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The nature of active galaxies

University of British Columbia

Many details of the structure of Active Galactic Nuclei (AGN) galaxies continue to elude researchers in the field. To shed light on some of the enigmas related to the fueling and classification of AGN, I have studied the core structure of a sample of 37 nearby Seyfert galaxies at high resolution using adaptive optics on the CFHT. This dataset consists of near-IR imaging from 1 to 3 μm (the J,H, and K bands). I first describe the instruments and observing techniques along with a presentation of the galaxy sample properties. I then outline the detailed data reduction and image processing required with adaptive optics observations, highlighting some of the associated unavoidable perils. The main difficulty with adaptive optics observations is a PSF artifact I discovered which is associated with guiding on galactic nuclei. This represents the main hurdle to identifying small-scale (<1") structures in the core regions of galaxies using adaptive optics. A detailed multi-wavelength study is pursued for two nearby Seyfert galaxies, NGC3227 and NGC2992. With these objects, the current ideas of Seyfert fueling and unification of Seyfert types are scrutinized, focusing on the high spatial resolution achieved using adaptive optics in the near-IR. The dynamical processes and differing classifications of these galaxies are substantially clarified through their core morphologies. These studies show that scientific results can be established with AO data, in spite of the above mentionned artifact. For NGC2992, a spiral structure within the central 6" and a 1" extended feature are traced down to the core at the resolution of our images. We speculate, based on these observed structures, that multiple radio components are superposed which contribute to the observed figure-8 morphology in the VLA images: one associated with the spiral structure in the galaxy disk, and another flowing out of the galaxy plane. With NGC3227, an assembly of star formation knots lying in a spiral/ring pattern is suggestive of embedded spiral arms within the larger-scale spiral of the outer galaxy. An elongation of PA ~ 40° may be a nuclear disk or bar structure, although the blue colors are suggestive of some scattered nuclear continuum emission. The extended radio feature observed with MERLIN is coincident with the near-IR region of knots assuming a spiral/ring pattern, but may still be associated with an outflow. I then address whether the classification of Seyfert galaxy types can be explained via patchy dust at fairly large distances (~ 100 pc) from the central engine. Maps of dust extinction are constructed with the deep view afforded by the near-IR. These are compared with optical images observed with the Hubble Space Telescope (HST) to aid in distinguishing dust absorption from other emission processes, as well as providing a dust diagnostic with a different range of sensitivity than the near-IR. In the absence of any standardized classification for dust absorption in galaxies, we have developed a graded system of two indices. A dust patchiness index (DPI) characterizes the dust morphology, and a global dust index (GDI) combines the DPI morphological index with a quantitative assessment of the extinction. Applying these indices to the Seyfert sample, we find that Seyfert type 2 galaxies are typically slightly more dusty than Seyfert type 1. However, when later intermediate-type Seyfert galaxies (Seyfert 1.8 and 1.9) and Seyfert 1 with extended nuclei are included with the Seyfert 2, the dust indices indicate a significant difference from the Seyfert types 1 through 1.5. The observational evidence is now such that an alternative should be considered to the standard classification model where the visibility of the type 1 AGN signature is the main differentiator. The new possibility is that Seyfert galaxies are drawn from two different populations: very dusty, and less/non-dusty. Sy2 are then thought almost always to be of the very dusty type. Whether or not the actual Syl nucleus is visible in the very dusty type is then merely a result of the current dust configuration of the Seyfert nucleus.

Thesis/Dissertation

application/pdf

2009-07-02

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http://hdl.handle.net/2429/9951

Physics

University of British Columbia

UBCV

DSpace