Active Galactic Nuclei (AGN) are accreting supermassive black holes at the centers of galaxies. According to the unified model, this accretion disk is surrounded by an obscuring torus of dust and gas. In Type 2, or obscured, AGN this torus is viewed edge on. When the column density of the torus exceeds 1/σt = 1.5×1021 cm–2, this obscuring medium becomes Compton-thick. Studies indicate that a significant fraction of Compton-thick Type 2 AGN exist but are under-represented in many current samples. We have studied two samples of local type 2 AGN (Seyfert 2 galaxies) to explore issues relevant to finding and characterizing the Compton-thick population. We have also investigated the relationship between type 2 AGN and the galaxies in which they live.
To find this Compton-thick population, selecting samples of AGN based on their inherent flux is necessary. We undertook an empirical approach in identifying the most reliable intrinsic AGN flux proxies. Using infrared spectroscopy from Spitzer, optical spectra from the Sloan Digital Sky Survey (SDSS) and the literature, and radio and hard X-ray (E > 10 keV) data from the literature, we demonstrated that the [OIV] 26μm, [OIII] 5007Å and MIR continuum fluxes agree the best among Type 1 and Type 2 Seyfert galaxies.
Utilizing 2-10 keV X-ray data from Chandra and XMM-Newton, we probed the amount of obscuration that may be present in these systems. We find that a majority of sources exhibit signatures of heavy, and possibly Compton-thick, obscuration: depressed 2-10 key X-ray emission when normalized by intrinsic AGN flux and large Fe Ka equivalent widths.
Using a sample of ∼250 star forming galaxies, ∼50 composite systems and an additional ∼20 Seyfert 2 galaxies, we examined the connection between AGN activity and star formation. We found that the SDSS derived star formation rates and [NeII] 12.8μm flux accurately probe starburst activity in both quiescent and active galaxies. Using these parameters and diagnostics that accurately trace AGN flux, we have shown that these processes are significantly correlated. This link suggests that supermassive black holes and their host galaxies grow simultaneously in the local universe.
|Advisor:||Heckman, Timothy, Ptak, Andrew|
|School:||The Johns Hopkins University|
|School Location:||United States -- Maryland|
|Source:||DAI-B 73/04, Dissertation Abstracts International|
|Keywords:||Active galactic nuclei, Black holes, Seyfert galaxies, Supermassive black holes|
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