The 2175 Angstrom Extinction Bump in QSO Spectra

K. M. Pitman, G. C. Clayton, & K. D. Gordon 2000, PASP, 112, 537

The 2175 Å absorption bump, a feature often ascribed to graphite grains and ubiquitous in the spectra of sightlines through the Galactic diffuse interstellar medium, is generally weak or non-existent for objects outside our Galaxy. Many active galaxies seem to have SMC-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 BAL 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.

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