The Black Hole in IC 1459 from Hubble Space Telescope Observations of the Ionized Gas Disk

The peculiar elliptical galaxy IC 1459 (MV = -21.19, D = 16.5 h-1 Mpc) has a fast counterrotating stellar core, stellar shells and ripples, a blue nuclear point source, and strong radio core emission. We present results of a detailed Hubble Space Telescope study of IC 1459, and in particular its central gas disk, aimed at constraining the central mass distribution. We obtained WFPC2 narrowband imaging centered on the Hα+[N II] emission lines to determine the flux distribution of the gas emission at small radii, and we obtained FOS spectra at six aperture positions along the major axis to sample the gas kinematics. We construct dynamical models for the Hα+[N II] and Hβ kinematics that include a supermassive black hole and in which the stellar mass distribution is constrained by the observed surface brightness distribution and ground-based stellar kinematics. In one set of models we assume that the gas rotates on circular orbits in an infinitesimally thin disk. Such models adequately reproduce the observed gas fluxes and kinematics. The steepness of the observed rotation velocity gradient implies that a black hole must be present. There are some differences between the fluxes and kinematics for the various line species that we observe in the wavelength range 4569–6819 Å. Species with higher critical densities generally have a flux distribution that is more concentrated toward the nucleus, and have observed velocities that are higher. This can be attributed qualitatively to the presence of the black hole. There is some evidence that the gas in the central few arcsec has a certain amount of asymmetric drift, and we therefore construct alternative models in which the gas resides in collisionless cloudlets that move isotropically. All models are consistent with a black hole mass in the range M• = 1–4 × 108 M⊙, and models without a black hole are always ruled out at high confidence. The implied ratio of black holes mass to galaxy mass is in the range 0.4–1.5 × 10-3, which is not inconsistent with results obtained for other galaxies. These results for the peculiar galaxy IC 1459 and its black hole add an interesting data point for studies on the nature of galactic nuclei.