Evidence for a massive black hole in the active galaxy NGC 4261 from Hubble Space Telescope images and spectra

We present HST/WFPC2 V, R, and I images and Hubble Space Telescope/FOS spectra of the active galaxy NGC 4261. The galaxy hosts a nuclear disk of dust (see papers by Jaffe et at), roughly perpendicular to the radio jet. The images presented here show evidence of a spiral-like structure in the disk that likely provides the means by which angular momentum is carried from the center outward, allowing the inner material to sink in. The disk is not in an equilibrium configuration: it is not coaxial with the major axis of the galaxy, and it is not centered on either the nucleus or on the isophotal center of the galaxy. This prompted us to conclude that the dust has probably an external origin. With respect to the isophotal center of the galaxy, the nucleus is displaced by about 3 pc (assuming a distance to NGC 4261 of 30 Mpc) in the direction of the radio lobes: such a displacement can be due to recoil from the radio jet. An Her map of the nuclear region is obtained from the R-band image by using the V and I images for the continuum determination. The ionized gas is concentrated in a resolved region with a FWHM of 0''.12, or 17 pc. FOS spectra were taken with the 0''.1 aperture in the wavelength region between 4570 and 6870 Angstrom in a grid of 13 aperture positions around and at the nuclear location. The [N II] lambda lambda 6548, 6584 + H alpha emission complex is detected at all but three of the aperture positions. Fainter emission in H beta, [O III] lambda lambda 4459, 5007, [O I] lambda lambda 6300, 6364, [S II] lambda lambda M6717, 6731, [N I] lambda lambda 5200, 5202 and [N II] lambda 5756 is also detected. The central velocities of the CN nl lines as a function of distance from the center can be accounted for by assuming that the ionized gas is confined in a disk in Keplerian motion around a central mass (4.9 +/- 1.0) x 10(8) M(.). By integrating the unreddened V luminosity density we find a mass to light ratio (M/L)(V) similar to 2100 M(.)/L(.) within the inner 14.5 pc. The large mass-to-light ratio, and the fact that NGC 4261 is a relatively strong radio galaxy, lead us to conclude that the majority of the central mass is concentrated in a (4.9 +/- 1.0) x 10(8) M(.) black hole.