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Cohen's current research is mainly concerned with active galactic nuclei. In these studies the first need is to describe the nuclear region. What components are there, what is the disposition of disks and torii, is the outflowing material in a wide cone or perhaps held close to the equatorial plane, what fraction of quasars and galaxies have extended scattering regions, etc. The second need, then, is to understand this phenomenology in terms of physics, and of course the two tracks are pursued simultaneously.

The observational program aimed at these goals is mostly built around spectropolarimetry at the Hale and Keck telescopes. As an example, the figure shows spectra for a broad-absorption-line quasar, 0105-265, taken at the Keck telescope. The top panel shows the total flux and the lower panel shows the fractional linear polarization. Five broad absorption troughs are visible; they are due to the atomic species C IV, Si IV, Ly alpha/N V, O VI, and S VI. The polarization increases in these absorption featuers, and decreases in the emission features. A simple explanation for this might involve two components. One is the unpolarized continuum+line radiation which passes through material flowing out near the equatorial plane; this material absorbs at the various resonance wavelengths. The other is continuum light from the nucleus which is reflected from electrons or dust located, perhaps, high on the axis. The reflected light is strongly polarized and makes up most of the light in the troughs, but only a fraction of the continuum light and an even smaller fraction of the total light in the emission lines. This simple picture turns out to be deficient in several ways, and a more complicated explanation is currently being investigated.


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