Reddening of the BLR and NLR in AGNs from a systematic analysis of Balmer decrement

Abstract

We selected an active galactic nuclei (AGN) sample ($0 < z \le 0.35$) from Sloan Digital Sky Survey Data Release 7, and measured the broad- (${\rm H\alpha^{b}/H\beta^{b}}$) and narrow-line Balmer decrements (${\rm H\alpha^{n}/H\beta^{n}}$) of 554 selected AGNs. We found that the distributions of Balmer decrements can be fitted by a Gaussian function and give the best estimates of ${\rm H\alpha^{b}/H\beta^{b}} = 3.16$ with a standard deviation 0.07 dex, and ${\rm H\alpha^{n}/H\beta^{n}} = 4.37$ with a standard deviation 0.10 dex. We inspected the distributions of ${\rm H\alpha^{b}/H\beta^{b}}$ and ${\rm H\alpha^{n}/H\beta^{n}}$ in the Baldwin$-$Phillips$-$Terlevich (BPT) diagram and found that only narrow-line Balmer decrements depend on the physical conditions of the narrow-line region (NLR). We tested the relationship between ${\rm H\alpha^{b}/H\beta^{b}}$ and ${\rm H\alpha^{n}/H\beta^{n}}$, and found that ${\rm H\alpha^{b}/H\beta^{b}}$ does not correlate with ${\rm H\alpha^{n}/H\beta^{n}}$. We investigated the relationship between Balmer decrements and Seyfert sub-type, and found that only broad-line Balmer decrements correlate with Seyfert sub-type, We also examined the dependency of Balmer decrements on AGN properties, and found that Balmer decrements have no correlation with optical luminosity, but show some dependence on accretion rate. These results indicate that the NLR is subject to more reddening by dust than the broad-line region (BLR).