The space between the stars (interstellar space) of the Milky Way Galaxy and other galaxies is filled with gaseous ions, atoms, molecules (interstellar gas) and tiny dust grains (interstellar dust). The first direct evidence which pointed to the existence of interstellar gas came from the ground-based detection of Na and Ca+ optical absorption lines (Hartmann 1904). 1 This did not gain wide acceptance until Struve (1929) showed that the strength of the Ca+ K-line was correlated with the distance of the star. The true interstellar nature of this gas was further supported by the detection of the first interstellar molecules CH, CH+ and CN (Swings & Rosenfeld 1937, McKellar 1940, Douglas & Herzberg 1941). The presence of dark, obscuring matter in the Milky Way Galaxy was also recognized at the beginning of the 20th century (e.g. see Barnard 1919). Trumpler (1930) first convincingly showed that this "Obscuring Matter" which dims and reddens starlight consists of small solid dust grains. The dust and gas are generally well mixed in the interstellar medium (ISM), as demonstrated observationally by the reasonably uniform correlation in the diffuse ISM between the hydrogen column density NH and the dust extinction color excess or reddening E(B - V) AB - AV: NH / E(B - V) 5.8 × 1021 mag-1 cm-2 (Bohlin et al. 1978), where AB and AV are the extinction at the B ( = 4400 Å) and V ( = 5500 Å) wavelength bands. From this correlation one can estimate the gas-to-dust ratio to be ~ 210 in the diffuse ISM. 2 Despite its tiny contribution to the total mass of a galaxy, 3 interstellar dust has a dramatic effect on the physical conditions and processes taking place within the universe, in particular, the evolution of galaxies and the formation of stars and planetary systems (see the introduction section of Li & Greenberg 2003).
In this review I will concentrate on the radiative properties of interstellar dust. I will distinguish the dust in terms of 3 components: cold dust with steady-state temperatures of 15 K T 25 K in thermal equilibrium with the solar neighbourhood interstellar radiation field, very cold dust with T < 10 K (typically T ~ 5 K), and warm dust undergoing "temperature spikes" via single-photon heating. I will first summarize in Section 2 the observational constraints on the physical and chemical properties of interstellar dust. I will then discuss in Section 3 the heating and cooling properties of the warm and cold interstellar dust components. In Section 4 I will demonstrate the robustness of the silicate-graphite-PAHs interstellar grain model by showing that this model closely reproduces the observed infrared (IR) emission of the Milky Way, the Small Magellanic Cloud, and the ringed Sb galaxy NGC7331. The possible existence of a population of very cold dust (with T < 10 K) in interstellar space will be discussed in Section 5. Whether ultrasmall grains can really cool down to T < 2.7 K and appear in absorption against the cosmic microwave background (CMB) radiation will be discussed in Section 6.