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

2.1.2. Far-IR Emission: Cold Dust and Galactic Extinction

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 [() ^- with ~ 1.5 to 2 in the range 20 µm; Draine & Lee 1984]. IRAS observations have established that the 100 µm dust emission in normal spirals comes from ~ 30 K dust grains with a total gas mass (HI + H2 + He) to dust mass ratio of 10³ (e.g. Devereux & Young 1990). This value is about an order of magnitude greater than that in the Milky Way (M_gas / M_dust ~ 170; cf Haas et al 1998b). The likely source of this discrepancy is that IRAS did not pick up the coldest dust that contains most of the mass (e.g. Xu & Helou 1996).

By extending the wavelength coverage to 200 µm, ISO has resolved this puzzle. ISOPHOT observations of a number of normal, inactive spirals have uncovered a dust component with typical temperatures ~ 20 K and a range between 10 K and 28 K (Alton et al 1998a, Davies et al 1999, Domingue et al 1999, Haas 1998, Haas et al 1998b, Hippelein et al 1996, Krügel et al 1998, Siebenmorgen et al 1999, Trewhella et al 1997, 1998, Tuffs et al 1996). With increasing activity, a second dust component (T ~ 30-40 K) becomes more prominent and also dominates the 60 µm and 100 µm IRAS bands (e.g. Siebenmorgen et al 1999). The cold dust has a larger radial extent than the stars (Alton et al 1998a, Davies et al 1999) and may be partially associated with extended HI disks.

Preliminary results from the 175 µm serendipity survey indicate that this is a general result for normal spirals (Stickel et al 1999). However, Domingue et al (1999) argue against any dominant part of the dust mass being much colder than 20 K, based on a direct comparison of visual and far-IR data in three overlapping galaxies. Particularly impressive is the case of M31 where Haas et al (1998b) presented a 175 µm map of the entire galaxy (Figure 2, see color insert). Across most of the disk the far-IR SED is still rising from 100 to 200 µm with a temperature of 16 ± 2 K (see also Odenwald et al 1998). The temperature of the cold dust component is consistent with theoretical predictions from the heating/cooling balance of equilibrium dust grains in the diffuse radiation field. The gas-to-dust ratio inferred for this cold dust component is very close to the galactic value in the Milky Way.

Figure 2. ISOPHOT 175 µm map of M31 (north up, east to the left). The emission is dominated by a 10 kpc ring, with numerous bright condensations and a fainter ring at 14 kpc (from Haas et al 1998b). The ratio of ring to nuclear emission is much greater than in the IRAS bands (Habing et al 1984).

An important consequence of the larger dust masses derived from the ISOPHOT data is that the corresponding dust extinctions in the optical become quite significant. The extinction-corrected morphologies of spirals can thus be quite different from the uncorrected ones (Trewhella et al 1997, Haas et al 1998b). For instance, by including extinction, M31 turns from an Sb into a ring galaxy (Figure 2) and NGC 6946 from an Sc into an Sb (Trewhella 1998). These findings qualitatively support earlier suggestions that spiral galaxies are significantly affected by dust (Disney et al 1989, Valentijn 1990).