One of the main components of the unified model of AGN (active galactic nuclei) is the circumnuclear dusty torus. The torus often obscures our view of the ''central engine" of the AGN, therefore it is important to understand its size and structure. Reverberation mapping can be used to constrain the size of the torus, and relies on the response of the torus dust emission to the variations in the driving optical/UV emission of the accretion disk. The time lag between the optical emission and the infrared response can be used to constrain the distance between the inner region of the AGN, and the IR emission region of the torus. This project focuses on a sample of 11 AGN, whose mid-IR (3.6 and 4.5 micron) light curves were observed with the Spitzer Space Telescope. The IR light curves were presented in Vazquez (2015). In this work, the optical light curves are constructed with a combination of data from three different ground-based telescopes, in the B, V, and G bands. Cross-correlation analysis has been conducted for four AGN, giving us reverberation lags for these objects. The reliability of the lags have been carefully evaluated by testing their dependence on different analysis procedures. One of these AGN is a changing-look AGN, NGC 6418, whose previous cross-correlation results have been reanalyzed. The optical-IR lag was measured both before and after a large optical flare, and was found to have increased after the flare, likely due to the sublimation of dust in the inner region of the torus. For eight of the AGN, HST images were used along with the galaxy decomposition program GALFIT to model the distribution of galaxy light and hence to remove the starlight contribution from the optical light curves. This is a necessary step in order to use the torus reverberation mapping code, TORMAC, to model the IR light curves, as the dust emission responds to the optical variations. TORMAC was used to model three of the AGN in our sample, which allows us to constrain other torus structural properties. Some of these properties include the radial cloud distribution, the radial extent of the torus, and the torus inclination angle. This dissertation produces reverberation mapping results for four AGN (KAZ 163, MRK 507, NGC 6418, and UGC 10697). micron flux with typical lags between 15-30 days. This project increases the number of AGN with mid-IR dust reverberation mapping measurements, for which there have been few previous studies. One of the AGN (NGC 6418) is found to be a rare changing-look AGN. Not only are we able to measure the size of the torus, but through the use of TORMAC we also have a more complete picture of the structure of the torus in our sample of AGN.
Astrophysical Sciences and Technology (Ph.D.)
Department, Program, or Center
Physics and Astronomy, School of
College of Science
Frank, Ashley, "Mid-Infrared Dust Reverberation Mapping of Active Galactic Nuclei" (2024). Thesis. Rochester Institute of Technology. Accessed from
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