The masses of black holes in the nuclei of spirals

We use the innermost kinematics of spirals to investigate whether these gal axies could host the massive black hole remnants that once powered the quas i-stellar object (QSO) phenomenon. Hundreds of rotation curves of early- an d late-type spirals are used to place upper limits on the central black hol e (BH) masses. We find that (i) in late-type spirals, the central massive d ark objects (MDOs) are about 10-100 times smaller than the MDOs detected in ellipticals, and (ii) in early-type spirals, the central bodies are likely to be in the same mass range as the elliptical MDOs. As a consequence, the contribution to the QSO/active galactic nuclei (AGN) phenomenon by the BH remnants eventually hosted in spirals is negligible: rho(BH)(Sb-Im) < 6 x 1 0(4) M. Mpc(-3). We find several hints that the MDO mass versus bulge mass relationship is significantly steeper in spirals than in ellipticals, altho ugh the very issue of the existence of such a relation for late Hubble type objects remains open. The upper limits on the masses of the BHs resident i n late-type spirals are stringent: M-BH less than or equal to 10(6)-10(7) M ., indicating that only low-luminosity activity could possibly have occurre d in these objects.