ABSTRACT

We find that Sy 2 galaxies have compact radio cores much more often than Sy 1 galaxies. This result is inconsistent with the popular unified models which explain the difference between Sy 1 and Sy 2 in terms of orientation. We propose a mechanism which is consistent with the observations while still supporting the unified model.

INTRODUCTION

The orientation unification scheme has enjoyed considerable success in accounting for the difference between Sy 1 and Sy 2 galaxies. This scheme suggests that the apparent differences between Sy 1 and Sy 2 galaxies are due simply to our viewing angle. It invokes a dense dusty torus which obscures our view of the broad-line region (BLR) when our line of sight lies close to the plane of the torus. In this case, only the lines from the more distant narrow-line region (NLR) are visible, and the galaxy then appears as a Sy 2. Light from the BLR is visible only when viewed from within a cone centred on the polar axis of the dust torus, and the galaxy then appears as a Sy 1.

This model has received support from observations such as those of Miller and Goodrich (1990), who found that the polarized emission from some Sy 2 galaxies has broad lines characteristic of a Sy 1 galaxy. They suggest that the light scattered from dust or electrons above and below the torus enable us to see the BLR in scattered light. This scheme predicts that, since the dust in the torus should be optically thin at radio wavelengths, Sy 1 and Sy 2 galaxies should appear identical at radio wavelengths.