Broad-line Balmer Decrements in Blue Active Galactic Nuclei

TL;DR

This study analyzes the broad-line Balmer decrements in a large sample of Seyfert 1 galaxies and QSOs from SDSS, finding a consistent intrinsic ratio and minimal correlation with other AGN properties, supporting its use as a dust extinction indicator.

Contribution

It provides the first large-scale, homogeneous analysis of broad-line Balmer decrements in AGN, establishing a baseline intrinsic ratio and assessing its dependence on AGN characteristics.

Findings

Intrinsic broad-line Halpha/Hbeta ratio peaks at 3.06

Balmer decrement shows no significant correlation with AGN luminosity or accretion rate

Radio-loud and double-peaked sources have slightly higher decrements

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.