Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved

3.3. Seyfert Galaxies and QSOs

3.3.1. UIB Features in Seyfert Galaxies: Confirmation of Unified Models

Clavel et al (1998) have used the UIB features as a new tool for testing unified schemes ⁷ in Seyfert 1 and 2 galaxies. The basic idea is that hot dust at the inner edge of the postulated circumnuclear dust/gas torus surrounding the central AGN (Antonucci 1993) re-emits the absorbed energy from the central AGN as a thermal continuum in the near- and mid-IR. In the torus models of Pier & Krolik (1992, 1993), Granato & Danese (1994), the torus is optically thick even at mid-infrared wavelengths. The hot dust emission is predicted to be much stronger in Seyfert 1 than in Seyfert 2 galaxies, because the mid-IR continuum is angle dependent as a result of the torus blockage/shadow. The fact that the infrared energy distributions of AGNs (Seyferts and QSOs) are fairly broad and the silicate absorption/emission is relatively weak in most of the objects in the Clavel et al sample (Figure 9) argues against very compact and very thick tori. The data are in better agreement with moderately thick and extended tori, or clumpy disk configurations, as was pointed out before the ISO mission by Granato et al (1997), Efstathiou & Rowan-Robinson (1995).

Figure 9. Average ISOPHOT-S spectra of 20 Seyfert 1 galaxies (top) and 23 Seyfert 2 galaxies (bottom) from the CFA sample (Clavel et al 1998; Schulz & Clavel, private communication). The wavelengths of the main UIB features and of the 9.7 µm silicate feature are marked by arrows. UIB features are relatively more prominent in Sey 2s than in Sey 1s, but that is mostly because of the stronger red continuum in Sey 1s. Silicate feature emission and absorption are weak.

Clavel et al carried out ISOPHOT-S spectrophotometry of a sample of 20 Seyfert 1-1.5 (< z > = 0.036) and 23 Seyfert 1.8-2 (< z > = 0.017) galaxies drawn from the CFA sample (Huchra & Burg 1992). Their main finding is that the equivalent width of the 7.7 µm UIB feature is much stronger in Seyfert 2 galaxies than in Seyfert 1 galaxies (Figure 9). The median 7.7 µm UIB strength ⁵ of Seyfert 2 galaxies is ~ 4 times larger than for Seyfert 1s (7.7 µm strength = 1.7 ± 0.37 vs. 0.4 ± 0.13). This difference is not a result of Seyfert 1s having weaker UIB emission but of their having a much stronger (red) continuum emission than Seyfert 2s (Figure 9). The distributions of UIB luminosities (and the ratios of UIB to far-IR luminosities) of the two Seyfert classes are similar (Figure 10). The UIB/far-IR ratio in Seyferts is also very similar to that in starburst galaxies (Figure 10).

Figure 10. Fraction of sources as a function of ratio of 7.7 µm UIB luminosity to 60 + 100 µm-band IRAS luminosity for ULIRGs (filled circles and thin continuous line), starbursts (open circles and thick line), Seyfert 2s (open rectangles and thick dashed) and Seyfert 1s (filled rectangles and thin long dashes) (data from Genzel et al 1998, Rigopoulou et al 1999, Clavel et al 1998, Schulz & Clavel, private communication).

The findings of Clavel et al (1998) agree with earlier ground-based, mid-IR photometry of several Seyfert samples (Maiolino et al 1995, Giuricin et al 1995, Heckman 1995). The results are in excellent agreement with the unified torus model discussed previously if the UIB emission is orientation independent. Spatially resolved ISOCAM CVF imaging in three nearby active galaxies (NGC 1068, Circinus and Cen A) shows that the UIB emission comes from kpc-scale star-forming regions in the disk, and not from the nucleus (Laurent et al 2000, Moorwood 1999). The similar UIB/far-IR ratios in Seyfert 1s, Seyfert 2s, and starburst galaxies (Figure 10) suggest that the UIB luminosity (and the far-IR continuum) in Seyfert (and other) galaxies is proportional to the sum of star formation activity and emission from the diffuse ISM (Section 3.3.2).

⁷ In unified models (e.g. Antonucci 1993) different types of AGNs are postulated to all harbor a central, accreting massive black hole surrounded by a broad line region. The AGN is proposed to be surrounded by an optically and geometrically thick gas/dust torus that anisotropically absorbs/shadows emission from the nuclear region. Broad-line Seyfert 1 galaxies (or QSOs) in this scheme are AGNs where the line of sight to the nucleus is not blocked. In narrow-line, Seyfert 2 galaxies (or radio galaxies) the line of sight to the nucleus is blocked by the intervening torus. Back.