The 3000 A bump in quasars

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Abstract
The nature of the broad, hump-like emission excess superposed on the power-law continuum of a quasar and centered at 3000 A is examined in a modeling approach. A synthetic spectrum including theoretically expected emission components (blended, high-order Balmer lines, optically thin Balmer continuum emission, two-photon emission and Fe II blends) as well as some dust reddening is calculated using mean line parameters, and shown to be in reasonable agreement with observed quasar spectra. Shortward of the Balmer edge, however, certain quasars do not show the downward slope change characteristic of the synthetic spectrum but instead keep rising with no obvious shape change. Detailed comparison of the spectra of such risers with the synthetic spectrum indicates that the emission excess is not due to emission from a single-temperature blackbody or enhanced two-photon emission, but may be attributable to partially optically thick Balmer continuum emission.

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