The Broad Line Region in NGC 4051: An Inflow Illuminated by a 10^5 K Accretion Disk

Abstract: Adopting a spherically symmetric steady-state ballistic inflow as the kinematic model for the gas distribution responsible for producing the H{\alpha} emission line and a central black hole (BH) mass of 1.7 x10^6 M_sun, determined from prior reverberation mapping, leads to the following dimensions for the size of the broad line region (BLR) in NGC 4051; an inner radius ~3 lt-days and a lower limit to the outer radius ~ 475 lt-days. Thus, the previously determined reverberation size for the BLR marks just the inner radius of a much larger volume of ionized gas. The number of ionizing photons required to sustain the H{\alpha} emission line luminosity exceeds the number observed to be available from the central AGN by a factor of 3 - 4. Such a large ionizing deficit can be reconciled if the BLR is ionized by a 10^5 K accretion disk that is hidden from direct view by the high opacity of intervening H gas. A new definition is introduced for the ionization parameter that acknowledges the fact that H opacity significantly attenuates the flux of ionizing photons in the large, partially ionized, nebula surrounding the AGN. Collectively, these results have important implications for BH masses estimated using reverberation radii and the structure of the BLR inferred from velocity-delay maps.