One of the key quests in modern astrophysics is to understand how galaxies form and evolve. Observations of their luminous constituents, the stars, provide important constraints on this problem. We can barely resolve individual stars in other galaxies, and thus have to rely on their composite, mass-function integrated, spectra. But, with the Milky Way we are in a unique position: one of the most massive members of the Local Group can be studied in great detail from the inside.

Modern telescopes are able to catch light from stars populating all galactic components: the bulge, the disk, and the halo. Using observed spectra and stellar evolution theory, we can determine the chemical composition and the ages of stars. With sufficiently large datasets, we can then reconstruct how chemical enrichment in each of the components varied over time, i.e. what nucleosynthesis processes took place and at which rate the heavy elements were injected to the interstellar medium. This information is complemented with stellar kinematics and forms the observational basis for studies of galactic chemo-dynamical evolution.

The Gaia-ESO Survey (GES) is based on this core idea. The survey is a spectroscopic extension with ground-based telescopes to the Gaia astrometric space mission (ESA). GES has been awarded 300 nights on the Very Large Telescope in Chile, the largest ever allocation on a 8-10m telescope, and will acquire spectra for about 100,000 stars, probing distances as large as 15 kpc. (For comparison: The distance from the Sun to the galactic centre is about 8 kpc or 26,000 light-years.) Fig. 2 shows the coverage map of the Milky Way field. The spectra will be uniformly analysed with state-of-the-art models of stellar atmospheres and stellar evolution to complement the data. The uniqueness of the survey comes from its ability to map the detailed chemistry of all stellar populations homogeneously and with good statistics.

The complete dataset will have enormous potential, but the first interesting results have already been obtained. The preliminary analysis has yielded detailed measurements of ages and metallicities for stars in the solar neighbourhood. Contrary to most previous surveys, we are now able to add new dimensions to this data, for example, the abundances of different elements such as magnesium (Mg).

In the metallicity range -0.5 < [Fe/H] < +0.5, which is typical of the Milky Way thin disk, stars can be found with any age between 1 and 10 Gyrs. But as the metallicity decreases further, the stars become markedly older and show significantly larger abundances of alpha elements, which are synthesized in nuclear alpha-capture reactions. Thus the elevated levels of magnesium are a clear signature that core-collapse supernovae dominate the enrichment of this stellar population. But also stars with low, solar-like [Mg/Fe]-ratios display a large range of ages (Fig. 3). Does this support the current views on the formation of our galaxy? Some models are indeed bolstered by these findings, such as for example, the models with radial migration. These predict a significant spread in magnesium content and age for any given metallicity in the disk, just as observed.

This is just the beginning. In a few years, GES will have accumulated sufficiently large, statistically-significant datasets to make firm statements about stellar evolution and chemical enrichment. They will undeniably set a new point of reference in observational galactic astronomy, perhaps breaking old paradigms and bringing fresh ideas to help understand how our Milky Way came to be.

Maria Bergemann and the Gaia-ESO Survey team

Note:

The GES team includes more than 300 co-Investigators from 90 institutes worldwide. The survey is lead by G. Gilmore (IoA, Cambridge) and S. Randich (INAF-Arcetri). MPA is represented by M. Asplund, M. Bergemann, K. Lind, A. Marino, G. Ruchti, A. Serenelli.

References:

Gilmore, Randich et al., ESO Messenger 147, 2012