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Session 56 - Active Galaxies: The Central Engine.
Oral session, Tuesday, January 16
Corte Real, Hilton
We present HST/WFPC2 V, R and I images and HST/FOS spectra of the active galaxy NGC 4261. An H\alpha 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 circular region with a FWHM of 0.12'', or 17 pc, assuming a distance to NGC\thinspace 4261 of 30 Mpc. FOS spectra were taken with the 0.1'' aperture in the wavelength region between 4570 Å\ and 6870 Åin a grid of thirteen aperture positions around and at the nuclear location. The [N\thinspace II]\lambda\lambda6548,6584+H\alpha emission complex is detected at all aperture positions. Fainter emission in H\beta, [O\thinspace III] \lambda\lambda4459,5007, [O\thinspace I] \lambda\lambda6300,6364, [S\thinspace II] \lambda\lambda6717,6731, [N\thinspace I]\lambda\lambda5200,5202 and [N\thinspace II]\lambda5756 is also detected. The central velocities of the [N\thinspace II] 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 (1.2 \pm 0.4) \times 10^9 M_ødot. By integrating the V luminosity density we find a mass to light ratio (M/L)_V \approx 5200 M_ødot/L_ødot within the inner 14.5 pc. The large mass to light ratio, and the fact that NGC\thinspace 4261 is a relatively strong radio galaxy, lead us to conclude that the majority of the central mass is concentrated in a (1.2 \pm 0.4) \times 10^9 M_ødot black hole.
This work was supported by NASA Grants NAS 5-1630, GO-2607.01-87A, and GO-05432.01-93A.
Program listing for Tuesday