Super-massive black holes (SMBHs) are amongst the most spectacular astrophysical objects in the Universe. This summer it will be a decade since the discovery of the M-sigma relation: the observed correlation between the masses of SMBHs and the velocity dispersions of their host spheroids. This observation revolutionized the field of galaxy formation since it implied a strong interplay between the evolution of black holes and their host galaxies. This discovery also indicates that SMBHs ordinarily dwell in the centers of nearby galaxies.
In Cold Dark Matter cosmologies, galaxies experience multiple mergers during their cosmic assembly. If SMBHs are common features of galaxy centers, and if their host galaxies experience multiple mergers during their lifetime, then SMBH pairs are therefore expected to be recurrent, albeit transient features of most galactic bulges. Observationally, only in the last couple of years double SMBHs have been discovered. If SMBHs are widespread in galaxies, and SMBHs co-evolve with their hosts along the cosmic merger history, why are SMBH pairs so rare, or so difficult to detect? It is well known that standard dynamical friction over a stellar background can not shrink the orbit of the two black holes below ∼ parsec scale (the “final parsec problem”). If pairs were long-lived we should be able to detect them. Nature has clearly found a dynamical route that scientists have not discovered yet.
This workshop will focus on the astrophysics of black holes at the extremes of the SMBH mass function, and their relation their host galaxies, with an emphasis on the dynamics of single and double systems. Some of the topics that will be addressed are:
•Do SMBHs really inhabit all or at least most galaxies? Is there a mass or luminosity threshold below which SMBHs are not expected to form in galaxies?
•Is the relationship between SMBHs and their host related to the morphology of the host galaxy? Since galaxies morphology is believed to depend on their merger history, this question feeds back into the issue of the relationship between galaxy mergers and SMBH mergers.
•What is the fate of SMBHs in galaxy mergers? Do SMBHs merge as efficiently as galaxies do? What are the dynamical drivers of the SMBH orbital decay? Several possibilities have been proposed (e.g., triaxiality, gas-dynamical torques) but no definite answer exists on how SMBH pairs can shrink to sub-parsec scales and merge.
•What are the expected observables? How can we link our theoretical expectations to SMBH pair detection? What would be the smoking gun?