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Astrophysics

Title: Broad-line Balmer Decrements in Blue Active Galactic Nuclei

Authors: Xiaobo Dong (1), Tinggui Wang (1), Jianguo Wang (2), Weimin Yuan (2), Hongyan Zhou (1), Haifeng Dai (1), Kai Zhang (1) ((1) Univ. of Sci. & Tech. of China, (2) Yunnan Observatory, China)
Abstract: We have investigated the broad-line Balmer decrements (Halpha/Hbeta) for a large, homogeneous sample of Seyfert 1 galaxies and 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 Halpha/Hbeta ratio can be well described by log-Gaussian, with a peak at Halpha/Hbeta=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 Halpha/Hbeta 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.
Comments: To appear in MNRAS. The data and the fitted parameters for the decomposed spectral components (continuum, FeII and other emission lines) of the 446 blue AGNs are available at this http URL
Subjects: Astrophysics (astro-ph)
DOI: 10.1111/j.1365-2966.2007.12560.x
Cite as: arXiv:0710.1458 [astro-ph]
  (or arXiv:0710.1458v1 [astro-ph] for this version)

Submission history

From: Xiaobo Dong [view email]
[v1] Mon, 8 Oct 2007 07:13:48 GMT (481kb)