Introduction

How disc galaxies form and evolve, and how their component stars and stellar populations form and evolve, are among the most fundamental questions in contemporary astrophysics. The high statistical weight (105 stars) spectroscopic Gaia-ESO survey (Gilmore, Randich et al., 2012, The Messenger, 147, 25) will contribute to answering these key questions. The survey has enormous stand-alone value, sampling all the main components of the Milky Way, from star-forming regions to ancient halo stars. However, when combined with Gaia astrometry (2016) and Gaia spectrophotometry and improved stellar parameters (2018), the Gaia-ESO products will revolutionise our knowledge of the formation and evolution of the Galaxy and its stars.

Understanding how galaxies actually form and evolve within our ΛCDM universe continues to be an enormous challenge (Peebles, 2011, Nature, 469, 305; Kormendy et al., 2010, ApJ, 723, 54). Theoretical models of galaxy formation, which necessarily involve modeling star formation and stellar evolution, rely more heavily on phenomenological models than on physical theory: these models require calibration with well-studied (nearby) test cases. Thus, observations are crucial to learning how galaxies and stars were formed and evolved, and what their structure now is. Observations of objects at high redshifts and long look-back times are important for this endeavour, as is detailed examination of our Galaxy, because such “near-field cosmology” gives insights into key processes that cannot be obtained by studying faint, poorly resolved objects with uncertain futures.

Just as the history of life was deduced by examining rocks, we expect to deduce the history of our Galaxy by examining its stars. Stars record the past in their ages, compositions, and in their kinematics. For example, individual accretion and cluster dissolution events can be inferred by detecting stellar streams from accurate phase-space positions. Correlations between the chemical compositions and kinematics of field stars will enable us to deduce the history of star formation and even the past dynamics of the disc. The kinematic structure of the bulge will reveal the relative importances in its formation of disc instability and an early major merger. Star clusters provide a snapshot of stellar evolution - thus studying open clusters at different ages and chemical compositions is crucial to understanding fundamental issues in stellar evolution, the star formation process, and the assembly and evolution of the Milky Way thin disc (Bland et al., 2010, ApJ, 713, 166).

The scale of the challenge of mapping the chemical elements, spatial distributions and kinematics (and ages, from Gaia) of distinct Galactic populations is enormous. Progress in formation and evolution of the Galaxy and its component stars and populations requires a spectroscopic survey returning data for a sample of ≥ 105 field stars and at least 100 open clusters . The Gaia-ESO Survey is that survey. It will also be the first survey yielding a homogeneous dataset for field and cluster stars, providing unique added value.

Page last updated: 02/09/2013