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3.1 Mid-Infrared Spectroscopy

Spectroscopic data on normal galaxies in the mid-infrared range were acquired with ISO-PHOT from 2.5 to 5 and from 5.7 to 11.6µm (Lemke et al. 1996); with ISO-CAM in the CVF mode from 5 to 16.5 (Césarsky et al. 1996); and to some extent with SWS from 2.5 to 45µm (de Graauw et al. 1996).

The interstellar medium emission from galaxies powered by star formation is strongly dominated by AFE (Figure 1). These appear in two main groups, one stretching from 5.5 to 9µm, with peaks at 6.3, 7.7 and 8.6, and the other one starting at 11µm and extending to 12.5µm (Helou et al. 1999). There is good evidence linking these features to Polycyclic Aromatic Hydrocarbons (PAH), but no rigorous spectral identification of specific molecules (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.

Figure 1

Figure 1. Two composite mid-infrared spectra, one for twelve FIR-cold galaxies (squares), and one for twelve FIR-warm ones (connected dots). Spectra were averaged after normalization to the 6.3µm peak.

The relative fluxes in individual features depend very weakly on galaxy parameters such as the far-infrared colors, direct evidence that the emitting particles are not in thermal equilibrium. Figure 1 shows that the difference between the galaxies coldest and warmest in the far infrared is very small in relative terms. In addition to the AFE, there is a continuum component with a slope close to zero in fnu vs. nu between 3 and 5µm, and whose extrapolation to longer wavelengths would place it below the observed flux density even at 9-10µm. This continuum component is almost certainly of non-stellar origin, and is probably due to fluctuating grains without aromatic features.

ISO-CAM CVF studies targeting the shape of the continuum emission and ionic fine-structure lines between 5 and 16.5µm are turning out to be powerful diagnostics of the radiation field in the disks of nearby galaxies, allowing us to disentangle the variations in heating intensity and hardness of interstellar radiation (Tran 1998; Contursi 1998).

The spectra reported here typify the integrated emission from the interstellar medium of the majority of star-forming galaxies, and could thus be used as a template to obtain redshifts of highly extincted galaxies with SIRTF. For instance, a galaxy at a redshift z = 3 with a flux density average of 0.5 mJy in the range 19-27µm and a total infrared luminosity comparable to Mkn 231 at ~3 x 1012 Lsun would be detected by SIRTF's IRS (Infrared Spectrometer; Roellig et al. 1998) in roughly 1000 seconds of integration (Weedman 1998).

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