6.7	Extinction Correction

Extinction can dramatically alter the counts expected from your source, particularly in the UV. Figure 6.3 shows Aλ/E(B–V) values applicable to the Galaxy, taken from Seaton (MNRAS, 187, 73p, 1979). An additional galactic extension curve is available taken from Cardelli, Calyton, & Mathis (ApJ, 345, 245, 1989) denoted gal3 in the ETC.

Extinction curves, however, have a strong metallicity and environment dependence, particularly at 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 Aλ/E(B–V) increases at UV wavelengths. However, as discussed by Gordon et al. 2003 (ApJ, 594, 279), the extinction curve for any sight line depends on the local environment, and even within the Magellenic clouds, may examples can be found that are closer to the typical Galactic extinction law than to the Koornneef & Code curve.

The easiest way to understand how to determine the extinction correction for your source is to work through an example; see Section 6.8.4. A choice of extinction laws is offered in the ETC.

Figure 6.3: Average Galactic Extinction versus Wavelength (figure courtesy of Seaton, M.J., see also Seaton. 1979, MNRAS,187, 73p)