Optical Polarization of Warm IR-Ultraluminous AGNs

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Session 94 -- Acceleration and Jets in AGN, High Redshift Quasars, Polarization
Oral presentation, Wednesday, 11, 1995, 2:00pm - 3:30pm

[94.04D] Optical Polarization of Warm IR-Ultraluminous AGNs

D.C.Hines (U.Texas)

Results are presented from an optical linear polarimetry survey of ``warm'' [0.25 $\le$ F$_\nu(25\mu$m)/F$_\nu(60\mu$m) $\le$ 3], Infrared Ultraluminous [log(L$_{\rm IR}$/L$_\odot) \ge 11.5$] AGNs. Half show high, non-time-variable polarizations p $\ge 3\%$, 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$_\lambda \approx const \le 5\%$, (B) p$_\lambda$ decreasing with wavelength and p $\ge 5\%$, 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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