Extinction can dramatically reduce the counts expected from your source, particularly in the ultraviolet. Figure 9.3 shows the average
Av/
E(
B–
V) values for our galaxy, taken from (Seaton,
MNRAS,
187, 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/
E(
B–
V) may increase monotonically at UV wavelengths.
