Optical linear polarimetry of warm, infrared ultraluminous active galactic nuclei: Orientation and the nature of the active nucleus

Details Optical linear polarimetry of warm, infrared ultraluminous active galactic nuclei: Orientation and the nature of the active nucleus Optical linear polarimetry of warm, infrared ultraluminous active galactic nuclei: Orientation and the nature of the active nucleus

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt........32h&link_type=abstract

PhD Dissertation, Texas Univ. Austin, TX United States

Computer Science

10

Active Galactic Nuclei, Optical Properties, Infrared Spectrophotometers, Interstellar Extinction, Continuous Spectra, Emission Spectra, Infrared Radiation, Polarimetry, Stellar Mass, Space Density

Scientific paper

Results are presented from an optical linear polarimetry survey of 'warm' (0.25 is less than or equal to Fnu (25mu m)/Fnu (60 mu m) is less than or equal to 3), Infrared Ultraluminous (log(LIR/L(solar mass)) is greater than 11.5) Active Galactic Nuclei (AGNs). Fifty percent of the objects observed, have high, non-time-variable polarizations Pmax is greater than 3%, in sharp contrast to previous surveys of objects selected by optical properties. The observed wavelength dependence of the polarization (P lambda) can be divided into three classes: (A) PA approximately constant and Pmax is less than or equal to 5%, (B) P lambda decreasing with wavelength and Pmax is greater than or equal to 5%, and (C) PA increasing with wavelength. All Class A objects have broad emission lines and strong featureless UV-optical continua in their observed total spectra (Type-1: traditionally called QSOs). Class B objects are dominated by Type-1 spectra, or by narrow emission lines and a weaker, reddened featureless continuum (Type-2 spectra). Class C objects are always dominated by narrow (Type-2) optical emission lines. Spectropolarimetry, and infrared spectrophotometry, reveals Type-1 nuclei in the Class B objects with Type-2 spectra, and in all Class C objects . These results are explained by a simple model, in which all Type-1 nuclei are surrounded by dusty tori. Class A objects are viewed pole-on, while Class B objects are viewed at intermediate latitudes, where our direct view is partially obscured by the torus. At low latitudes, the direct nuclear light may be completely extinguished, so the observed spectrum will be dominated by narrow-line emission from regions above the torus, by unpolarized starlight light from the host galaxy, and by nuclear light that is scattered near the poles of the torus, into our line-of-sight (Class C). This scattered light may also be reddened if it passes through the torus. Thus the three polarization classes represent a sequence in the orientation of the nucleus with respect to our line-ofsight. These results show that there is a population of luminous Type-1 AGNs that have previously been missed by optical selection techniques. This in turn implies that the space density of QSOs may have been underestimated, by a factor of two.

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