AGN Reddening and Ultraviolet Extinction Curves from Hubble Space Telescope Spectra

TL;DR

This study derives intrinsic extinction curves for 14 AGNs, revealing mostly flat UV extinction with some showing SMC or LMC-like features, and discusses implications for AGN dust properties and previous extinction models.

Contribution

First detailed analysis of intrinsic AGN extinction curves from Hubble spectra, showing mostly flat UV extinction and challenging previous radio-loud AGN extinction assumptions.

Findings

Most extinction curves are flat in the UV, lacking the 2175 Angstrom feature.

Some AGNs exhibit SMC or LMC-like extinction curves, including the 2175 bump.

Variations in spectra are mainly due to dust, not intrinsic AGN differences.

Abstract

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 2175 Angstrom 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 2175 Ang. 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 2175 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/lambda < 6.5 inverse microns 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 alpha 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".