2175 angstrom feature

The 2175 angstrom feature, a "bump" in the interstellar medium's (ISM's) absorption-by-wavelength. The mechanism remains of research interest though it was first detected by ultraviolet astronomy by sounding rockets in the 1970s and before. Current theories include the notion that it is caused by polycyclic aromatic hydrocarbons (PAHs). The feature is a minor contributor to the ISM's reddening and the most prominent absorption feature within the visible light-to-near-UV regime (though reddening generally stems from a large-scale slope in the amount of absorption over this whole regime, which is more absorption with smaller wavelengths, leading to a peak at about 800 angstroms). The 2175 feature has also been detected in the interplanetary medium (IPM) and intergalactic medium (IGM) and cosmic dust within other galaxies. It is referred to using various phrases, among them, the 2175 angstrom absorption feature, the 217.5 nm feature, the 217.5 nm absorption feature, and the 220 nm feature.