A black hole expelled from the center of its galaxy after a merger could trigger several detectable phenomena around it.
The scenario begins with two massive black holes coming together at the heart of a galaxy. When they merge, they emit gravitational waves, distortions in space that carry energy. If this emission is stronger in one direction, the resulting black hole receives a push in the opposite direction. It may then leave the galactic center.
Erwan Hochart and Simon Portegies Zwart simulated this abrupt departure in a very dense cluster of stars surrounding the black hole. Their models use black holes with masses of 100,000 and 400,000 Suns. Their simulations impose recoil speeds of 300 or 600 km/s, then track the movements of nearby objects for 100,000 years.
The movement immediately changes the trajectories of the stars around the black hole. Some also pass close enough to be torn apart by its gravity. Such a phenomenon, called a tidal disruption event, releases matter that could produce a luminous signal. The simulation predicts a rapid increase in the number of these encounters after the recoil.
Other very dense objects present in the cluster may also be drawn toward the wandering black hole. Thus, the same initial movement could generate several categories of signals, at positions offset from the center of the galaxy where the merger occurred.
In the simulations, more events occur when the black hole is more massive, when its recoil speed is lower, and when the stars are highly concentrated near the center. The initial distribution of the stars therefore becomes crucial for predicting what an observatory might detect.
The authors also estimate how many such events could be observable in the Universe. Depending on their assumptions about the concentration of stars, the maximum estimate varies considerably. It reaches about 290 events per year in a dense scenario, compared with about 30 for a less concentrated distribution.
This dependence could turn future signals into tools for observing the immediate surroundings of black holes. Their number and distribution would provide clues about the density of stars after a merger and the speed of the recoil.