Spectropolarimetry of a complete infrared-selected sample of Seyfert 2 galaxies

Abstract

We report the results of a spectropolarimetric survey of a complete far-infrared-selected sample of Seyfert 2 galaxies. We have found polarized broad Ha emission in one new source, NGC 5995. In the sample as a whole, there is a clear tendency for galaxies in which we have detected broad Ha in polarized light to have warm mid - far-infrared colours (F60 μm/F25 μm ≲ 4), in agreement with our previous results. However, a comparison of the optical, radio and hard X-ray properties of these systems leads us to conclude that this is a secondary consequence of the true mechanism governing our ability to see scattered light from the broad-line region. We find a strong trend for galaxies showing such emission to lie above a critical value of the relative luminosity of the active core to the host galaxy (as measured from the [O III] 5007-Å equivalent width) which varies as a function of the obscuring column density as measured from hard X-ray observations. The warmth of the infrared colours is then largely due to a combination of the luminosity of the active core, the obscuring column and the relative importance of the host galaxy in powering the far-infrared emission, and not solely orientation as we inferred in our previous paper. Our data may also provide an explanation as to why the most highly polarized galaxies, which appear to have tori that are largely edge-on, are also the most luminous and have the most easily detectable scattered broad Hα.