On the dark side of quasar evolution

Recent improved determinations of the mass density ρBH of supermassive black holes (SMBHs) in the local universe have allowed accurate comparisons of ρBH with the amount of light received from past quasar activity. These comparisons support the notion that local SMBHs are "dead quasars" and yield a value epsilon ≳ 0.1 for the average radiative efficiency of cosmic SMBH accretion. BH coalescences may represent an important component of the quasar mass assembly and yet not produce any observable electromagnetic signature. Therefore, ignoring gravitational wave (GW) emission during such coalescences, which reduces the amount of mass locked into remnant BHs, results in an overestimate of epsilon. Here we put constraints on the magnitude of this bias. We calculate the cumulative mass loss to GWs experienced by a representative population of BHs during repeated cosmological mergers, using loss prescriptions based on detailed general relativistic calculations. Despite the possibly large number of mergers in the assembly history of each individual SMBH, we find that near-equal mass mergers are rare; therefore, the cumulative loss is likely to be modest, amounting at most to a 20% increase in the inferred epsilon value. Thus, recent estimates of epsilon ≳ 0.1 appear robust. The space interferometer LISA should provide empirical constraints on the dark side of quasar evolution by measuring the masses and rates of coalescence of massive BHs to cosmological distances.