High-resolution mid-infrared imaging of active galactic nuclei.

摘要

Type 1 AGN are characterized by broad emission lines in addition to narrow permitted and forbidden lines, whereas as Type 2 AGN reveal only narrow emission lines. The “grand unified model” suggests these AGN represent the same phenomenon; a super massive black hole (SMBH) surrounded by a dusty torus. In the case of Type 2 AGN the line-of-sight intercepts the torus (edge-on) and obscures the broad emission lines. In Type 1 the torus is orientated such that the line of sight is perpendicular to the plane of the torus (pole-on) allowing observations of the broad emission lines close to the SMBH. However, the idea that AGN classification depends solely on the line of sight to the central engine has been called into question by recent surveys of AGN. These surveys reveal that the host galaxies of Type 2 AGN typically have an increased level of star formation as compared to Type 1. Several authors suggest that this is due to an evolutionary link between Type 1 and 2 AGN. In these models material associated with circumnuclear starbursts in addition to the torus acts to obscure Type 2 AGN. As these starbursts evolve over time and expel excess material through superwinds, the central AGN is more likely to be classified as a Type 1.; A key prediction of evolutionary models is a greater occurrence of circumnuclear starbursts with Type 2 AGN in comparison to Type 1 AGN. Previous observations suggest the size of such circumnuclear starburst may be on order of a few hundred parsecs. A high resolution mid-IR survey of AGN which can trace the peak emission of such circumnuclear starbursts on the scales necessary to test evolutionary predictions is needed. This dissertation outlines the process necessary to conduct such a mid-IR survey and presents three preliminary observations of AGN. These AGN (NGC 4151, Cygnus A, and Mrk 231) show no strong circumnuclear starburst activity but are too few to accurately test evolutionary predictions statistically. However these observations show the advantages of mid-IR imaging in studying the relationship between AGN and starburst activity in the central kiloparsec of active galaxies.