Detectability of Double White Dwarfs in the Local Group with LISA

Detached double white dwarf (DWD) binaries are one of the main science cases for the Laser Interferometer Space Antenna (LISA). As the most numerous LISA sources, they will provide important contributions toward understanding binary evolution, supernovae Type Ia (SNIa) formation channels, and the structure of the Milky Way. So far only detection prospects for the Milky Way have been predicted. In this Letter we show that LISA has the potential to detect DWDs in neighboring galaxies up to the border of the Local Group. In particular, we compute quantitative estimates for the number of detections in M31. We expect between a dozen to several tens of DWDs above the nominal detection threshold, for a mission duration between 4 and 10 years. We show that detectable extra-galactic DWDs include those with the shortest orbital periods (P < 10 minutes) and with the highest chirp masses (${ \mathcal M }\gt 0.6\,{M}_{\odot }$). This subgroup represents candidates for SNIa progenitors. These binaries are virtually undetectable at those distances in optical, implying that LISA could be the best instrument able to provide SNIa merger rates across the Local Group.