1.5.1 Normal Galaxies

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, 1998), which confirms a long-standing hypothesis derived from ground-based and IRAS data (Sellgren, 1984; Beichman, 1987). The clusters of particles are 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 centred at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 $\mu$m. These unidentified infrared bands (UIBs) are thought to result from C-C and C-H stretching/bending vibrational bands in aromatic hydrocarbons. The most popular model for the carrier of the bands 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 $\mu$m, accompanied by strong fine structure line emission, in particular [NeII] and [NeIII]. This continuum component is characteristic of active star forming regions. Desert (1990) attributed this continuum to very small grains (VSG) of size $\leq 10$ nm.

The transition from stellar emission to interstellar dust emission in galaxies occurs in the mid-IR band and depends on the star forming activity. Three main components of dust emission contribute to the mid-IR spectra of galaxies. The first is the UIB dominated, mid-IR SED of galaxies. The SED and ratios of the different UIB features remain constant in galaxies with a wide range of radiation fields and properties. The second components, present only in some galaxies or regions of galaxies, is the steeply rising VSG continuum longward of 10 $\mu$m discussed above. It is characteristic of intense star forming regions. The third is near-radiation-equilibrium emission from hot dust particles (150 to 1700 K). This near-IR/mid-IR bump is characteristic of dusty tori in AGN. A 3-5 $\mu$m continuum component may also come from a fluctuating dust component without PAH features (Helou, 1999).

UIB emission a is good tracer of normal and moderately active star formation activity in spiral and irregular galaxies (Vigroux, 1999; Helou, 1999). In such systems the luminosity in the ISOCAM LW2 filter (containing the 6.2, 7.7 and 8.6 $\mu$m UIB features) is well correlated with the longer-wavelength mid-IR LW3 filter at 15 $\mu$m. Both correlate with the far-IR and the H$\alpha$ line luminosities (Metcalfe, 1996; Vigroux, 1999; Roussel, 1999; Rouan, 1996). In normal galaxies, with low activity, the excitation of the UIB features by other than UV photons (e.g., optical photons) may play an important role (Pagani, 1999). At higher luminosities and activity, the $\lambda \geq 10$ $\mu$m measured in the LW3 filter increases relative to the UIB emission. At radiation fields $\geq10$ times the local radiation field, such as in the central parsec of the Galaxy or in active galactic nuclei, the UIB emission strength goes down due to the destruction of its carriers. As a result, the LW3/LW2 ratio is an interesting diagnostic of the radiation environment, as illustrated by Figure 1.8.

Figure 1.8: ISOCAM/IRAS Colour-Colour Diagram for Different Types of Galaxies. The decrease in the LW2/LW3 ratio 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 band). From Genzel (2000).

Figures 1.9 and 1.10 present synthetic spectra of the mid-IR emission of galactic components between 1 and 20 $\mu$m. Figure 1.9 shows a spectrum of a starburst galaxy, taken from Moorwood (1996) for the 1 to 5 $\mu$m part and from Tran (1998) for the 5 to 18 $\mu$m part. Unidentified infrared bands (UIB) and ionization lines of Neon are clearly visible, as well as a hot dust continuum and silicate absorption. In this case, the derived temperature for the hot dust is about 200 K, using the model from Desert (1990), and the extinction due to dust is about 3 mag, representative of a starburst region in a galaxy. The dotted line is a spectrum dominated by very hot dust heated at 1000 K, that could be typical of the mid-IR emission in the central core of an AGN, as observed by Aussel (1998). Figure 1.10 depicts emission dominated by old population stars as modelled by Bressan (1998), and is thus relevant for early-types galaxies.

Figure 1.9: Mid-IR Emission of a Starburst Galaxy and of an AGN, compared with ISOCAM LW2 and LW3 Filters Bandpasses at Various Redshifts. Solid line: total mid-IR emission of a starburst galaxy. Dashed line: contribution to the mid-IR emission of the hot dust. This dust is formed by very small grains (VSG) (Desert, 1990) heated to a few hundred degrees. Dotted-dashed line: Possible contribution of a continuum associated to the carriers of UIB. Dotted line: Hot dust (1000 K) mid-IR emission of an AGN central part, as observed by Aussel (1998).

The mid-IR colours of galaxies are not simple, because the observed emission of a galaxy is a mixture of all the components shown on Figures 1.9 and 1.10. According to the type of the object (early-type, AGN, starburst or normal galaxy), it is expected that a given type of emission will dominate (e.g. stellar, very hot dust or UIB). This is not always the case, as shown by Madden (1999) on a sample of elliptical galaxies: most exhibit stellar colours in the mid-IR but some present mid-IR spectral energy distributions (SED) typical of UIB emission. Thus, the relative fluxes in Figures 1.9 and 1.10 are not to be compared directly.

Figure 1.10: Mid-IR Emission of an Old Stellar Population, compared with ISOCAM LW2 and LW3 Filters Bandpasses at Various Redshifts. Solid line: model of the mid-IR emission of a 5 Gyr old stellar simple population, where dust envelopes of Mira and OH/IR stars are neglected (Bressan, 1998). Dashed line: model of the mid-IR emission of a 5 Gyr old stellar simple population, for which the dust envelopes have been taken into account for the AGB phase, using silicate grains (Bressan, 1998). The arbitrary units used for fluxes cannot be compared with those of Fig. 1.9, the relative importance of these emissions in various galaxies varies by factor of order 100 (Aussel, 1999).

In the rest frame, the LW2 filter is dominated by UIB features while the emission in the LW3 filter is due to the hot dust continuum, Neon ionization lines and the 12.7 $\mu$m UIB plus the associated continuum. This picture changes dramatically when the redshift rises towards $z=1$: the contribution of the stellar continuum, especially from old population stars, overtakes UIB features in the LW2 band, which in turn are shifted to the LW3 band, together with the silicate absorption band at 9.7 $\mu$m. This indicates that in order to interpret the mid-IR observations and disentangle the various contributions, it the help of data at other (e.g. optical and near-IR) wavelengths is necessary.

In the far-IR, the emitted spectrum results from the radiation equilibrium between absorbed short-wavelength radiation and grey-body emission with a wavelength dependent emissivity. ISOPHOT observations of a number of normal, inactive spirals have uncovered a dust component with typical temperatures $\sim 20$K. With increasing activity, a second dust component ( $T_D\sim 30-40$K) becomes more prominent and also dominates the 60 and 100 $\mu$m bands (e.g. Siebenmorgen, 1999). The cold dust has a larger radial extent than the stars and may be partially associated with extended HI disks. Preliminary results from the 175 $\mu$m serendipity survey indicate that this is a general result for normal spirals (Stickel, 1999).