Optical Polarization of Warm IR-Ultraluminous AGNs

Details Optical Polarization of Warm IR-Ultraluminous AGNs Optical Polarization of Warm IR-Ultraluminous AGNs

Dec 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...185.9404h&link_type=abstract

American Astronomical Society, 185th AAS Meeting, #94.04; Bulletin of the American Astronomical Society, Vol. 26, p.1474

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Results are presented from an optical linear polarimetry survey of ``warm'' [0.25 <= F_nu (25microns)/F_nu (60microns) <= 3], Infrared Ultraluminous [log(L_IR/L_sun) >= 11.5] AGNs. Half show high, non-time-variable polarizations p >= 3%, in sharp contrast to previous surveys of optically selected QSOs. The observed polarization wavelength dependence (p_lambda ) can be divided into three classes: (A) p_lambda ~ const <= 5%, (B) p_lambda decreasing with wavelength and p >= 5%, and (C) p_lambda increasing with wavelength. All Class A objects have broad emission lines and strong featureless UV-optical continua (FCs) in their observed total spectra (Type-1: QSOs). Class B are either dominated by Type-1 spectra, or by narrow emission lines and a weaker, reddened FC (i.e. Type-2). Class C always have Type-2 total spectra. However, spectropolarimetry, and infrared spectrophotometry, reveals Type-1 spectra in all of the Class B & C objects. These results are explained by a simple model in which all luminous Type-1 nuclei are surrounded by dust (perhaps in a torus). Class A objects are viewed along unobscured lines-of-sight (pole-on), while Class B are seen such that our direct view is partially obscured (intermediate latitudes). For Class C, the direct nuclear light may be completely extinguished (edge-on) --- the observed spectrum is dominated by narrow-line emission from regions beyond the obscuring material, by unpolarized host-galaxy star light, and by nuclear light that is scattered (near the poles of the torus) into our line-of-sight. This scattered light may also be reddened if it is partially obscured. Thus, it is suggested that the observationally defined polarization classes represent progressively stronger obscuration of the direct line-of-sight, but in all classes some views are unobscured. If the obscuring material is distributed in a torus, the classes represent a sequence in orientation with respect to our direct line-of-sight. Implications for the space density of QSOs, and for the nature of Broad Absorption Line QSOs (which show Class A polarization) are briefly discussed.

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