NIR and MIR spectroscopic study of the dust properties of Seyfert galaxies and the implications on AGN unification

The goal of this thesis is to investigate the properties of dust grains in active galaxies using near-infrared (NIR) and mid-infrared (mid-IR) spectroscopy. Understanding dust properties in these regions is important as dust is believed to play a crucial role in determining whether an active galactic nucleus (AGN) is perceived as a broad-line object or a narrow-line object. The near-infrared spectroscopy provides the estimate of the reddening toward the light emitting regions, broad-line regions (BLRs) and narrow-line regions (NLRs), by comparing the line ratios of the observed hydrogen recombination lines, Pbeta/Bgamma, with the case B conditions while the mid-infrared spectroscopy investigates the dust in AGN environment using the silicate feature at 9.7mum. The size distribution of dust can be probed as both reddening and the 9.7mum silicate feature are produced by small grains only. By comparing the dust properties of Seyfert 1 galaxies with those of Seyfert 2's, the standard model of AGN unification can also be tested.;In the near-infrared, we report the spectra of 11 Seyfert galaxies (6 Seyfert 1's, 1 Seyfert 1.9 and 4 Seyfert 2's) obtained using the Gemini twin channel near-infrared camera on the Shane 3m telescope at Lick Observatory. With the unique design of the Gemini camera, full J and K spectra were taken simultaneously through the same slit. This produced accurate line ratios of hydrogen recombination lines over a large wavelength baseline. For the Seyfert 1's (≤1.5), the line ratios of Pabeta/Brgamma are not only comparable in both broad and narrow-line regions but also consistent with case B recombination indicating little or no reddening in both narrow and broad line regions. Seyfert 2 (>1.5) galaxies, however, show substantial reddening toward the narrow-line regions. We compare optical reddening data from the literature and find significant support for the dichotomy between Seyfert 1's and 2's at least in low luminosity objects. Two different scenarios are explored to explain the observed difference in reddening: difference in reddening by a extended dusty torus under AGN unification or by different grain size distribution between the two Seyfert types. (Abstract shortened by UMI.).