ABSTRACT

I discuss the various processes which may affect the transfer of angular momentum in gas within the inner parsec surrounding a supermassive black hole. Even after gas has been brought into the gravitational sphere of influence of the black hole, it still has 100–1000 times too much angular momentum to reach the event horizon. Angular momentum loss cannot be accommodated in a scaled-up standard thin accretion disk model, because local self-gravitational instabilities will lead to fragmentation of the disk, which will decrease the efficiency of angular momentum transport. Instead, some nonlocal mechanism or “external” trigger for angular momentum transfer is needed, such as a large-scale magnetized wind, stirring by winds from massive stars and supernovae, or global gravitational instabilities.

INTRODUCTION

Most participants in this conference seem to agree that large-scale, non-axisym-metric gravitational disturbances can be very effective in transferring angular momentum on scales of hundreds of parsecs or larger. These disturbances might be driven by tidal encounters or may be “self-starting”, as in the “bars-in-bars” scenario. What is less certain is whether similar mechanisms can be effective all the way into the nucleus, say, into the inner parsec surrounding a supermassive black hole. Gravitational triggers for inflow usually involve at least a mild form of self-gravitational instability, which requires that the stars plus gas constitute a significant fraction of the total mass enclosed within the region under consideration (see, e.g., Friedli, these proceedings).