The Center for Gravitational Wave Physics at Penn State ran a series of focus sessions in 2002 (see article by Sam Finn in Issue 19 of Matters of Gravity, and Éanna Flanagan in Issue 20). The November 2002 focus session was held the week of the 18th to the 21st, focused on the topic of the astrophysics of the coalescence of supermassive black holes. The session was organised by Ramesh Narayan, chair of the Center's advisory board, Chris Mihos and myself. Focus sessions are, by design, limited to a small number of invited attendees, and student applicants sponsored by their advisors. A web site with most of the presentations made at the meeting is at http://cgwp.gravity.psu.edu/events/MBHMergers/.
The astrophysical processes that can remove energy and angular momentum from a pair of supermassive black holes, bound to each other, but embedded in a surrounding stellar population with significant total mass, have been investigated to various levels of approximation for many decades. Despite a lot of research on the issues, the fundamental problem of how supermassive black holes can transit from the ``dynamical friction regime'' to the ``gravitational radiation dominated regime'' remains intractable. The fundamental problem, as noted by Begelman, Blandford and Rees (1980, see Quinlan 1996 for historical review), is that once the BH binary becomes ``hard'', further increases in binding energy is primarily achieved by ejecting stars, which depletes the population of central stars available to interact with the BH binary. Relaxation processes can generally refill the so-called ``loss-cone'' - the region in phase space where particles can interact strongly with the binary, but only on very long time scales. In simulations, the BH binary ``stalls'' at the ``last parsec'', and additional physics is required if the binary is to merge.
While BH binaries are observed (cf Komossa et al 2003, in fact the announcement of the first confirmed detection of a bound binary supermassive black hole was made, independently by NASA, during the focus session), there is not an obvious, large population of tightly bound binaries in the ``loss-cone'' regime, suggesting that there are processes which enable rapid transit of the last parsec. Candidate processes include rapid refilling of the loss-cone by dynamical processes and hydrodynamical processes. There was also some discussion of whether angular momentum extraction could be efficient, leading to high eccentricity binaries (cf Aarseth 2002).
The session was split into a series of talks reviewing the physical issues and discussing some recent results by some of the groups working on the problem. Peter Bender, Doug Richstone and Tim de Zeeuw reviewed the observational situation; Josh Barnes, Scott Tremaine and myself reviewed some of the dynamical issues; Jeremy Goodman and Ramesh Narayan reviewed the hydrodynamical issues and Martin Haehnelt review the cosmological context and presented some new results on estimated net merger rates in the context of hierarchical galaxy formation. Qingjuan Yu, Pinaki Chatterjee, Priya Natarajan, Milos Milosavljevic, Andres Escala, Kelly Holley-Bockelmann and Sverre Aarseth presented some very interesting new results from recent research.
The meeting finished with a very lively discussion led by Scott
Tremaine, and a review of the science and policy issues by Tom Prince.
Scott Tremaine summarised the work of the meeting, by preparing a list of issues that
are perceived to be decidable in by current research efforts.
We thank Ramesh Narayan and Chris Mihos for their efforts in organizing the meeting.
Aarseth, S.J., ``Black Hole Binary Dynamics'', in ``Fred Hoyle's Universe, eds. G. Burbidge, J.V. Narlikar and N.C. Wickramasinghe, 2003, astro-ph/0210116.
Begelman, M.C., Blandford, R.D. & Rees, M.J., Nature, 287, 307-309, 1980
Komossa, S., Burwitz, V., Hasinger, G., Predehl, P., Kaastra, J.S. & Ikebe, Y., ApJL, 582, L15-19, 2003
Quinlan, G.D., New Astronomy, 1, 35-56, 1996