Properties of galactic nuclei inferred from line spectra

Abstract

This work explores how certain properties of galactic nuclei can be understood on the basis of the available data. Current evidence for the presence of large central masses in these regions, believed to be supermassive black holes, is first reviewed. Methods for estimating the mass are discussed, and a new algorithm is presented for implementing reverberation techniques with time-variable broad line data from active nuclei. The effectiveness of this new algorithm is first tested on sample data sets; it is then applied to actual data. Next, a model is presented for the formation of the cool, dense clouds responsible for the broad line emission, involving the rapid cooling of shocked gas embedded in a quasi-spherical, turbulent accretion flow. As an illustrative example, fitting of the model (with simplify assumptions) is performed on data pertaining to the Seyfert nucleus NGC 5548. Accretion flows in two specific objects are then discussed. First, a cool, spherical accretion flow is argued for the non-active nucleus of M31 on the basis of the observed broad-band spectrum. In addition to comparisons of the model with the currently available data, we provide detailed predictions of the UV and optical line spectra, correcting for extinction due to intervening dust and cold gas. Then, a turbulent disk structure is argued for the weakly-active nucleus in the radio galaxy NGC 4261. This structure is capable of producing both the observed broad line spectrum and radio absorption, and may have application to the nuclei of other radio galaxies. Finally, the iron Kalpha emission from Sgr B2, a giant molecular cloud located in the Galactic Center region, is reviewed. While many argue that this suggests recent activity associated with the radio source Sgr A*, our modeling indicates that the data are also consistent with a time-variable illuminator embedded within the cloud. The observations and modeling suggest that turbulence may be a key component to accretion in active nuclei, facilitating the transfer of angular momentum and allowing the greater accretion rates needed to fuel the central engines. Future 3D-hydrodynamical simulations are required to test this assertion.