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

TL;DR

This paper models the broad line region in NGC 4051 as an inflow illuminated by a hidden 10^5 K accretion disk, revealing a larger BLR size and ionization structure than previously thought, impacting black hole mass estimates.

Contribution

It introduces a spherically symmetric inflow model for the BLR, accounting for a hidden ionizing source and revised ionization parameters, challenging prior reverberation-based size estimates.

Findings

BLR extends from ~3 to >475 light-days.

Ionizing photon deficit suggests a hidden hot accretion disk.

Reverberation size marks only the inner BLR radius.

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.