Gravitational waves from galaxy encounters

We discuss the emission of gravitational radiation produced in encounters of dark matter galactic halos. To this aim we perform a number of numerical simulations of typical galaxy mergers, computing the associated gravitational radiation waveforms as well as the energy released in the processes. Our simulations yield dimensionless gravitational wave amplitudes of the order of ${10}^{-13}$ and gravitational wave frequencies of the order of ${10}^{-16}\mathrm{Hz}$, when the galaxies are located at a distance of 10 Mpc. These values are of the same order as those arising in the gravitational radiation originated by strong variations of the gravitational field in the early Universe, and therefore, such gravitational waves cannot be directly observed by ground-based detectors. We discuss the feasibility of an indirect detection by means of the $B$-mode polarization of the cosmic microwave background (CMB) induced by such waves. Our results show that the gravitational waves from encounters of dark matter galactic halos leave much too small an imprint on the CMB polarization to be actually observed with ongoing and future missions.

Figure 1

Dynamical evolution of the three merger simulations as seen from a point of view parallel to the orbital angular momentum. Left column: M1, middle column: M2, right column: M3. Top row initial conditions, middle row right after the first pass through pericenter, bottom row at the end of the simulation.

Figure 2

Gravitational waveform for the three models: M1 (top), M2 (middle), and M3 (bottom). Only the “$+$” polarization mode is shown. Galaxies are located at $D=10\mathrm{Mpc}$ from the observer.

Figure 3

Gravitational wave luminosity as a function of time for the three models: M1 (top), M2 (middle), and M3 (bottom).