On the Polarization of H-alpha Lines Scattered by Neutral Hydrogen in Active Galactic Nuclei

Abstract

Raman scattering by atomic hydrogen converts the UV continuum around Ly$\beta$ into optical continuum around H$\alpha$, and the basic atomic physics has been discussed in several works on symbiotic stars. We propose that the same process may operate in active galactic nuclei (AGN) and calculate the linear polarization of the broad emission lines Raman-scattered by a high column neutral hydrogen compnent. The conversion efficiency of the Raman scattering process is discussed and the expected scattered flux is computed using the spectral energy distribution of an AGN given by a typical power law. The high column H {\sc i} component in AGN is suggested by many observations encompassing radio through UV and X-ray ranges. When the neutral hydrogen component with a column density $\sim 10^{22}c{m}^{-2}$ is present around the active nucleus, it is found that the scattered H$\alpha$ is characterized by a very broad width $\sim 20,000km/s$ and that the strength of the polarized flux is comparable to that of the electron-scattered flux expected from a conventional unified model of narrow line AGN. The width of the scattered flux is mainly determined by the column density of the neutral scatterers where the total scattering optical depth becomes of order unity. The asymmetry in the Raman scattering cross section around Ly$\beta$ introduces red asymmetric polarized profiles around H$\alpha$. The effects of the blended Ly$\beta$ and O {\sc vi} 1034 doublet are also investigated. We briefly discuss the spectropolarimetric observations performed on the Seyfert galaxy IRAS 110548-1131 and the narrow line radio galaxy Cyg A.

11 pages, 6 figures, accepted for publication in MNRAS

Cited by in corpus (8)

• The Polarization of Scattered Lyman Alpha Radiation Around High-Redshift Galaxies
• Raman-Scattering Wings of H alpha in Symbiotic Stars
• Broad H$\alpha$ Wing Formation in the Planetary Nebula IC 4997
• Toward the Evidence of the Accretion Disk Emission in the Symbiotic Star RR Tel
• Spectropolarimetry of low redshift Quasars: origin of the polarization and implications for black hole mass estimates
• Impact of Resonance, Raman, and Thomson Scattering on Hydrogen Line Formation in Little Red Dots
• Rayleigh and Raman scattering cross-sections and phase matrices of the ground-state hydrogen atom, and their astrophysical implications
• Polarization Structures in the Thomson-Scattered Emission Lines in Active Galactic Nuclei