The nuclear disk of NGC 4261: Hubble Space Telescope images and ground-based spectra

We present Hubble Space Telescope Planetary Camera images and ground-based spectra of the nuclear region of NGC 4261 = 3C 270. The images reveal a sharply defined disk of dust whose projected major and minor axes are 1.''79 and 0.''77, respectively. The minor axis of the disk is nearly aligned with the radio axis of 3C 270. Our analysis shows that the disk is physically thin (less than or similar to 20 pc) with an optical depth near unity. It appears displaced from the isophotal center of the galaxy by approximately 5 pc. The optical nucleus, with m(v) = 23.6, is unresolved. The spectra show strong Ha, [S II] and [N II] emission lines with narrow cores and unusually broad bases. The broad wings of the forbidden lines suggest a central mass (at r < 0.1 pc) of similar to 4 x 10(7) M. and a central stellar M/L similar to 20 (V band, solar units). We argue that the visible disk is most probably related to the nuclear activity, feeding material by viscous transport of angular momentum into a much more compact accretion disk surrounding the central black hole. The values of the disk surface density and kinetic viscosity that we derive from our observations are consistent with the radio and optical line luminosities of the galaxy.