The Contribution of Halo White Dwarf Binaries to the LISA Signal

TL;DR

This study uses population synthesis models to accurately estimate the halo white dwarf population's gravitational wave signal, finding it negligible compared to the disk and bulge contributions for LISA detection.

Contribution

First calculation of halo white dwarf population's gravitational wave signal using population synthesis, challenging previous assumptions of a dominant halo contribution.

Findings

Halo white dwarf signal is lower than disk+bulge signal by a factor of 10.

Halo contribution remains negligible even with maximum baryonic mass assumptions.

Halo white dwarfs do not significantly affect LISA's detection of other sources.

Abstract

Galactic double white dwarfs were postulated as a source of confusion limited noise for LISA, the future space-based gravitational wave observatory. Until very recently, the Galactic population consisted of a relatively well studied disk population, a somewhat studied smaller bulge population and a mostly unknown, but potentially large halo population. It has been argued that the halo population may produce a signal that is much stronger (factor of ~5 in spectral amplitude) than the disk population. However, this surprising result was not based on an actual calculation of a halo white dwarf population but was derived on (i) the assumption that one can extrapolate the halo population properties from those of the disk population and (ii) the postulated (unrealistically) high number of white dwarfs in the halo. We perform the first calculation of a halo white dwarf population using population synthesis models. Our comparison with the signal arising from double white dwarfs in the Galactic disk+bulge clearly shows that it is impossible for the double white dwarf halo signal to exceed that of the rest of the Galaxy. Using microlensing results to give an upper limit on the content of white dwarfs in the halo (~30 % baryonic mass in white dwarfs), our predicted halo signal is a factor of 10 lower than the disk+bulge signal. Even in the implausible case where all of the baryonic halo mass is found in white dwarfs, the halo signal does not become comparable to that of the disk+bulge, and thus would still have a negligible effect on the detection of other LISA sources.