LISA's view of the Galactic Halo: forecasts for the Galactic double white dwarf population using Gaia data

Abstract

The population of close double white dwarfs (DWDs) in the Milky Way will make up the largest population of sources resolved by LISA, with a subset of the population having three-dimensional position and chirp mass measurements obtained from LISA observations. Because white dwarfs are the bulk of the Milky Way's stellar remnant population, the positions and masses of close DWDs resolved by LISA are defined by the stellar population properties that host them. Recent Gaia data has unveiled a triaxial accreted component of the Galactic stellar halo: the Gaia-Sausage-Enceladus, which contains stars that are more metal-rich than the extremely metal-poor population of stars residing in the stellar halo beyond 30 kpc. In this work, we assess the size and characteristics of the population of close DWDs using an empirically motivated Galactic model which incorporates the GSE and compare to the classical Galactic model that contains only a single very metal-poor halo population. To do this, we simulate a realistic present-day Galactic DWD population and determine its gravitational wave signal in LISA using LEGWORK. We find that incorporating the metal-rich population from the GSE imprints significant differences in the chirp mass and distance distributions of resolved DWDs, but that the strength and height of the gravitational wave foreground remains unchanged.