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

2. NORMAL GALAXIES

2.1. Infrared Emission from Normal Galaxies

2.1.1. Mid-Infrared Emission: Unidentified Bands and Very Small Dust Grains

ISO studies of our Galaxy have demonstrated that the bulk of the mid-IR emission from the interstellar medium results from transient heating of small clusters of particles (Boulanger et al 1998), which confirms a long-standing hypothesis derived from ground-based and IRAS data (Sellgren 1984, Beichman 1987). The clusters are stochastically heated by single photons and as a result exhibit large temperature fluctuations. The resulting mid-IR flux is simply proportional to the underlying radiation field intensity. The spectra are surprisingly regular, exhibiting almost invariably a family of features centered at 3.3, 6.2, 7.7, 8.6, 11.3, and 12.7 µm (cf Allamandola et al 1995, D Cesarsky et al 1996a, b, Verstraete et al 1996, Mattila et al 1996, 1999, Onaka et al 1996). These unidentified infrared bands (UIBs) are thought to result from C-C and C-H stretching/bending vibrational bands in aromatic hydrocarbons (Puget & Leger 1989, Duley & Williams 1991, Tielens et al 1999). The actual carrier of the bands is still uncertain. One possibility is that it consists of large, carbon-rich ring molecules [e.g. polycyclic aromatic hydrocarbons (PAHs), size a few nm]. Alternatively, the carrier may be very small, amorphous carbon dust grains that are exposed to moderately intense UV (and possibly visible) radiation (Boulade et al 1996, Uchida et al 1998, Pagani et al 1999). The most popular model is the PAH interpretation, but no rigorous identification with specific molecules has yet been established. The second component of the interstellar mid-IR emission manifests itself as a steeply rising continuum longward of 10 µm, accompanied by strong fine structure line emission, in particular [NeII] and [NeIII]. This continuum component is characteristic of active star-forming regions, such as the Galactic HII region M17 (Verstraete et al 1996, D Cesarsky et al 1996b). Desert et al (1990) attributed this continuum to very small fluctuating grains (VSGs: size 10 nm); this hypothesis is consistent with the ISOCAM CVF spectral maps obtained around M17 and the photometry of NGC 7023 (Tran 1998, Laureijs et al 1996).

The transition from stellar emission to interstellar dust emission in galaxies occurs in the mid-IR (3-30 µm) band and depends on the star-forming activity. This is well demonstrated by a detailed ISOCAM study of ~ 10² Virgo cluster galaxies (Boselli et al 1997, 1999). Three main components of dust emission contribute to the mid-IR spectra of galaxies. The first is a UIB-dominated, mid-IR spectral energy distribution (SED) up to 13 microns. The SED and the ratios of the different UIB features remain constant in galaxies with a wide range of radiation fields and properties (Helou 1999, Helou et al 1999). The second component, present only in some galaxies or regions of galaxies, is the steeply rising (VSG) continuum longwards of 10 microns discussed above. It is characteristic of intense star-forming regions. The third is near-radiation equilibrium emission from hot (150 to 1700 K) dust particles. This near-IR/mid-IR "bump" is characteristic of dust tori in AGNs (Section 3.3.1). A 3-5 µm continuum component may also come from a fluctuating dust component without PAH features (Helou et al 1999).

UIB emission is a good tracer of normal and moderately active star formation activity in spiral and irregular galaxies (Helou 1999, Vigroux et al 1999). In such systems the luminosity in the ISOCAM LW2 filter (containing the 6.2, 7.7, and 8.6 µm UIB features) is well correlated with the longer wavelength mid-IR (LW3 filter: 15 µm; Figure 1). Both correlate with the far-IR and the H line luminosities (Rouan et al 1996, Metcalfe et al 1996, Smith 1998, Vigroux et al 1999, Roussel et al 1999a). In normal galaxies with low activity, as in NGC 891, NGC 7331, M31, or in parts of the LMC, the mid-IR emission mainly traces the distribution of (molecular + atomic) gas that is bathed in the diffuse radiation field (Mattila et al 1999, Smith 1998, Pagani et al 1999, Contursi et al 1998). Under these conditions excitation of the UIB features other than by UV photons (e.g. visible photons) may play an important role (Pagani et al 1999). At higher luminosities and activity, however, the 10 µm continuum (from warm dust in PDRs and HII regions) measured in the LW3 filter increases relative to the UIB emission (Figure 1; Vigroux et al 1999). At radiation fields 10⁵ times the local interstellar radiation field, such as in the central parsec of the Galaxy (Lutz et al 1996b) or the M17 HII region (Verstraete et al 1996), in active galactic nuclei (see Section 3.1), or in metal-poor environments (such as the metal-poor blue compact dwarf galaxy SBS0335-052; Thuan et al 1999), the UIB emission strength plummets, presumably because of the destruction of its carriers. As a result, the LW3/LW2 ratio is an interesting diagnostic of the radiation environment (Figure 1). This ratio decreases outward from the nuclei in disk galaxies (Dale et al 2000).

Figure 1. ISOCAM/IRAS color diagram for different types of galaxies. The decrease in LW2 / LW3 ratio for 60 / 100 flux ratios > 0.5 (AGN/starbursts and dwarf galaxies) can be explained by a combination of the destruction of the UIB features (LW2 band) in the intense radiation field and increased emission from (very small) warm grains (LW3) (from Vigroux et al 1999).