It is thought that galaxy mergers may play a significant role in the triggering of Active Galactic Nuclei (AGN). Tidal torques and shocking result in galactic-scale flows of gas toward the center of the galaxy. This gas can then feed onto the supermassive black hole at the galaxy center, triggering an AGN. While there is observational support for this hypothesis, many of the details of the process are not understood. Perhaps the largest problem for AGN fueling is presented by angular momentum. Gas at galactic distances from the black hole typically has to loose very large amounts of angular momentum in order to be accreted on the black hole.
The figure on the left shows the gas within 0.5 kpc in a merger remnant 0.5 Gyr after the final coalescence. Arrows indicate direction and magnitude of the velocities of the gas. Black and green arrows represent particles from progenitor one and two respectively. The histogram depicts the distribution of angular momentum of the gas particles within 0.5 kpc with respect to the critical angular momentum. Particles below the critical angular momentum (j/2cRs < 1) would readily accrete onto the central black hole. However, the vast majority of the particles have j>>jcrit. These particles will form a toroidal cloud and will not accrete unless some mechanism of angular momentum loss is introduced (such as viscosity or magnetic fields).
In a project with Peter Anninos, Jay Salmonson, and Wil van Breugel of Lawrence Livermore National Laboratory I am working to understand some of these difficulties. We are using gas distributions from GADGET2 galaxy merger simulations as boundary conditions for more detailed simulations of black hole accretion. The more detailed simulations are being run using the COSMOS++ code developed by Anninos. Preliminary results show that the gas from the mergers will not accrete onto the central black hole unless some mechanism for angular momentum transport is included. Currently we are examining the effects of various potential mechanisms.
