Recoiling Massive Black Holes in Gas-Rich Galaxy Mergers
by Guedes, Javiera and Madau, Piero and Mayer, Lucio and Callegari, Simone
15 pages, submitted to ApJ
The asymmetric emission of gravitational waves produced during the coalescence of a massive black hole (MBH) binary imparts a velocity “kick” to the system that can displace the hole from the center of its host. Here we study the trajectories and observability of MBHs recoiling in three (one major, two minor) gas-rich galaxy merger remnants that were previously simulated at high resolution, and in which the pairing of the MBHs had been shown to be successful. We run new simulations of MBHs recoiling in the major merger remnant with Mach numbers in the range 1<M<6 km/s, and use simulation data to construct a semi-analytical model for the orbital evolution of MBHs in gas-rich systems. We show that: 1) in major merger remnants the energy deposited by the moving hole into the rotationally supported, turbulent medium makes a negligible contribution to the thermodynamics of the gas. This contribution is more significant in minor merger remnants, potentially allowing for electromagnetic signatures in this case; 2) in major mergers, the drag from high-density gas allows even MBHs with kick velocities of 1200 km/s to remain within 1 kpc from the host's center; 3) kinematically offset nuclei can be observed for timescales of a few Myr in major merger remnants in the case of recoil velocities in the range 700-1000 km/s; 4) in minor mergers remnants the effect of gas drag is weaker, and MBHs with recoil speeds in the range 300-600 km/s will wander through the host halo and may be detectable as spatially-offset active nuclei.