Exploring AGNs in the Local Universe through High Angular Resolution Spectroscopy and Optical Variability Monitoring

This dissertation presents the results of several observational projects designed to explore AGNs, on scales of about 100 pc and smaller, in nearby galaxies. High angular resolution spectroscopy acquired from HST STIS and Keck LGS AO OSIRIS observations are used to study the kinematic structure of gas disks and stars on scales comparable to the gravitational sphere of influence of the supermassive black hole. Specifically, I use multi-slit STIS data of low-luminosity AGNs to map out the emission-line kinematics, and to look for regularly rotating velocity fields suitable for future gas dynamical black hole mass measurements. The HST data is further useful for searching for electron-density gradients and for examining how the emission-line velocity dispersion varies as a function of aperture size. In another project, I aim to resolve a discrepancy between two previous gas dynamical measurements of the black hole in the elliptical galaxy M84. I perform new measurements of the gas kinematics from archival multi-slit STIS data, and carry out a more comprehensive dynamical model of the emission-line disk than had been previously attempted. With the most recent project, I measure high-resolution stellar kinematics from LGS AO OSIRIS data and large-scale kinematics from long-slit LRIS data of the S0 galaxy NGC 3998. Using triaxial orbit-based stellar dynamical models, I determine the black hole mass. The stellar dynamical value is then compared to a previous gas dynamical determination in order to test the consistency between these two main mass measurement methods.;In addition to using high spatial resolution spectroscopy, AGN variability can be used to probe the inner regions of AGNs. The Lick AGN Monitoring Project targeted 12 galaxies expected to harbor low-mass black holes. Through reverberation mapping, the collaboration measured the masses of nine black holes, and learned about the geometry and kinematics of the broad-line region in several of the objects. I discuss my contribution to the project: measuring the AGN continuum from broadband B and V images. I also examine continuum variability characteristics, search for time lags between B- and V-band variations, and look for fluctuations in the AGN color.