Constrain the Size of Broad Line Region in NGC 1068 Using Sub-millimeter Excess

Abstract Constraining the size of broad-line region (BLR) close to the accretion disk in active galactic nuclei (AGNs) is key to understand the central engine and evolution of AGN. We propose a new method to constrain the BLR size, based on millimeter to sub-millimeter spectral energy distribution (SED), from which a sub-millimeter excess may be found. Such excess can be used to determine the solid angle of the free-free emission region from BLR, as a region with high electron density and optically thick at sub-millimeter wavelength for free-free continuum. Millimeter to sub-millimeter SED analysis toward the central source S1 in a nearby type II AGN NGC 1068 was done, which gave an upper limit on the solid angle of BLR region as (4.94 ± 0.61) × 10−6 arcsec2, corresponding to a radius no more than 0.085 pc. Such a constraint on the BLR size is important for estimating the masses of central supermassive black holes in AGNs.