Supermassive Black Hole Research in the Post-HST Era

Thanks to its unprecedented spatial resolution, the Hubble Space Telescope has ended a 20-year long stalemate by detecting the dynamical signature of nuclear supermassive black holes (SBHs) in a sizeable number of nearby galaxies. These detections have revealed the existence of a symbiotic relationship between SBHs and their hosts, changing the way we view SBH and galaxy formation. In this contribution I review which are the most pressing outstanding issues in SBH research, and what are the technological requirements needed to address them. 1. The Current Status of SBH Searches The study of supermassive black holes is one of the areas of modern astrophysics which has benefited most from the launch of HST. After two decades of tantalizing but inconclusive ground-based studies, the HST/FOS observations of M87 (Harms et al. 1994) and NGC 4261 (Ferrarese et al. 1996) provided the first firm measurements of SBH masses in galactic nuclei. In the years that followed, FOS and STIS data lead to detections in ten additional galaxies (Bower et al. 1998; van der Marel & van den Bosch 1998; Ferrarese & Ford 1999; Emsellem et al. 1999; Cretton & van den Bosh 1999; Verdoes Kleijn et al. 2000; Gebhardt et al. 2000a; Joseph et al. 2001; Barth et al. 2001; Sarzi et al. 2001). The superiority of HST over ground based facilities in this field is easily understood. Only dynamical evidence, either from gas or stellar kinematics, can yield compelling proof of the existence of SBHs. With rare exceptions (e.g. M31, M87), ground based telescopes lack the spatial resolution necessary to resolve the SBH “sphere of influence”, i.e. the region of space within which the SBH gravitational influence dominates that of the surrounding stars: rh = GM•/σ 2 ∼ 11.2(M•/10 8 M⊙)/(σ/200 km s ) pc, (1) with σ the stellar velocity dispersion and M• the SBH mass. Resolving rh is a necessary condition for a SBH detection to be made; not meeting it leads to spurious detections and biased masses (Merritt & Ferrarese 2001a). With over a dozen secure measurements, it has become possible to search for correlations between M• and the overall properties of the host galaxies. The first relation to emerge was one between M• and the blue luminosity LB of the surrounding bulge (Kormendy & Richstone 1995). A much tighter correlation