The starburst model for active galactic nuclei: the broad-line region as supernova remnants evolving in a high-density medium

Abstract

It is shown that the broad permitted and semipermitted emission lines of the broad-line region (BLR) of radio-quiet active galactic nuclei (AGN) can be generated by strongly radiative (rapidly cooling) supernova remnants expected to occur in the central regions of early-type galaxies undergoing a starburst. Supernova remnants interacting with a circumstellar medium with a density of about |$n\,\sim\,{10}^{7}\,\text{cm}^{-3}$| become strongly radiative while still expanding at several thousand km s–1, and miss the adiabatic Sedov track. Radiative cooling becomes important well before the thermalization of the ejecta is completed, and the shocked matter undergoes a fast condensation behind both the outgoing forward shock and the reverse shock. Two concentric, high-density, and fast-moving thin shells are then formed. Both cool dense shells, as well as the expanding ejecta, the unshocked circumstellar material and a section of the interstellar gas, are irradiated and ionized by the photon field produced by the radiative shocks. Our model, in which the only free parameter is the density of the circumstellar medium, combines analytic and numerical hydrodynamic simulations, together with static photoionization computations, to reproduce most of the basic properties of the BLR in low-luminosity AGN.