Broad-line Balmer decrements in blue active galactic nuclei |
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| Authors: | Xiaobo Dong Tinggui Wang Jianguo Wang Weimin Yuan Hongyan Zhou Haifeng Dai Kai Zhang |
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| Institution: | Centre for Astrophysics, University of Science and Technology of China, Hefei, Anhui, 230026, China;Joint Institute of Galaxies and Cosmology, Shanghai Observatory and University of Science and Technology China;National Astronomical Observatories/Yunnan Observatory, Chinese Academy of Sciences, PO Box 110, Kunming, Yunnan 650011, China;Department of Physics, Yunnan University, 650031, Kunming, China;Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany |
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| Abstract: | We have investigated the broad-line Balmer decrements (Hα/Hβ) for a large, homogeneous sample of Seyfert 1 galaxies and quasi-stellar objects (QSOs) using spectroscopic data obtained in the Sloan Digital Sky Survey. The sample, drawn from the Fourth Data Release, comprises 446 low-redshift ( z ? 0.35) active galactic nuclei (AGN) that have blue optical continua as indicated by the spectral slopes in order to minimize the effect of dust extinction. We find that (i) the distribution of the intrinsic broad-line Hα/Hβ ratio can be well described by log-Gaussian, with a peak at Hα/Hβ= 3.06 and a standard deviation of about 0.03 dex only; (ii) the Balmer decrement does not correlate with AGN properties such as luminosity, accretion rate and continuum slope, etc.; (iii) on average, the Balmer decrements are found to be only slightly larger in radio-loud sources (3.37) and sources having double-peaked emission-line profiles (3.27) compared to the rest of the sample. We therefore suggest that the broad-line Hα/Hβ ratio can be used as a good indicator for dust extinction in the AGN broad-line region; this is especially true for radio-quiet AGN with regular emission-line profiles, which constitute the vast majority of the AGN population. |
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| Keywords: | quasars: emission lines quasars: general |
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