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Session 50 - Active Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
Ever since the identification of low-ionization nuclear emission-line regions (LINERs), the nature of the ionization mechanism in these galactic nuclei has remained controversial. Although models involving collisional ionization can reproduce the observed emission-line intensities in LINERs, other evidence suggests that photoionization by a low-luminosity active galactic nucleus (AGN) is also a likely explanation in some cases.
If some LINERs are indeed low-luminosity AGNs, then the central engine that powers the observed activity should be similar to that in AGNs. The leading model for the AGN `central engine' is the accretion of matter into the relativistically deep potential well of a black hole (BH) with a mass \sim 10^7 - 10^9 \ M_ødot. Thus, the stellar kinematics in nearby LINERs should be measured with high resolution to search for evidence of supermassive BHs.
We obtained spectroscopic observations of M81's nuclear region with the Faint Object Spectrograph aboard the Hubble Space Telescope during March 1995. Although the data have only modest S/N in the stellar continuum, we have obtained preliminary measurements of the stellar kinematics, suggesting Keplerian rotation within \approx 5 pc of the nucleus and a dramatic increase in stellar velocity dispersion. A preliminary calculation demonstrates that these kinematics indicate the presence of a \sim 10^7 \ M_ødot BH.
In addition, our spectra also include several optical emission lines, resulting in the serendipitous discovery described here of a double-peaked emission-line component in the broad line region. Double-peaked emission lines in other AGNs can be modeled in terms of a Keplerian accretion disk with radius \sim 10^3 GM/c^2 orbiting about the central BH.
