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On the nature of the red, 2MASS-selected AGN in the local Universe

Rose, Marvin (2012) On the nature of the red, 2MASS-selected AGN in the local Universe. PhD thesis, University of Sheffield.

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Abstract

A key result of the Two Micron All Sky Survey (2MASS) was the discovery of a popula­ tion of AGN that appear redder than their traditional UV/optical selected counterparts at IR wavelengths. In order to investigate the nature of 2MASS­selected AGN, in this thesis I present optical spectra and near­ to mid­IR photometry for a representative sam­ ple of 29 nearby (z<0.28) 2MASS­selected AGN with red near­IR colours (J­K S # > 2.0), comparing my findings with those obtained for comparison samples of UV/optical se­ lected AGN. The spectra show a remarkable variety, including moderately reddened type 1 objects (58%), type 1 objects that appear similar to traditional UV/optically selected AGN, narrow­line Seyfert 1 AGN (14%), type 2 AGN (21%) and HII/composite objects (7%). Although the median Balmer decrement suggests significant optical reddening compared to the comparison samples, some objects show relatively little (if any) reddening in their optical spectra. In addition, the near­ to mid­IR photometric results reveal that a fraction of the broad­line objects in the 2MASS sample (26%) have unusually blue 2.17­3.4µm colours when compared to the rest of the sample, suggesting that their red near­IR colours are due, at least in part, to emission by a component of hot dust with an unusually high covering factor. Overall, a variety of mechanisms is required to explain the red near­IR colours of the 2MASS objects: although some are moderately reddened type 1 objects, or `genuine' type 2 objects that are highly obscured by the central torus structure, others show unusually strong emission from dust close to the AGN. This is confirmed by simple modelling of the colours of the 2MASS and comparison samples However, I find no clear evidence that the 2MASS objects are young, dust enshrouded AGN. In addition to the analysis of the full 2MASS sample, I present a detailed study of one of the red 2MASS AGN (J1131+16) which shows unusually strong forbidden high ionization lines. This study demonstrates the potential of optical spectroscopic studies of the FHILs for understanding the nature of the obscuring dust structure of AGN.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Science (Sheffield)
The University of Sheffield > Faculty of Science (Sheffield) > Physics and Astronomy (Sheffield)
Identification Number/EthosID: uk.bl.ethos.589142
Depositing User: Mr Marvin Rose
Date Deposited: 17 Jan 2014 15:14
Last Modified: 03 Oct 2016 11:03
URI: http://etheses.whiterose.ac.uk/id/eprint/4490

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