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

2.2. Extinction

In our Galaxy, the extinction curve of the diffuse ISM has been known for a few decades. The average optical depth perpendicular to the disk of our Galaxy in the solar vicinity is small (A_V 0.2) and typical of spiral galaxies. The average optical depth increases to a few in large molecular cloud complexes. It can become very large in galactic nuclei. Finally it should also be remembered that the optical depth in the UV is typically 10 times larger than that in optical wavelengths. The conversion of star light into infrared radiation will thus depend strongly on the location of the stars and their spectral types.

In external galaxies, modeling the extinction is very hard because it strongly depends on the geometric distribution of the ISM and of the chemical abundances. Simple models have been used to take this into account to first order. Galaxies can be modeled as an oblate ellipsoid where absorbers (dust) and sources (stars) are homogeneously mixed; the dust absorption can be computed in a "screen" or "sandwich" geometry (dust layers in front of the stars or sandwiched between two star layers). As a consequence, the reddening curve average over a whole galaxy appears to vary within a class of objects and between the different classes, from normal star-forming galaxies to highly concentrated starburst. It is thus very difficult to derive the total dust optical depth (e.g., Calzetti et al. 1994). In the local Universe, the average extinction per galaxy is quite low. About one third of the bolometric flux is emitted in the far-infrared, and this is typical of our Galaxy. In more actively star-forming galaxies, up to 70% of the bolometric flux is emitted in the far-infrared. In some of these, the starburst activity is mostly in the disk (like in M51). For a given total luminosity, the radiation energy density is lower than in the case of a starburst concentrated in a small volume in the nucleus. In this case, the dust will be hotter due to the larger energy density and the conversion of stellar light to infrared will be more efficient. Some ULIRGs emit more than 95% of their energy in the far-infrared (e.g., Arp 220). Such galaxies are very compact, dusty starbursts where dust optical depths are very large. In such galaxies, fine structure and recombination line ratios imply an equivalent "screen" dust extinction between A_v ~ 5 and 50. The result is that the SED is significantly distorted in the opposite way from the higher dust temperature (less mid-infrared emission). In the following, we will refer to "infrared galaxies" and to "optical galaxies" to designate galaxies in which the infrared emission, respectively optical emission dominates. Different typical spectra of galaxies are shown in Figure 1 from the UV to the millimeter. We clearly see the variation of the optical to infrared energy ratio as starburst activity increases.

Figure 1. Spectral energy distributions of galaxies from UV to the millimeter. The ULIRG is observed at redshift z = 0.66 and is represented here in the rest-frame (from Galliano 2004).