We present imaging and spectro- polarimetric observations of the ultraluminous infrared galaxy IRAS P09104+4109 using the Keck 10-m Telescope .
We detect the clear presence of broad H \beta , H \gamma , and Mg II \lambda 2800 emission lines in the polarized flux spectra of the nucleus and of an extranuclear emission region \sim 4 \arcsec away , confirming the presence of a hidden central quasar .
The polarization of the broad Mg II emission line is high ( \sim 29 % ) , consistent with the remarkably high polarization ( \sim 30 % –40 % ) observed in the extended continuum emission .
This indicates that the off-nuclear continuum is dominated by light scattered from the hidden quasar , most probably by dust mixed with the line emitting gas .
The high polarizations , combined with the “ foreshortened ” morphology of the polarized brightness distribution allow us to constrain the scattering biconical structure to be at inclination i~ { } \approx~ { } 50 \arcdeg with a half-opening cone angle \theta _ { c } ~ { } \approx 40 \arcdeg .

The narrow emission lines are polarized in a stratified fashion , with the high ionization lines ( [ O III ] , [ Ne V ] , [ Fe VII ] ) being polarized 0.7 % –1.7 % and [ O II ] essentially unpolarized .
The line polarizations are positively correlated with critical density , ionization potential , and velocity width of the emission lines .
This indicates that , as is the case with the narrow-line radio galaxies , which also often contain powerful quasars , the narrow-emission line region may be partially shadowed by the putative torus , with the higher ionization lines originating closer to the nucleus .

One notable characteristic of the extranuclear knot is that all species of Fe are markedly absent in its spectrum , while they appear prominently in the nucleus .
In addition , narrow Mg II is observed to be much weaker than predicted by ionization models .
Our favored interpretation is that there is a large amount of dust in the extranuclear regions , allowing gaseous refractory metals to deposit .
Near the nucleus , dust is destroyed in the strong radiation field of the quasar , inhibiting metal depletion onto grains .
The extended emission regions are most likely material shredded from nearby cluster members and not gas condensed from the cooling flow or expelled from the obscured quasar .

The higher temperature inferred from [ O III ] lines compared to that from [ N II ] and the general better agreement with models of line ratios , especially [ O III ] \lambda 5007/ \lambda 4363 and He II/H \beta , provide strong evidence for matter-bounded clouds in addition to ionization-bounded clouds in the NLR .
Ionization by pure velocity shocks can be ruled out .
Shocks with photoionizing precursors may be present , but are probably not a dominant contributor to the energy input .