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 Hα, Hβ and Paα 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 HI being roughly proportional to N 1/2 HI . In particular, with N HI = 10 23 cm −2 the Hα wings typically show a width ∼ 2 × 10 4 km s −1 . We also find that Hα and Paα wing profiles are asymmetric with the red part stronger than the blue part and an opposite behavior is seen for Hβ wings.
Lyα-emitting galaxies and giant Lyα blobs (LABs) have been extensively observed to study the formation history of galaxies. However, the origin of their extended Lyα emission, especially of LABs, remains controversial. Polarization signals from some LABs have been discovered, and this is commonly interpreted as strong evidence supporting that the extended Lyα emission originates from the resonance scattering. The Monte Carlo Lyα radiative transfer code LaRT is updated to investigate the polarization of Lyα using the Stokes vector formalism. We apply LaRT to a few models to explore the fundamental polarization properties of Lyα. Interestingly, individual Lyα photon packets are found to be almost completely polarized by a sufficient number of scatterings (N scatt ≳ 104–105 in a static medium) or Doppler shifts induced by gas motion, even starting from unpolarized light. It is also found that the polarization pattern can exhibit a nonmonotonically increasing pattern in some cases, other than the commonly known trend that the polarization monotonically increases with radius. The polarization properties are primarily determined by the degree of polarization of individual photon packets and the anisotropy of the Lyα radiation field, which are eventually controlled by the medium’s optical depth and velocity field. Once Lyα photon packets achieve ∼100% polarization, the radial profile of polarization appears to correlate with the surface brightness profile. A steep surface brightness profile tends to yield a rapid increase of the linear polarization near the Lyα source location. In contrast, a shallow surface brightness profile gives rise to a slowly increasing polarization pattern.
The unification scheme of active galactic nuclei (AGNs) invokes an optically thick molecular torus component hiding the broad emission line region. Assuming the presence of a thick neutral component in the molecular torus characterized by a H i column density > 10 22 cm −2 , we propose that far UV radiation around Lyα can be significantly polarized through Rayleigh scattering. Adopting a Monte Carlo technique we compute polarization of Rayleigh scattered radiation near Lyα in a thick neutral region in the shape of a slab and a cylindrical shell. It is found that radiation near Lyα Rayleigh reflected from a very thick slab can be significantly polarized in a fairly large range of wavelength ∆λ ∼ 50Å exhibiting a flux profile similar to the incident one. Rayleigh transmitted radiation in a slab is characterized by the central dip with a complicated polarization behavior. The optically thick part near Lyα center is polarized in the direction perpendicular to the slab normal, which is in contrast to weakly polarized wing parts in the direction parallel to the slab normal. A similar polarization flip phenomenon is also found in the case of a tall cylindrical shell, in which the spatial diffusion along the vertical direction near the inner cylinder wall for core photons leads to a tendency of the electric field aligned to the direction perpendicular to the vertical axis. Observational implications are briefly discussed including spectropolarimetry of the quasar PG 1630+377 by Koratkar et al. in 1990 where Lyα is strongly polarized with no other emission lines polarized.
T Tauri stars produce broad Lyα emission lines that contribute ∼88% of the total UV flux incident on the inner circumstellar disks. Lyα photons are generated at the accretion shocks and in the protostellar chromospheres and must travel through accretion flows, winds, and jets, the protoplanetary disks, and the interstellar medium before reaching the observer. This trajectory produces asymmetric, double-peaked features that carry kinematic and opacity signatures of the disk environments. To understand the link between the evolution of Lyα emission lines and the disks themselves, we model HST-COS spectra from targets included in Data Release 3 of the Hubble UV Legacy Library of Young Stars as Essential Standards program. We find that resonant scattering in a simple spherical expanding shell is able to reproduce the high-velocity emission line wings, providing estimates of the average velocities within the bulk intervening H i. The model velocities are significantly correlated with the K-band veiling, indicating a turnover from Lyα profiles absorbed by outflowing winds to emission lines suppressed by accretion flows as the hot inner disk is depleted. Just 30% of targets in our sample have profiles with redshifted absorption from accretion flows, many of which have resolved dust gaps. At this stage, Lyα photons may no longer intersect with disk winds along the path to the observer. Our results point to a significant evolution of Lyα irradiation within the gas disks over time, which may lead to chemical differences that are observable with ALMA and JWST.
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