Understanding the Broad-line Region of Active Galactic Nuclei with Photoionization. I. the Moderate-Accretion Regime
arxiv(2024)
Abstract
Over three decades of reverberation mapping (RM) studies on local broad-line
active galactic nuclei (AGNs) have measured reliable black-hole (BH) masses for
> 100 AGNs. These RM measurements reveal a significant correlation between
the Balmer broad-line region size and the AGN optical luminosity (the R-L
relation). Recent RM studies for AGN samples with more diverse BH accretion
parameters (e.g., mass and Eddington ratio) reveal a substantial intrinsic
dispersion around the average R-L relation, suggesting variations in the
overall spectral energy distribution shape as functions of accretion
parameters. Here we perform a detailed photoionization investigation of
expected broad-line properties as functions of accretion parameters, using the
latest models for the AGN continuum implemented in qsosed. We compare
theoretical predictions with observations of a sample of 67 z≲0.5
reverberation-mapped AGNs with both rest-frame optical and UV spectra in the
moderate-accretion regime (Eddington ratio λ_ Edd≡ L/L_
Edd<0.5). The UV/optical line strengths and their dependences on accretion
parameters can be reasonably well reproduced by the locally-optimally-emitting
cloud (LOC) photoionization models. We provide quantitative recipes that use
optical/UV line flux ratios to infer the ionizing continuum, which is not
directly observable. In addition, photoionization models with universal values
of ionization parameter (log U_ H=-2) and hydrogen density (log
n( H)=12) can qualitatively reproduce the observed global R-L relation
for the current AGN sample. However, such models fail to reproduce the observed
trend of decreasing BLR size with L/L_ Edd at fixed optical luminosity,
which may imply that the gas density increases with the accretion rate.
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