Relationship of Black Holes to Bulges

Supermassive black holes appear to be uniquely associated with galactic bulges. The mean ratio of black hole mass to bulge mass was until recently very uncertain, with ground based, stellar kinematical data giving a value for 〈M•/Mbulge〉 roughly an order of magnitude larger than other techniques. The discrepancy was resolved with the discovery of the M• − σ relation, which simultaneously established a tight corrrelation between black hole mass and bulge velocity dispersion, and confirmed that the stellar kinematical mass estimates were systematically too large due to failure to resolve the black hole’s sphere of influence. There is now excellent agreement between the various techniques for estimating 〈M•/Mbulge〉, including dynamical mass estimation in quiescent galaxies; reverberation mapping in active galaxies and quasars; and computation of the mean density of compact objects based on integrated quasar light. All techniques now give 〈M•/Mbulge〉 ≈ 10−3 and ρ• ≈ 3×10M⊙/Mpc. Implications of the M• − σ relation for the formation of black holes are discussed.