Systematic study of gravitational waves from galaxy mergers

Abstract

A systematic study of gravitational waves from galaxy mergers, through N -body simulations, was performed. In particular, we investigated the relative importance of galaxy components (disk, bulge and halo) and effects of initial relative velocity, relative angular momentum and mass ratio of the galaxies. We found that the features of light curve of gravitational waves, such as peak width and luminosity, are reliably simulated with particle numbers larger than ∼ 10 4 . Dominant contribution to gravitational wave emission came from the halo component, while peak luminosity amounted to 10 31 erg/sec for the collision of two halos with mass 3.8 × 10 12 h −1 M⊙. We also found that the initial relative velocity in the direction of the initial separation did not significantly affect gravitational wave emission, while the initial relative angular momentum broadened the peak width and suppressed the luminosity. Mass dependence of the peak luminosity was also investigated, and we obtained evidence that the luminosity is proportional to the cubic mass when the scaling relation is satisfied. This behavior was considered by a simple analysis.PACS numbers:

Stochastic gravitational wave background originating from halo mergers

“…This spectral function must cover a wide range of masses and mass ratios, and in [12] we found a scaling relation of the spectrum with respect to them. The spectrum of gravitational waves from a merger of equal-mass halos can be written as E GW;0 ðf; M 0 Þ, where f and M 0 are the frequency and the fiducial mass.…”

“…Fortunately, as we showed in [12], the initial relative velocity does not affect the gravitational wave emission so much. Specifically, the difference in the emitted energies is about 20% between models with zero initial relative velocity and 220 km=s, which is the maximum initial relative velocity of two gravitationally bound halos.…”

“…In The merger history obtained here provides only evolution of the halo mass and does not give spatial information such as the relative velocity and the angular momentum of halos, which one may consider as important ingredients to estimate the gravitaional wave emission. Fortunately, as we showed in [12], the initial relative velocity does not affect the gravitational wave emission so much. Specifically, the difference in the emitted energies is about 20% between models with zero initial relative velocity and 220 km/s which is the maximum initial relative velocity of two gravitationally-bound halos.…”

“…As the second step, we sum up the gravitational waves from dark halo mergers following the merger history obtained above, using the result of [12] which provides the gravitational-wave spectrum from a single merger as a function of the halo masses. This spectral function must cover a wide range of masses and mass ratios and in [12] we found a scaling relation of the spectrum with respect to them. The spectrum of gravitational waves from a merger of equal-mass halos can be written as E GW,0 (f, M 0 ) where f and M 0 are the frequency and the fiducial mass.…”

“…A large amounts of gravitational waves from the process would be emitted because the process is a highly nonlinear event. In [12], we studied the gravitational waves emitted from a single galaxy merger with N-body simulations. The peak luminosity and total emitted energy were found to reach about 10 31 erg/sec and 10 47 erg, respectively for a collision of two galaxies with masses 3.8 × 10 12 M ⊙ .…”