3.2. IR continuum
Spectroscopic observations from ISO (Clavel et al. 1998) show a stronger near-mid IR continuum in Sy1 than in Sy2 galaxies causing a lower observed equivalent width 7.7 µm feature in Sy1s. Orientation dependent near-IR emission is also clear from comparison of Sy2s with and without hidden broad line regions (Heisler et al. 1997). Further comparison with Sy1s and quasars (Figure 4, Kuraszkiewicz et al. 2003) indicate obscuring column densities, log NH ~ 23, as opposed to ~ 24 in earlier torus models (Pier & Krolik 1992). Lower column density (Kuraszkiewicz et al. 2003) or clumpy (Zier & Biermann 2002, Nenkova et al. 2002) models for the obscuring material reproduce the full range of temperatures in AGN IR continua with orientation primarily responsible for the range in IR SED shapes. This removes the need for a starburst-related component (Kuraszkiewicz et al. 2003), although a two-component model is required in some low-redshift AGN (Genzel & Cesarsky 2000).
Figure 4. Left: Mid-IR colors for quasars compared with Seyfert 2 galaxies (Heisler et al. 1997) showing that redenning of up to log NH ~ 23 can explain the progression (Kuraszkiewicz et al. 2003, Fig.11). Right: The effects of host galaxy emission on the mid-IR colors of low luminosity AGN (Lumsden & Alexander 2001).
An alternative suggestion invokes evolution to explain at least part of the range IR continuum shapes (Haas et al. 2003). In this scenario (Sanders et al. 1988) IR galaxies are young quasars/AGN where the central engine is obscured by dust. The SED then changes from a young state where the far-IR continuum dominates, through an AGN state in which the IR continuum is hotter and the Big Blue Bump appears and on to a dead quasar where the latter component disappears. Although such a model can, qualitatively, explain the range of SEDs observed in the PG sample (Haas et al. 2003), there is no definitive evidence connecting SED shape to the evolution/age of a source.