On the evolution of gas clouds exposed to AGN radiation – I. Three-dimensional radiation hydrodynamic simulations

Abstract

We perform three-dimensional radiation hydrodynamic simulations of uniform dusty gas clouds irradiated by an active galactic nucleus (AGN) to investigate the depen-dence of evolution of clouds on the ionization parameter U and the Strömgren number NS. We find that the evolution can be classified into two cases depending on U. In low U cases (U ≈ 10−2), the evolution is mainly driven by photo-evaporation. A ap-proximately spherically-symmetric evaporation flow with velocity of 100- 150 km s−1 is launched from the irradiated face. The cloud is compressed by a D-type shock with losing its mass due to photo-evaporation and is finally turned into a dense filament by t. 1.5tsc. In high U cases (U ≈ 5 × 10−2), radiation pressure suppresses photo-evaporation from the central part of the irradiated face, reducing photo-evaporation rate. A evaporation flow from the outskirts of the irradiated face is turned into a high velocity (. 500 km s−1) gas wind because of radiation pressure on dust. The cloud is swept by a radiation pressure-driven shock and becomes a dense gas disk by t ≈ tsweep. Star formation is expected in these dense regions for both cases of U. We discuss the influences of the AGN radiation on the clumpy torus. A simple estimate suggests that the clumps are destroyed in timescales shorter than their orbital periods. For the clumpy structure to be maintained over long period, the incident radiation field needs to be sufficiently weaken for most of the clumps, or, some mechanism that creates the clumps continuously is needed. Key words: hydrodynamics – radiative transfer – methods: numerical – ISM: clouds – galaxies: active 1