Stochastic gravitational wave background anisotropies in the mHz band: astrophysical dependencies

Abstract

We show that the anisotropies of the astrophysical stochastic gravitational wave background in the mHz band have a strong dependence on the modelling of galactic and sub-galactic physics. We explore a wide range of self-consistent astrophysical models for stellar evolution and for the distribution of orbital parameters, all calibrated such that they predict the same number of resolved mergers to fit the number of detections during LIGO/Virgo O1+O2 observations runs. We show that different physical choices for the process of black hole collapse and cut-off in the black hole mass distribution give fractional differences in the angular power spectrum of anisotropies up to 50% on all angular scales. We also point out that the astrophysical information which can be extracted from anisotropies is complementary to the isotropic background and individual mergers. These results underline the interest in the anisotropies of the stochastic gravitational wave background as a new and potentially rich field of research, at the cross-road between astrophysics and cosmology.

Detecting the anisotropic astrophysical gravitational wave background in the presence of shot noise through cross-correlations

“…spatial shot noises of auto and cross-correlations (Eqs. (14)(15)(16)) have exactly the same expressions (up to normalisation factors 4π/ ΩGW ). In detail this is not exactly the case since the full expressions for galaxy numbers and for the GW background also involve sub dominant metric and velocity contributions, as well as the dominant galaxy overdensity term in eqs.…”

“…This is an intrinsic (irreducible) background. The shot noise in the LISA band will therefore only be due to the discreteness in space of the GW sources, and will be a subdominant contribution to the total error budget (see also [16]).…”

“…(3), ( 5) and ( 6) which characterize the statistical properties of the data [δ Ω GW (e), ∆ G (e)]. Measuring these spectra can give us a wealth of information about the underlying processes that lead to the generation of gravitational waves in the late Universe, see [9,16].…”

High angular resolution gravitational wave astronomy

“…The latest observational upper limits from the first and second Advanced LIGO runs [4] are derived for multipoles only up to = 4 and are several orders of magnitude above current theoretical predictions [45][46][47]80]. Major advances both in detector sensitivity and in analysis methods are required in order to be able to detect the anisotropic component of the stochastic background.…”

“…Astrophysical sources that contribute to the stochastic background in the millihertz-decihertz frequency range (stellar-mass compact binaries, intermediate-mass BH binaries, extreme mass ratio inspirals) reside in galaxies, and it is therefore expected that the intensity of the background will depend on sky direction, analogously to the cosmic infrared background. As shown in [46,47], both the amplitude and the shape of the astrophysical component of the stochastic background anisotropies depend on the formation and evolution processes of binary compact objects (such as the properties of their stellar progenitors, supernova explosion mechanism etc.). Crucially, the detection of individual merging binaries will only provide information on the brightest sources in the population, in contrast to the stochastic background.…”