|COS Instrument Handbook for Cycle 25|
Extinction can dramatically reduce the observed intensity of your source, particularly in the ultraviolet. Figure 7.3 shows Aλ/AV values applicable to our Galaxy, taken from Cardelli, Clayton, & Mathis (1989, ApJ, 345, 245) assuming RV = 3.1. This corresponds to the Milky Way Diffuse (Rv=3.1) selection of the ETC.Extinction curves have a strong metallicity dependence, particularly at ultraviolet wavelengths. Sample extinction curves are presented in Gordon et al. [2003, ApJ, 594, 279 (LMC Average, LMC 30 Dor Shell, and SMC Bar)], Calzetti et al. [2000, ApJ, 533, 682 (starburst galaxies)], and references therein. At lower metallicities, the 2200 Å bump that is so prominent in the Galactic extinction curve disappears, and Aλ/E(B–V) increases at shorter UV wavelengths.The ETC allows the user to select among a variety of extinction curves and to apply the extinction correction either before or after the input spectrum is normalized. Be aware that not all extinction laws in the ETC extend below 1200 Å, which may cause incorrect calculations for the 1222, 1055, and 1096 Å central wavelengths.