THE HIGHLY POLARIZED HIDDEN QUASAR IRAS-09104- A DOUBLE-LOBED RADIO-SOURCE IN A RICH CLUSTER(4109 )

IRAS 09104+4109 is the second most luminous active galactic nucleus (A GN) so far discovered by IRAS, with L(bol) approximately 10(12.6) h-2 L.. Kleinmann et al. have shown it to have a strong Seyfert 2 emission -line spectrum (z = 0.442), with a range of excitations and very large equivalent widths but no broad lines. They suggested that this Y appr oximately 19 cD galaxy hides a luminous QSO that ionizes the narrow-li ne region and heats dust within approximately 130 pc of the central en gine. Here we further investigate the geometry and environment of this extreme AGN by means of broad-band polarimetry, spectropolarimetry, n ew spectroscopy, and radio imaging. The observed polarization is very high and strongly wavelength-dependent, increasing from 4% at 7500 ang strom to 21% at 3600 angstrom. The position angle is wavelength-indepe ndent with +/-5-degrees. The optical spectrum is dominated by unpolari zed narrow-line emission and starlight. After correcting for dilution by these unpolarized components, we show that the remaining spectrum i s highly polarized (approximately 18%), and, within the uncertainties, this polarization is wavelength-independent. Spectrophotometry shows an ultraviolet excess and broad Mg II lambda2798 emission that provide strong evidence that we are viewing a luminous quasar obscured in dir ect light but visible in light polarized by scattering. The polarizati on position angle is perpendicular to the position angle of an extende d high-ionization [O III] emission region discovered by Kleinmann et a l., so we postulate that this extended emission is ionized by the same collimated nuclear continuum source that gives rise to the scattered light. Assuming that the polarization arises in an optically thin scat tering cone, we derive an opening angle less than 39-degrees and incli nation approximately 35-degrees, and compare this geometry with the du st covering factor implied by the infrared luminosity. If the observed wavelength dependence of polarization for IRAS 09104+4109 is typical of radio galaxies, it can explain the trend of increasing polarization with increasing redshift found by Tadhunter et al., strongly supporti ng their suggestion that this trend is the result of decreasing diluti on of scattered light by unpolarized starlight, toward the shorter wav elengths. IRAS 09104+4109 is also unusual in being the most powerful r adio source associated with an IRAS-discovered object, and our new rad io images show classic linear core and double-lobed structure. IRAS 09 104+4109 is also a dominant cD galaxy in a rich, flattened galaxy clus ter of intermediate redshift whose major axis is perpendicular to the radio jet. These facts suggest that this object may provide a clue to the relation between galaxy environment and strong radio jets.