Milky Way disc & Bulge in situ populations: ESO white paper - Expanding horizons call

TL;DR

The paper argues that unlocking the Milky Way’s formation history requires a Galaxy-wide, high-precision chemo-dynamical map spanning the disc, bulge, and bar. It proposes a future wide-field, massively multiplexed, 10m-class spectroscopic facility with both high- and low-resolution modes, thousands of high-res and tens of thousands of low-res fibers, and full-sky coverage to depths of $m_G \gtrsim 20$, enabling homogeneous abundances to $<$0.05 dex and precise 3D kinematics. This facility would reconstruct birth radii, radial migration, and the Galactic potential, tying baryonic processes to the Galaxy’s dynamical evolution and providing a benchmark for extragalactic studies. The work highlights the practicality of leveraging existing technologies and data infrastructures to deliver a transformative dataset that informs models of early disc formation, bulge assembly, and the interpretation of high-redshift discs.

Abstract

The formation and evolution of the Milky Way's disc, bar, and bulge remain fundamentally limited by the lack of a contiguous, Galaxy-wide, high-precision chemo-dynamical map. Key open questions - including the survival or destruction of the primitive discs, the origin of the bulge's multi-component structure, the role of mergers and secular processes, and the coupling between stellar chemistry, dynamics, and the Galactic potential - cannot be fully resolved with current or planned facilities. Existing spectroscopic surveys provide either high resolution for small samples or wide coverage at insufficient resolution and depth, and none can obtain homogeneous abundances, precise 3D kinematics, and reliable ages for the millions of stars required, particularly in the obscured midplane, the far side of the bar, or the outer, low-density disc. A new wide-field, massively multiplexed, large-aperture spectroscopic facility, capable of both high- and low-resolution spectroscopy over tens of thousands of square degrees, is therefore essential. Such a facility would deliver the statistical power, sensitivity, and completeness needed to reconstruct the Galaxy's assembly history, constrain its gravitational potential, and establish the Milky Way as the definitive benchmark for galaxy evolution.