The Galactic White Dwarf Population

The ESA Gaia mission has revolutionized our understanding of the white dwarf population, delivering an unprecedented census of these nearby remnants and revealing previously unseen structures in the Hertzsprung-Russell (HR) diagram. However, while Gaia has expanded the scope of white dwarf astrophysics, it has also exposed new questions related to atmospheric composition, spectral evolution, crystallization, magnetism, and merger-driven pathways. Many of these open problems are encoded in the detailed morphology of the Gaia HR diagram, where precise spectroscopic characterization is essential for interpreting the underlying physical processes. Spectroscopic characterization, obtainable with current and future ESO facilities, can provide the effective temperatures and surface gravities that are required to derive accurate white dwarf masses, cooling ages, and luminosities. These fundamental parameters not only enable studies of spectral evolution, interior physics, and the origin of magnetic and high-mass white dwarfs, but also guarantee the construction of robust mass distributions and luminosity functions, essential for constraining the initial-to-final mass relation, probing the initial mass function, and reconstructing the star formation history of the local Galaxy, among other applications. Looking toward the 2040s, future multi-fiber spectrographs operating in survey mode on 10--15 meter class telescopes will be able to collect a complete spectroscopic sample of white dwarf, enabling the detailed characterization of their population. Achieving spectroscopic completeness for the nearby Galactic population and securing high signal-to-noise, moderate-to-high resolution spectra across the HR diagram with ESO instrumentation will be critical steps toward resolving these longstanding questions in white dwarf astrophysics.