Revealing the Physical Driver of the Baldwin Effect: Gas Density in the Broad-Line Region
The Baldwin effect --- the inverse correlation between the equivalent width of emission lines and the continuum luminosity in active galactic nuclei (AGNs) --- has been known for nearly five decades, yet its physical origin remains poorly understood. Using a sample of 41,159 radio quasars constructed from the Sloan Digital Sky Survey and the Low-Frequency Array Two-metre Sky Survey, we investigate the origin and underlying physics of the Baldwin effect of MgII broad emission lines in both radio-quiet (RQ) and radio-loud (RL) quasars. We find that the slope $β$ of the Baldwin effect is positively correlated with the Eddington ratio $λ_{\rm Edd}$ in both populations, and RL quasars exhibit steeper $β$ than their RQ quasars at fixed $λ_{\rm Edd}$. Photoionization simulations reveal that the $β$ is primarily governed by the gas density in the broad-line region (BLR): lower gas densities yield steeper slopes. This density-driven mechanism naturally connects the Baldwin effect to the broader AGN evolutionary context. Specifically, higher $λ_{\rm Edd}$ drive stronger accretion disk winds, leading to denser BLRs and shallower $β$. Our findings indicate that BLR gas density serves as the primary physical driver underlying the "global" Baldwin effect, offering a physically grounded framework for interpreting AGN accretion states and their coupled evolution with host galaxies.