Supermassive black holes in the Sbc spiral galaxies NGC 3310, NGC 4303 and NGC 4258

We have undertaken an HST Space Telescope Imaging Spectrograph survey of 54 late type spiral galaxies to study the scaling relations between black holes and their host spheroids at the low mass end. Our aim is to measure black hole masses or to set upper limits for a sizeable sample of spiral galaxies. In this paper we present new Space Telescope Imaging Spectrograph (STIS) observations of three spiral galaxies, NGC 4303, NGC 3310 and NGC 4258. The bright optical emission lines H alpha lambda 6564 angstrom, [NII] lambda lambda 6549, 6585 angstrom and [SII] lambda lambda 6718, 6732 angstrom were used to study the kinematics of the ionized gas in the nuclear region of each galaxy with a similar to 0.07 '' spatial resolution. Our STIS data for NGC 4258 were analyzed in conjunction with archival ones to compare the gas kinematical estimate of the black hole mass with the accurate value from H2O-maser observations. In NGC3310, the observed gas kinematics is well matched by a circularly rotating disk model but we are only able to set an upper limit to the BH mass which, taking into account the allowed disk inclinations, varies in the range 5.0 x 10(6)-4.2 x 10(7) M-circle dot at the 95% confidence level. In NGC 4303 the kinematical data require the presence of a BH with mass M-BH = (5.0)(-2.26)(+0.87) x 10(6) M-circle dot (for a disk inclination i = 70 deg) but the weak agreement between data and disk model does not allow us to consider this measurement completely reliable. If the allowed inclination values are taken into account, M-BH varies in the range 6.0 x 10(5)-1.6 x 10(7) M-circle dot at the 95% confidence level. In NGC 4258, the observed kinematics require the presence of a black hole with M-BH = (7.9)(-3.5)(+6.2) x 10(7) M-circle dot (i = 60 deg) and, taking into account reasonable limits for the inclination, M-BH is in the range 2.5 x 10(7)-2.6 x 10(8) M-circle dot at the 95% confidence level. This result is in good agreement with the published value (3.9 +/- 0.1) x 10(7) M-circle dot, derived from H2O-maser observations. As in the case of NGC 4303, the agreement between observed and model kinematics is not strong but this does not affect the recovery of the correct M-BH value. Our attempt at measuring BH masses in these 3 late type Sbc spiral galaxies has shown that these measurements are very challenging and at the limit of the highest spatial resolution currently available. Nonetheless our estimates are in good agreement with the scaling relations between black holes and their host spheroids suggesting that (i) they are reliable and (ii) black holes in spiral galaxies follow the same scaling relations as those in more massive early-type galaxies. A crucial test for the gas kinematical method, the correct recovery of the known BH mass in NGC 4258, has been successful.