The Nuclear Reddening Curve for Active Galactic Nuclei and the Shape of the Infrared to X‐Ray Spectral Energy Distribution

We present extinction curves derived from the broad emission lines andcontinua of large samples of both radio-loud and radio-quiet AGNs. The curvesare significantly flatter in the UV than are curves for the local ISM. Thereddening curves for the radio-quiet LBQS quasars are slightly steeper thanthose of the radio-loud quasars in the UV, probably because of additionalreddening by dust further out in the host galaxies of the former. The UVextinction curves for the radio-loud AGNs are very flat. This is explicablewith slight modifications to standard MRN dust models: there is a relative lackof small grains in the nuclear dust. Our continuum and broad-emission linereddening curves agree in both shape and amplitude, confirming that thecontinuum shape is indeed profoundly affected by reddening for all but thebluest AGNs. With correction by our generic extinction curve, all of theradio-loud AGNs have continuous optical-UV spectra consistent with a singleshape. We show that radio-quiet AGNs have very similar intrinsic UV to opticalshape over orders of magnitude in luminosity. We also argue that radio-loud andradio-quiet AGNs probably share the same underlying continuum shape and thatmost of the systematic differences between their observed continuum shapes aredue to higher nuclear reddening in radio-selected AGNs, and additionalreddening from dust further out in the host galaxies in radio-quiet AGNs. Ourconclusions have important implications for the modelling of quasar continuaand the analysis of quasar demographics.Comment: 41 pages, including 6 figures and 3 tables. To appear in ApJ vol. 614, October 20 issue. Some slight wording changes. Some additional references added. Small changes in the model fit in section 6.2, to the analytical fit in the Appendix, and to the tabulated reddening curve in the Appendi