4. THE SILICATE-GRAPHITE-PAHS INTERSTELLAR DUST MODEL

Various models have been proposed for interstellar dust (see Li & Greenberg 2003, Li 2004a, Draine 2004 for recent reviews). In general, these models fall into three broad categories: the silicate core-carbonaceous mantle model (Li & Greenberg 1997), the silicate-graphite-PAHs model (Li & Draine 2001b, Weingartner & Draine 2001a) and the composite model (Mathis 1996, Zubko, Dwek & Arendt 2004). In this review I will focus on the IR emission calculated from the silicate-graphite-PAHs model and refer those who are interested in a detailed comparison between different models to my recent review articles (Li 2004a, Li & Greenberg 2003).

The silicate-graphite-PAHs model, consisting of a mixture of amorphous silicate dust and carbonaceous dust - each with an extended size distribution ranging from molecules containing tens of atoms to large grains 1 µm in diameter, is a natural extension of the classical silicate-graphite model (Mathis, Rumpl, & Nordsieck 1977; Draine & Lee 1984). We assume that the carbonaceous grain population extends from grains with graphitic properties at radii a 50 Å, down to particles with PAH-like properties at very small sizes.

With the temperature (energy) probability distribution functions calculated for ultrasmall grains undergoing "temperature spikes" and equilibrium temperatures calculated for large grains illuminated by the local ISRF, the silicate-graphite-PAHs model with grain size distributions consistent with the observed R_V = 3.1 interstellar extinction (Weingartner & Draine 2001a), is able to reproduce the observed near-IR to submillimeter emission spectrum of the diffuse ISM, including the PAH emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm. This is demonstrated in Figure 4 and Figure 5 for the high-latitude "Cirrus" cloud and 2 regions in the Galactic plane (see Li & Draine 2001b for details).

Figure 4. Comparison of the model to the observed emission from the diffuse ISM at high galactic latitudes (| b| 25°). Curves labelled Bsil and Bcarb show emission from "big" (a 250 Å) silicate and carbonaceous grains; curves labelled Ssil and Scarb show emission from "small'" (a < 250 Å) silicate and carbonaceous grains (including PAHs). Triangles show the model spectrum (solid curve) convolved with the DIRBE filters. Observational data are from DIRBE (diamonds) and FIRAS (squares). Taken from Li & Draine (2001b).

Figure 5. Infrared emission from dust plus starlight for two regions in the Galactic plane: (a) the MIRS region (44° l 44°40', -0°40' b 0°), and (b) the NIRS region (47°30' l 48°, | b| 15'). The starlight intensity heating the dust has been taken to be twice the MMP ISRF. The solid curve shows the overall model spectrum; triangles show the model spectrum convolved with the DIRBE filters. DIRBE observations are shown as diamonds. For the MIRS field we show the IRTS MIRS 5-12 µm spectrum (thin solid line). For the NIRS field we show the IRTS NIRS 2.8-3.9 µm spectrum (thin solid line, also shown as cross-dotted curve in inset). Taken from Li & Draine (2001b).

The silicate-graphite-PAHs model, with size distributions consistent with the SMC Bar extinction curve (Weingartner & Draine 2001a), is also successful in reproducing the observed IR emission from the SMC (Li & Draine 2002c), as shown in Figure 6. The dust in the SMC is taken to be illuminated by a distribution of starlight intensities. Following Dale et al. (2001), we adopt a simple power-law function for the starlight intensity distribution. The SMC, with a low metallicity (~ 10% of solar) and a low dust-to-gas ratio (~ 10% of the Milky Way), has a very weak or no 2175 Å extinction hump in its extinction curves for most sightlines (see Section 2.1) and very weak 12 µm emission (see Fig. 6) which is generally attributed to PAHs, supporting the idea of PAHs as the carrier for the 2175 Å extinction hump. ¹⁷

Figure 6. Comparison of the model (solid line) to the observed emission from the SMC obtained by COBE/DIRBE (diamonds) and IRAS (squares) averaged over a 6.25 deg2 region including the optical bar and the Eastern Wing. Triangles show the model spectrum convolved with the DIRBE filters. Stellar radiation (dot-dashed line) dominates for 6 µm. Grains are illuminated by a range of radiation intensities dNH / dU U-1.8, 0.1 U 102.75, with NHtot 5.4 × 1021 cm-2. Taken from Li & Draine (2002c).

Very recently, the silicate-graphite-PAHs model has also been successfully applied to NGC7331, a ringed Sb galaxy. Using the same set of dust parameters determined for the Milky Way diffuse ISM (R_V = 3.1; Weingartner & Draine 2001a), as shown in Figure 7, this model fits the IR emission observed by the IRAC instrument at 3.6, 4.5, 5.8 and 8 µm and the MIPS instrument at 24, 70 and 160 µm aboard the Spitzer Space Telescope and the 450 and 850 µm SCUBA submillimeter emission observed by JCMT, both for the ring and inside star-forming region and for the galaxy as whole (see Regan et al. 2004 for details). The model also closely reproduces the observed 6.2, 7.7, 8.6, 11.3 and 12.7 µm PAH emission features (see Fig.2 of Smith et al. 2004).

Figure 7. IR emission and model fits to the NGC7331 ring (a) and the entire galaxy (b). The thick solid lines and triangles are the model-predicted fluxes, and the squares are the observed fluxes. The broken curves indicate the contributions of the different model components. Taken from Regan et al. (2004).