Agn Reddening and Ultraviolet Extinction Curves from Hubble Space Telescope Spectra

We present intrinsic extinction curves for 14 AGNs. The AGNs have reddenings, E(B-V), of up to 0.36 mag. The majority (13 out of 14) of the extinction curves are not steep in the UV. Of the seven best determined extinction curves, five have extinction curves that are as flat as the standard Galactic curve in the optical and near UV, but flatter in the far UV, and without the λ2175 feature. One AGN, B3 0754+394, has a steep SMC-like extinction curve, and another, Mrk 304, has an LMC-like extinction curve, including a probable λ2175 bump. The remaining seven, lower-quality, extinction curves have overall shapes that are consistent with an LMC-like shape or a flatter shape. Two have possible λ2175 features, and one might be identical to the Galactic curve. The flatter curves that predominate in our best determined extinction curves are not as flat as the Gaskell et al. (2004) extinction curve for radio-loud AGNs. This suggests that the previous radio-loud extinction curve might be slightly too flat in the range 4 < 1/λ < 6.5 μm because of luminosity-dependent reddening biases in the composite spectra, but further investigation is needed. We present a parameterized average AGN extinction curve. Observed variations in the continuum properties of the AGNs are inconsistent with intrinsic object-to-object variations because observed differences are least in the far UV where changes in the accretion disk spectrum should be greatest. We suggest that the steepening of AGN spectra around Lyman α is the result of a small amount of SMC-like dust (E(B-V) ∼ 0.03). We find the largest object-to-object differences in spectral shape to be in the Fe II emission of the “small blue bump”. Subject headings: galaxies:active — ISM:dust, extinction galaxies:quasars:general — black hole physics — accretion: accretion disk