those of L* field galaxies, optical colors of Sbc spirals, and stellar spectral features consistent with a continuous star formation history with a mean stellar age of less than 1 Gyr.