Constraints on the Broad Line Region Properties and Extinction in Local Seyferts

TL;DR

This study uses high-resolution optical to near-infrared spectra of nearby Seyfert 1 galaxies to determine the physical conditions, extinction, and location of the broad line region, revealing its proximity to the dust sublimation radius and the nature of obscuring structures.

Contribution

First simultaneous optical to near-infrared observations combined with photoionization models to constrain BLR properties and extinction in Seyfert galaxies.

Findings

BLR gas density around 10^11 cm^-3

Extinction A_V ≤ 3 in Seyfert 1-1.5s, 4-8 in Seyfert 1.8-1.9s

BLR obscuration occurs on scales smaller than the NLR

Abstract

We use high spectral resolution (R > 8000) data covering 3800-13000\r{A} to study the physical conditions of the broad line region (BLR) of nine nearby Seyfert 1 galaxies. Up to six broad HI lines are present in each spectrum. A comparison - for the first time using simultaneous optical to near-infrared observations - to photoionisation calculations with our devised simple scheme yields the extinction to the BLR at the same time as determining the density and photon flux, and hence distance from the nucleus, of the emitting gas. This points to a typical density for the HI emitting gas of 10$^{11}$cm$^{-3}$ and shows that a significant amount of this gas lies at regions near the dust sublimation radius, consistent with theoretical predictions. We also confirm that in many objects the line ratios are far from case B, the best-fit intrinsic broad-line H$\alpha$/H$\beta$ ratios being in the range 2.5-6.6 as derived with our photoionization modeling scheme. The extinction to the BLR, based on independent estimates from HI and HeII lines, is A$_V$ $\le$ 3 for Seyfert 1-1.5s, while Seyfert 1.8-1.9s have A$_V$ in the range 4-8. A comparison of the extinction towards the BLR and narrow line region (NLR) indicates that the structure obscuring the BLR exists on scales smaller than the NLR. This could be the dusty torus, but dusty nuclear spirals or filaments could also be responsible. The ratios between the X-ray absorbing column N$_H$ and the extinction to the BLR are consistent with the Galactic gas-to-dust ratio if N$_H$ variations are considered.