4.2.1. The Aromatic Features
IRAS data had already indicated (Beichman 1987; Puget & Léger 1989; Boulanger & Cox in this volume) that the mid-infrared emission from the ISM was dominated by small fluctutating grains and Aromatic Features in Emission (AFE). ISO has not only established into fact what had been hypothesis, but is also allowing us to address quantitatively the mid-infrared energy budget across various emission components, and to investigate the variation of this budget from galaxy to galaxy. ISO data are generally consistent with older data from ground observations, including early M82 spectra by Willner et al. (1977), ground-based surveys (Roche et al. 1991), and IRAS-LRS data (Cohen & Volk 1989).
The AFE appear in two main groups, one stretching from 5.5 to 9 µm, with peaks at 6.2, 7.7 and 8.6, and the other one starting at 11 µm and extending to 12.5 µm (Figure 4; Lu et al. 1999; Helou et al. 2000). The shape and relative strengths of the features are quite similar to ``Type A sources'' which are the most common non-stellar objects in the Milky Way: reflection nebulae, planetary nebulae, molecular clouds, diffuse atomic clouds, and HII region surroundings (Geballe 1997, Tokunaga 1997, and references therein). Quantitatively similar spectra have been reported from spectroscopic observations with PHT-S, ISO-CAM CVF or ISO-SWS on a variety of Galactic sources (Roelfsema et al. 1996; Verstraete et al. 1996; Césarsky et al. 1996a; 1996b; Boulanger et al. 1996; Mattila et al. 1996; Beintema et al. 1996; Uchida, Sellgren & Werner 1998) and a number of galaxies (Boulade et al. 1996; Vigroux et al. 1996; Acosta-Pulido et al. 1996; Metcalfe et al. 1996). ISO-SWS spectra with greater spectral resolution show AFEs with the same shape, a clear indication that they are spectrally resolved by PHT-S data at a resolution of ~ 20.
Figure 4. A composite mid-infrared spectrum obtained from a straight average of a set of PHT-S spectra of 28 galaxies (Helou et al. 1999). Error bars indicate the dispersion among the avaraged spectra when they are all normalized to the flux integral between 6 and 6.6 µm. Note the ordinate is the flux density per frequency interval.
There is good evidence linking the AFE to Polycyclic Aromatic Hydrocarbons (PAH), but no rigorous spectral identification of specific molecules (Tielens 1999; Puget & Léger 1989; Allamandola et al. 1989). It is generally agreed that the emitters are small structures, no more than a few hundred atoms, transiently excited to high energy levels by single photons. The relative fluxes in individual AFE, and the general shape of the specturm, depend very weakly on galaxy parameters such as the far-infrared colors (Figure 5). This is direct evidence that the emitting particles are not in thermal equilibrium. As an estimate of the AFE relative strength, the integrals from 5.8 to 6.6 µm, 7.2 to 8.2 µm, and 8.2 to 9.3 µm are in the ratio 1:2:1 in the PHT-S spectra obtained under the Key Project on Normal Galaxies (Helou et al. 2000). The strongest detectable variation in the same data set is a slightly stronger 11.3 µm AFE in the colder galaxies (Lu et al. 1999). Since this feature is linked primarily to neutral PAHs as opposed to the shorter wavelength features which have a strong ionized PAH contribution (Tielens, 1999), one is tempted to interpret this trend as a result of the more active galaxies having a greater contribution to their luminosity originating in regions heated by harder radiation fields.
Figure 5. To illustrate the very stable spectral shapes in the PHT-S data, Lu et al. (2000) co-added eleven FIR-cold galaxies (R(60, 100) < 0.4, solid line) and nine FIR-warm galaxies (0.6 < R(60, 100) > 0.9, dashed line) separately, and found very little difference between the two resulting class averages. The only significant difference is a slightly stronger 11.3 µm feature in the cold galaxies with respect to the 6-9 µm features. The significance of this difference is discussed in the text.
An important consequence of the invariant shape of the spectrum up to 11 µm,, even as the infrared-to-blue ratio reaches high values, is that the 10 µm, trough is best interpreted as a gap between AFE rather than a silicate absorption feature. An absorption feature would become more pronounced in galaxies with larger infrared-to-blue ratios, and that is not observed (Sturm et al. 2000).
The fraction of starlight processed through AFE has been under debate since the IRAS mission (Helou, Ryter & Soifer 1991), and can now be directly estimated using the new ISO data for the sample described above. AFE account for about 65% of the total power within the 3 to 13 µm range, and about 90% of the total power in the 6 to 13 µm range. The AFE between 6 and 13 µm carry 25 to 30% of L(FIR) in quiescent galaxies, or 12% of the total infrared dust luminosity between 3 µm and 1 mm, whereas all ISM emission at < 13 µm comes up to ~18% of the total dust emission. The ratio AFE-to-FIR gradually drops to less than 10% in the most actively star forming galaxies, i.e. those with the greatest L(IR) / L(B) ratio or IRAS color R(60/100), following the trend already noted in Helou, Ryter & Soifer (1991). Boselli et al. (1997 and 1998) have interpreted similar trends as evidence for the destruction of AFE carriers in more intense radiation fields. The 3.3 µm, feature carries about 0.5% of the total AFE luminosity longwards of 5 µm,, a significantly smaller value than that reported by Willner et al. (1982) for M 82.