Velocity-Resolved Ionization Mapping of Broad Line Region. I. Insights into Diverse Geometry and Kinematics

TL;DR

This paper introduces a novel velocity-resolved ionization mapping technique to study the geometry and kinematics of the broad-line region in AGNs, revealing asymmetries that impact black hole mass estimates.

Contribution

It presents a new method for mapping BLR gas distribution using ionization ratios, uncovering asymmetries in the region's geometry and dynamics.

Findings

Discovered asymmetric BLR structure using ionization mapping.

Found anticorrelation between Balmer decrement and continuum.

Observed deviations from symmetric virialized BLR profiles.

Abstract

Broad emission lines of active galactic nuclei (AGNs) originate from the broad-line region (BLR), consisting of dense gas clouds in orbit around an accreting supermassive black hole. Understanding the geometry and kinematics of the region is crucial for gaining insights into the physics and evolution of AGNs. Conventional velocity-resolved reverberation mapping may face challenges in disentangling the degeneracy between intricate motion and geometry of this region. To address this challenge, new key constraints are required. Here, we report the discovery of an asymmetric BLR using a novel technique: velocity-resolved ionization mapping, which can map the distance of emitting gas clouds by measuring Hydrogen line ratios at different velocities. By analyzing spectroscopic monitoring data, we find that the Balmer decrement is anticorrelated with the continuum and correlated with the lags across broad emission line velocities. Some line ratio profiles deviate from the expectations for a symmetrically virialized BLR, suggesting that the red-shifted and blue-shifted gas clouds may not be equidistant from the supermassive black hole (SMBH). This asymmetric geometry might represent a formation imprint, provide new perspectives on the evolution of AGNs, and influence SMBH mass measurements.