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 beta of the Baldwin effect is positively correlated with the Eddington ratio lambda Edd in both populations, and RL quasars exhibit steeper beta than their RQ quasars at fixed lambda Edd. Photoionization simulations reveal that the beta 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 lambda Edd drive stronger accretion disk winds, leading to denser BLRs and shallower beta. 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.