The 2175 Å extinction bump in QSO spectra
The 2175 Å absorption bump, a feature often ascribed to graphite grains and ubiquitous in the spectra of sight lines through the Galactic diffuse interstellar medium, is generally weak or nonexistent for objects outside our Galaxy. Many active galaxies seem to have Small Magellanic Cloud-type dust extinction, suggesting that the presence of the bump in our Galaxy may be exceptional. Recently, it was suggested that the spectrum of the high-ionization broad absorption line QSO UM 425 shows a 2175 Å feature. This apparent feature seen in UM 425 and the rest frame spectra of other QSOs is intrinsic to the QSO spectrum. It is the result of a relative absence of emission near 2200 Å. Thus far, no significant detection of the 2175 Å bump in a QSO has been reported.