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

3. ACTIVE GALAXIES

The 2-200 µm band accessible to the ISO spectrometers (6.5 octaves!) contains a plethora of atomic, ionic, and molecular spectral lines spanning a wide range of excitation potential, along with various solid-state features from dust grains of different sizes (Section 2.1.1, Figure 3). These lines sample widely different excitation/ionization states and are characteristic tracers of different physical regions: photodissociation regions (PDRs ⁴), shocks, X-ray excited gas, HII regions photoionized by OB stars, and coronal gas photoionized by a hard central AGN source (see Genzel 1992 for a review).

Figure 3. Combined LWS+SWS spectra of galaxies (> 6 octaves). Left: Combined SWS/LWS spectrum of the Circinus galaxy (Moorwood 1999, Sturm et al 1999b). The H2 lines and low-excitation atomic/ionic fine structure lines ([FeII], [SiII], [OI], [CII]) sample photodissociation regions (PDRs4; Sternberg & Dalgarno 1995, Hollenbach & Tielens 1997), shocks (Draine et al 1983, Hollenbach & McKee 1989), or X-ray excited gas (Maloney et al 1996). Hydrogen recombination lines and low-lying ionic fine structure lines (excitation potential < 50 eV: [ArIII], [NeII], [NeIII], [SIII], [OIII], [NII]) sample mainly HII regions photoionized by OB stars (Spinoglio & Malkan 1992, Voit 1992), although ionizing shocks may contribute in some sources (e.g. Contini & Viegas 1992, Sutherland et al 1993). Ionic lines from species with excitation potentials up to ~ 300 eV (e.g. [OIV], [NeV], [NeVI], [SiIX]) probe highly ionized coronal gas and require very hard radiation fields (such as the accretion disks of AGNs) or fast ionizing shocks. Line ratios give information about the physical characteristics of the emitting gas. Extinction corrections are small (A() / A(V) ~ 0.1 to 0.01 in the 2-40 µm region). Right: The starburst galaxy M82 (top): low excitation lines, strong UIBs/PAHs; and the AGN NGC 1068 (bottom): high excitation, no UIBs/PAHs (Sturm et al 1999b, Colbert et al 1999, Spinoglio et al 1999). Sudden breaks in the SEDs are the result of different aperture sizes at different wavelengths. Local bumps and unusual slopes in the Circinus spectrum (12-20 µm and 35 µm) may be caused by residual calibration uncertainties.

As an example of the rich spectra that were obtained with ISO, we show in Figure 3 the combined SWS/LWS spectrum of the Circinus galaxy, which displays the entire set of spectral characteristics just discussed. Circinus is the closest Seyfert 2 galaxy (D = 4 Mpc) and has an active circumnuclear starburst (Moorwood et al 1996, Moorwood 1999, Sturm et al 1999b).

⁴ PDRs are the origin of much of the infrared radiation from the interstellar medium (ISM). PDRs are created when far-UV radiation impinges on (dense) neutral interstellar (or circumstellar) clouds and ionizes/photodisscoiates atoms and molecules. The incident UV (star) light is absorbed by dust grains and large carbon molecules (such as PAHs) and is converted into infrared continuum and UIB features. As much as 0.1-1% of the absorbed starlight is converted to gas heating via photoelectric ejection of electrons from grains or UIBs (Hollenbach & Tielens 1997, Kaufman et al 1999). Back.