Annu. Rev. Astron. Astrophys. 2005. 43: xxx-xxx Copyright © 2005 by Annual Reviews. All rights reserved

2. DUST IN THE LOCAL UNIVERSE

A fraction of the stellar radiation produced in galaxies is absorbed by dust and re-radiated from mid-infrared to millimeter wavelengths. Understanding dust properties and the associated physics of the absorption and emission are thus essential. These determine the Spectral Energy Distribution (SED) of the galaxies.

2.1. Dust Particles

Small dust particles with sizes ranging from a nanometer to a fraction of micrometer are ubiquitous in the interstellar medium. They result from natural condensation in cool stellar atmospheres, supernovae, and the interstellar medium of the heavy elements produced by the nucleosynthesis in stars and released to the diffuse medium by late type stars and supernovae explosions. Interstellar grain models have been improved for 30 years in order to fit all observational constraints: elemental abundances of the heavy elements, UV, visible and infrared absorption and scattering properties, infrared emission, polarization properties of the absorbed and emitted light. The models include a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains 0.1 µm in diameter that can be coated with ices in dense clouds and/or organic residues (e.g., Désert et al. 1990; Li & Draine 2001). It is now widely accepted that the smallest carbonaceous grains are Polycyclic Aromatic Hydrocarbons (PAHs) that emit a substantial fraction of the energy in a set of features between 3 and 17 µm (3.3, 6.2, 7.7, 8.6, 11.3, 12.7, 16.3, 17 µm for the main ones) that used to be known as the UIB for Unidentified Infrared Bands. These features result from C-C and C-H stretching/bending vibrational bands excited by the absorption of a single UV or optical photon and are a good tracer of normal and moderately active star formation activity in spiral and irregular galaxies (e.g., Helou et al. 2000; Peeters et al. 2004). For radii a 50 Å, the carbonaceous grains are often assumed to have graphitic properties. The so-called very small grains of the interstellar medium are small enough to have very low heat capacity, so their temperature are significantly affected by single-photon absorption. In the diffuse ISM of our Galaxy, they dominate the infrared emission for wavelengths smaller than about 80 µm. At longer wavelengths, the infrared spectrum is dominated by the emission of the larger grains at their equilibrium temperatures. Considering the energy density of the radiation in a galactic disc like ours, the temperature of the larger grains is rather low; 15 to 25 K. For these grains, the far-infrared emissivity decreases roughly as the square of the wavelength. This in turn makes the temperature dependence on the radiation energy density u very weak (T u^1/6). For a galaxy like the Milky Way, the infrared part of the SED peaks at 170 µm whereas for an Ultra Luminous Infrared Galaxy (ULIRG) it peaks at about 60 µm: a factor 3 in temperature for a factor 10³ in energy density or luminosity. At long wavelengths in the submillimeter and millimeter, the intensity should increase like I ⁴.