Seyfert (Sy) galaxies have commonly been classified into types 1 or 2 depending on whether their spectrophotometrically detectable permitted emission lines are much broader or of similar width respectively relative to the forbidden lines. The similarities and differences between Seyferts of the two types are commonly understood in the framework of the Unified Scheme (US) which hypothesizes the presence of a ubiquitous optically and geometrically thick torus around the central region; when the torus is edge-on, it obscures the central continuum and broad emission-line clouds seen in a Seyfert type 1, resulting in a Seyfert type 2.
Evidence in support of the US includes (i) the discovery of polarized broad permitted lines from Sy 2s (Antonucci & Miller, 1985; Moran 2000), (ii) the detection in Sy 2s of broad Paschen- lines, to which the torus is expected to be transparent (Veilleux et al.1997), and (iii) the ``double-cone" morphology of the narrow emission line structures, interpreted as due to shadowing of the central ionizing photons by the torus (Wilson, 1996). Challenges to the US remain, however, such as (i) Sy 1 host galaxies are of earlier Hubble-type than those of Sy 2s (Malkan et al.1998), (ii) Sy 2s have an excess of nearby companions over Sy 1s (Dultzin-Hacyan et al.1999), and (iii) the Sy 2s appear more likely to show compact radio emission than Sy 1s (Roy et al.1994).
In our study, we sought to improve upon the single-baseline interferometric investigation of Roy et al.(1994) by (a) performing higher resolution global VLBI imaging, and (b) rigorously selecting a sample to specifically test the US.