Tidally disrupted dusty clumps as the origin of broad emission lines in active galactic nuclei

Type 1 active galactic nuclei display broad emission lines, which are regarded as arising from photoionized gas moving in the gravitational potential of a supermassive black hole(1,2). However, the origin of this broad-line region gas is unresolved so far(1-3). Another component is the dusty tome beyond the broad-line region probably an assembly of discrete clumps(5-7) which can hide the region from some viewing angles and make them observationally appear as type 2 objects. Here, we report that these clumps moving within the dust sublimation radius, such as the molecular cloud G2 discovered in the Galactic Centres, will be tidally disrupted by the black hole, resulting in some gas becoming bound at smaller radii while other gas is ejected and returns to the torus. The clumps fulfill necessary conditions to be photoionizeds. Specific dynamical components of tidally disrupted clumps include spiral-in gas as inflow, circularized gas and ejecta as outflow. We calculated various profiles of emission lines from these clouds, and found that they generally agree with H beta profiles of Palomar-Green quasars's. We found that the asymmetry, shape and shift of the profiles strongly depend on [O-III] luminosity, which we interpret as a proxy of dusty torus angles. Tidally disrupted clumps from the torus may represent the source of the broad-line region gas.