Structure in the Distribution of the Dust and Its Impact on Extragalactic Studies
The apparent morphology of galaxies (and therefore their classification scheme) appears to be much influenced by the presence of dust extinction, as indicated by near infrared (NIR) observations (see e.g. Block et al., 1994). Further, in many studies of galaxies, it is critical to determine the intrinsic (i.e. unextinguished) spectral energy distribution emerging from the whole system. Indeed, the latter carries essential information about the old and young stellar populations and hence about the star formation activity. Since internal extinction may induce a very substantial attenuation, it is important to correct the observed energy distribution in an appropriate way. The so-called “uniform screen” model has long been employed to this purpose. In the latter, extinction corrections can be made as for nearby stars extinguished by interstellar clouds. Indeed, in both cases the observed flux (or brightness) writes F obs = F 0 e −τ . As a consequence, a simple relation exists between reddening and extinction and estimates of the former from colors or line ratio can then be used to infer the extinction. Although it was clear that the uniform screen model is a gross oversimplification of the real situation, it was thought that the corrections thus obtained were roughly correct. Further, this procedure requires only one parameter, the slab opacity, while more realistic modelling involves many other (unknown) quantities.
KeywordsMolecular Cloud Galaxy Disc Interstellar Cloud Extinction Curve Dust Distribution
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