The Nuclear Disk of NGC 4261: Hubble Space Telescope Images and Ground-based Spectra

Abstract

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 (<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_nu_ = 23.6, is unresolved. The spectra show strong Hα, [S II] and [NII] 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 ~4 x 10^7^ M_sun_ and a central stellar M/L ~ 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.