The Milky Way'S Bar Structural Properties From Gravitational Waves

The Laser Interferometer Space Antenna (LISA) will enable Galactic gravitational wave (GW) astronomy by individually resolving >10(4) signals from double white dwarf (DWD) binaries throughout the Milky Way. Since GWs are unaffected by stellar crowding and dust extinction unlike optical observations of the Galactic plane, in this work, we assess for the first time the potential of LISA to map the Galactic stellar bar and spiral arms. To achieve this goal, we combine a realistic population of Galactic DWDs with a high-resolution N-body Galactic simulation in good agreement with current observations of the Milky Way. We then model GW signals from our synthetic DWD population and reconstruct the structure of the simulated Galaxy from mock LISA observations. Our results show that while the low-density contrast between the background disc and the spiral arms hampers our ability to characterize the spiral structure, the stellar bar will clearly appear in the GW map of the bulge. The axial ratio derived from the synthetic observations agrees within 1 sigma with the reference value, although the scale lengths are underestimated. We also recover the bar viewing angle to within 1 degrees and the bar's physical length to within 0.2 kpc. This shows that LISA can provide independent constraints on the bar's structural parameter, competitive compared to those from electromagnetic tracers. We therefore foresee that synergistic use of GWs and electromagnetic tracers will be a powerful strategy to map the Milky Way's bar and bulge.