Supermassive black hole systems

We now turn to the basic phenomenology of supermassive black holes (SMBHs). Although it is artificial to place strict bounds on any definition, the term SMBH usually refers to black holes with a mass greater than $10^5\hbox{$\rm\thinspace M_{\odot}$}$. These objects are always found to reside at the dynamical center of their host galaxy. This is readily understood through the action of dynamical friction [83]. Suppose the SMBH was not located at the center of the host galaxy and, instead, was orbiting within the galaxy's gravitational potential. As the SMBH moves through and gravitationally interacts with the background of stars (which have much lower mass) and dark matter, it gravitationally induces a wake in its trail in which the stellar and dark matter densities are enhanced. The resulting gravitational force between the wake and the SMBH itself acts as a drag force on the SMBH. The effective drag force scales as $M^2$ and hence, for a sufficiently massive black hole, is able to extract energy from the orbit of the SMBH on a fairly short time scale and make it come to rest at the bottom of the gravitational potential well, i.e., at the dynamical center of the galaxy.

Unlike the case of typical stellar mass black holes, it is far from clear how supermassive black holes formed. While it is clear that accretion has been responsible for significant growth of supermassive black holes [84], the origin of the initial massive ``seed'' black holes is highly controversial. The interested reader is pointed to the seminal review by Martin Rees [85].