The Effect of Radiation Pressure on Virial Black Hole Mass Estimates and the Case of Narrow‐Line Seyfert 1 Galaxies

Abstract

We consider the effect of radiation pressure from ionizing photons on black hole (BH) mass estimates based on the application of the virialtheorem to broad emission lines in AGN spectra. BH masses based only onthe virial product Δ V2R and neglecting the effect ofradiation pressure can be severely underestimated, especially in objectsclose to the Eddington limit. We provide an empirical calibration of thecorrection for radiation pressure, and we show that it is consistentwith a simple physical model in which BLR clouds are optically thick toionizing radiation and have average column densities of NH ~1023 cm -2. This value is remarkably similarto what is required in standard BLR photoionization models to explainobserved spectra. With the inclusion of radiation pressure, thediscrepancy between virial BH masses based on single-epoch spectra andon reverberation mapping data drops from 0.4 to 0.2 dex rms. The use ofsingle-epoch observations as surrogates of reverberation mappingcampaigns can thus provide more accurate BH masses than previouslythought. Finally, we show that narrow-line Seyfert 1 (NLS1) galaxieshave apparently low BH masses because they are radiating close to theirEddington limit. After the radiation pressure correction, NLS1 galaxieshave BH masses similar to other broad-line AGNs and follow the sameMBH - σe/Lsph relations asother active and normal galaxies. Radiation forces arising from ionizingphoton momentum deposition constitute an important physical effect whichmust be taken into account when computing virial BH masses.