Formation of Raman Scattering Wings around H alpha, H beta and Pa alpha in Active Galactic Nuclei

TL;DR

This paper investigates the formation of broad Raman scattering wings around Balmer and Paschen lines in AGNs, using Monte Carlo simulations to understand their properties and dependence on neutral hydrogen column density.

Contribution

It introduces a model for Raman scattering in AGNs' molecular tori and analyzes how the wing profiles depend on physical parameters, providing new insights into emission line broadening.

Findings

Wing widths scale with the square root of neutral hydrogen column density.

Hα and Paα wings are asymmetric with stronger red parts, opposite to Hβ wings.

Typical Hα wing width is about 20,000 km/s at N_HI=10^23 cm^-2.

Abstract

Powered by a supermassive black hole with an accretion disk, the spectra of active galactic nuclei (AGNs) are characterized by prominent emission lines including Balmer lines. The unification schemes of AGNs require the existence of a thick molecular torus that may hide the broad emission line region from the view of observers near the equatorial direction. In this configuration, one may expect that the far UV radiation from the central engine can be Raman scattered by neutral hydrogen to reappear around Balmer and Paschen emission lines which can be identified with broad wings. We produce H$\alpha$, H$\beta$ and Pa$\alpha$ wings using a Monte Carlo technique to investigate their properties. The neutral scattering region is assumed to be a cylindrical torus specified by the inner and outer radii and the height. While the covering factor of the scattering region affects the overall strengths of the wings, the wing widths are primarily dependent on the neutral hydrogen column density $N_{\rm HI}$ being roughly proportional to $N_{\rm HI}^{1/2}$. In particular, with $N_{\rm HI}=10^{23}{\rm\ cm^{-2}}$ the H$\alpha$ wings typically show a width $\sim 2\times 10^4{\rm\ km\ s^{-1}}$. We also find that H$\alpha$ and Pa$\alpha$ wing profiles are asymmetric with the red part stronger than the blue part and an opposite behavior is seen for H$\beta$ wings.