Active galactic nuclei with candidate intermediate-mass black holes

We present an initial sample of 19 intermediate-mass black hole candidates in active galactic nuclei culled from the first data release of the Sloan Digital Sky Survey. Using the line width-luminosity mass scaling relation established for broad-line active nuclei, we estimate black hole masses in the range of M_BH ≈ 8 × (10⁴-10⁶) M _⊙, a regime in which only two objects are currently known. The absolute magnitudes are faint for active galactic nuclei, ranging from M _g ≈ -15 to -18 mag, while the bolometric luminosities are all close to the Eddington limit. The entire sample formally satisfies the line width criterion for so-called narrow-line Seyfert 1 galaxies; however, they display a wider range of Fe II and [O III] λ5007 line strengths than is typically observed in this class of objects. Although the available imaging data are of insufficient quality to ascertain the detailed morphologies of the host galaxies, it is likely that the majority of the hosts are relatively late-type systems. The host galaxies have estimated g-band luminosities ∼1 mag fainter than M* for the general galaxy population at z ≈ 0.1. Beyond simply extending the known mass range of central black holes in galactic nuclei, these objects provide unique observational constraints on the progenitors of supermassive black holes. They are also expected to contribute significantly to the integrated signal for future gravitational wave experiments.