Extinction can dramatically reduce the counts expected from your source, particularly in the ultraviolet. Figure 9.3
shows the average Av
) values for our galaxy, taken from (Seaton, MNRAS
, 73P, 1979). Large variations about the average are observed (Witt, Bohlin, & Stecher, ApJ
, 279, 698, 1984).
Extinction curves have a strong metallicity dependence, particularly in the UV wavelengths. Sample extinction curves can be seen in Koornneef and Code, ApJ
, 247, 860 1981 (LMC); Bouchet et al., A&A
, 149, 330 1985 (SMC); and Calzetti, Kinney and Storchi-Bergmann, ApJ
, 429, 582, 1994, and references therein. At lower metallicities, the 2200 Å bump which is so prominent in the Galactic extinction curve disappears; and Av
) may increase monotonically at UV wavelengths.