A new sample of low-mass black holes in active galaxies

We present an expanded sample of low-mass black holes (BHs) found in galactic nuclei. Using standard virial mass techniques to estimate BH masses, we select from the Fourth Data Release of the Sloan Digital Sky Survey all broadline active galaxies with masses <2 x 10⁶ M_⊙. BHs in this mass regime provide unique tests of the relationship between BHs and galaxies, since their late-type galaxy hosts do not necessarily contain classical bulges. Furthermore, they provide observational analogs of primordial seed BHs and are expected, when merging, to provide strong gravitational signals for future detectors such as LISA. From our preliminary sample of 19, we have increased the total sample by an order of magnitude to 174, as well as an additional 55 (less secure) candidates. The sample has a median BH mass of (M_BH) = 1.3 x 10⁶ M_⊙, and in general the objects are radiating at high fractions of their Eddington limits. We investigate the broad spectral properties of the sample; 55 are detected by ROSAT, with soft X-ray luminosities in the range 10⁴⁰ to 7 x 10 ⁴³ ergs s^_1. Much like the preliminary sample, these objects are predominantly radio-quiet (R ≡ f_6cm/f _400Å < 10), but 11 objects are detected at 20 cm, with radio powers (10²¹-10²³ W Hz^-1) that may arise from either star formation or nuclear activity; only 1% of the sample is radio-loud. We further confirm that, with (M₉) = -19.3 and (g -r) = 0.7 mag, the host galaxies are low-mass, late-type systems. At least 40% show disklike morphologies, and the combination of host galaxy colors and higher order Balmer absorption lines indicate intermediateage stellar populations in a subset of the sample.