9.5 Extinction Correction

Extinction can dramatically reduce the counts expected from your source, particularly in the ultraviolet. Figure 9.3 shows the average A_V/E(B-V) values for our galaxy, taken from Seaton 1979, MNRAS, 187, 73. Large variations about the average are observed (Witt, Bohlin, & Stecher 1984, ApJ, 279, 698).

Extinction curves have a strong metallicity dependence, particularly in the UV wavelengths. Sample extinction curves can be seen in Koornneef & Code 1981, ApJ, 247, 860 (LMC); Bouchet et al. 1985, A&A, 149, 330 (SMC); and Calzetti, Kinney, & Storchi-Bergmann 1994, ApJ, 429, 582, and references therein. At lower metallicities, the 2200 Å bump, which is so prominent in the Galactic extinction curve, disappears; and A_V/E(B-V) may increase monotonically at UV wavelengths.

To take into account the effects of foreground Galactic extinction, the ETC allows users to supply a value of E(B-V) and choose from various extinction curves. Estimates of E(B-V) can be found as a function of sky position from e.g., Shlafly & Finkbeiner (2011).

Figure 9.3: Extinction versus wavelength.

The mean UV interstellar extinction curve of the Milky Way as a function of wavelength. The curve features a prominent bump at 2200 Å, which has been shown to disappear at lower metallicities.