UNDERSTANDING BLACK HOLE MASS ASSEMBLY VIA ACCRETION AND MERGERS AT LATE TIMES IN COSMOLOGICAL SIMULATIONS

Accretion is thought to primarily contribute to the mass accumulation history of supermassive black holes (SMBHs) throughout cosmic time. While this may be true at high redshifts, at lower redshifts and for the most massive black holes (BHs) mergers themselves might add significantly to the mass budget. We explore this in two disparate environments-a massive cluster and a void region. We evolve SMBHs from 4 > z > 0 using merger trees derived from hydrodynamical cosmological simulations of these two regions, scaled to the observed value of the stellar mass fraction to account for overcooling. Mass gains from gas accretion proportional to bulge growth and BH-BH mergers are tracked, as are BHs that remain orbiting due to insufficient dynamical friction in a merger remnant, as well as those that are ejected due to gravitational recoil. We find that gas accretion remains the dominant source of mass accumulation in almost all SMBHs; mergers contribute 2.5% +/- 0.1% for all SMBHs in the cluster and 1.0% +/- 0.1% in the void since z = 4. However, mergers are significant for massive SMBHs. The fraction of mass accumulated from mergers for central BHs generally increases for larger values of the host bulge mass: in the void, the fraction is 2% at M-*,(bul) = 10(10) M-circle dot, increasing to 4% at M-*,(bul) greater than or similar to 10(11)M(circle dot), and in the cluster it is 4% at M-*, (bul) = 10(10) M-circle dot and 23% at 10(12) M-circle dot . We also find that the total mass in orbiting SMBHs is negligible in the void, but significant in the cluster, in which a potentially detectable 40% of SMBHs and approximate to 8% of the total SMBH mass (where the total includes central, orbiting, and ejected SMBHs) is found orbiting at z = 0. The existence of orbiting and ejected SMBHs requires modification of the Soltan argument. We estimate this correction to the integrated accreted mass density of SMBHs to be in the range 6%-21%, with a mean value of 11% +/- 3%. Quantifying the growth due to mergers at these late times, we calculate the total energy output and strain from gravitational waves emitted by merging SMBHs, and obtain a signal potentially detectable by pulsar timing arrays.