  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.
