Broad-line region structure and kinematics in the radio galaxy 3C 120

TL;DR

This study investigates the structure and kinematics of the broad-line region in the radio galaxy 3C 120 using reverberation mapping, revealing a stratified BLR with outflow motions and a vertical structure consistent with other AGN.

Contribution

It provides detailed reverberation mapping analysis of 3C 120, demonstrating the stratified nature and outflow signatures of its BLR, and compares its vertical structure with other AGN.

Findings

BLR in 3C 120 is stratified with respect to distance and line width.

Hα and Hβ line wings respond faster than the core, indicating disk-like emission.

Blue wing responses suggest outflow motions during high activity states.

Abstract

Broad emission lines originate in the surroundings of supermassive black holes in the centers of active galactic nuclei (AGN). One method to investigate the extent, structure, and kinematics of the BLR is to study the continuum and line profile variability in AGN. We selected the radio-loud Seyfert 1 galaxy 3C 120 as a target for this study. We took spectra with a high signal-to-noise ratio of 3C 120 with the 9.2m Hobby-Eberly Telescope between Sept. 2008 and March 2009. In parallel, we photometrically monitored the continuum flux at the Wise observatory. We analyzed the continuum and line profile variations in detail (1D and 2D reverberation mapping) and modeled the geometry of the line-emitting regions based on the line profiles. We show that the BLR in 3C 120 is stratified with respect to the distance of the line-emitting regions from the center with respect to the line widths (FWHM) of the rms profiles and with respect to the variability amplitude of the emission lines. The emission line wings of H{\alpha} and H{\beta} respond much faster than their central region. This is explained by accretion disk models. In addition, these lines show a stronger response in the red wings. However, the velocity-delay maps of the helium lines show a stronger response in the blue wing. Furthermore, the HeII{\lambda}4686 line responds faster in the blue wing in contradiction to observations made one and a half years later when the galaxy was in a lower state. The faster response in the blue wing is an indication for central outflow motions when this galaxy was in a bright state during our observations. The vertical BLR structure in 3C 120 coincides with that of other AGN. We confirm the general trend: the emission lines of narrow line AGN originate at larger distances from the midplane than AGN with broader emission lines.