Symbiotic starburst-black hole active galactic nuclei - I. Isothermal hydrodynamics of the mass-loaded interstellar medium

Compelling evidence associates the nuclei of active galaxies and massive st arbursts. The symbiosis between a compact nuclear starburst stellar cluster and a massive black hole can self-consistently explain the properties of a ctive nuclei. The young stellar cluster has a profound effect on the most i mportant observable properties of active galaxies through its gravity, and by mass injection through stellar winds, supernovae and stellar collisions. This mass loss, injected throughout the nucleus, creates a hot nuclear int erstellar medium (nISM). The cluster both acts as an optically thin fuel re servoir and enriches the nISM with the products of nucleosynthesis. The nIS M flows under gravitational and radiative forces until it leaves the nucleu s or is accreted on to the black hole or accretion disc. The radiative force exerted by the black hole-accretion disc radiation fiel d is not spherically symmetric. This results in complex flows in which regi ons of inflow can coexist with high Mach number outflowing winds and hydrod ynamic jets. We present two-dimensional hydrodynamic models of such nISM fl ows, which are highly complex and time-variable. Shocked shells, jets and e xplosive bubbles are produced, with bipolar winds driving out from the nucl eus. Our results graphically illustrate why broad-emission-line studies hav e consistently failed to identify any simple, global flow geometry. The rea l structure of the flows is inevitably yet more complex. The structure of these nISM flows is principally determined by two dimensio nless quantities. The first is the magnitude of the stellar cluster velocit y dispersion relative to the sound speed in the nISM. These speeds measure the gravitational and thermal energies in the nISM respectively, and, there fore, whether the gas is initially bound or escapes in a thermal wind. The second parameter is the Mach number of the ill-collimated nISM flow which i s driven away from the central black hole. We discuss a two-parameter class ification based on this observation which, in future papers, we will relate to empirical classifications. The interplay between the nucleus and the wider galaxy depends critically o n the exchange of radiative and mechanical energy. The outbound mechanical energy transfer is governed by the nuclear stellar cluster. Active galactic nuclei will only be understood once the symbiotic relationships between th e black hole, the stellar cluster and the galaxy are considered. It is impo ssible to treat correctly any isolated component. Our conceptually simple a nd self-consistent symbiotic model explains the observed complexity of acti ve galaxies without ad hoc measures.