GCDS SURVEY SCIENCE
2 - 4 November, 2010 - Nice France -
Gaia will provide an unprecedented map of the positions, kinematics and spectrophotometry of a billion stars in the Milky Way, allow a quantum leap on our knowledge of the Galaxy and its stellar populations. We will be able to dissect the different Galactic components and confront this detailed structure with models of galaxy formation and their cosmological context. The full three dimensional velocity vector of a large number of stars, and their detailed chemical composition will allow to address crucial questions such as the amount and distribution of dark matter in the Galaxy and the formation and evolution of the different Galactic components. Nonetheless, our picture of the Milky Way will be hampered in two ways: the low resolution spectrograph on-board (RVS) will not reach the same limiting magnitude as the rest of the Gaia instruments and its resolution (R=11 500 for stars brighter than about 14 and R=6 500 for fainter stars) and small wavelength coverage, are not well suited for detailed studies of elemental abundances.
Indeed, the radial velocity measurements for faint stars (i.e. distant, but also the much more numerous less evolved stars) do not match the accuracies obtained for the tangential motions (proper motions), and in particular, for very faint stars (V > 17) no radial information will be available. As a consequence, the dynamical map of the Milky Way will be incomplete and restricted to the nearby few kiloparsecs limiting the scientific impact on certain high-priority questions.
Furthermore, the metallicity measurements obtained from the low resolution spectra means that the metallicity distribution function will be available from Gaia only within this sphere of influence, leading to a highly incomplete, and biased, view of the Milky Way.
Secondly, to underpin the evolution of the Milky Way requires knowledge of a time variable, which can be provided by detailed elemental abundance studies. This is much needed to understand the star formation, chemical evolution history and the coupling to the dynamics, all processes that have determined the way the Milky Way looks today. Since elemental abundances (α-elements which tell about the supernova type II, iron-peak elements which are related mostly to supernova type Ia, r- and s- process which measure the impact of winds from evolved stars) are either not provided at all by Gaia, or restricted to a small number of elements (Mg, Si, Ca, Ti, and Fe) in a limited sample of the cool stars brighter than about 12.
For all of the above reasons, wide field high resolution spectroscopic surveys from ground based facilities appear as a necessary complement to guarantee the maximum scientific throughput of the Gaia mission. This need has been recognised in a number of reports, and highlighted in the Science Vision report by Astronet: “It is crucial to supplement the Gaia data-set with dedicated ground-based spectroscopic programmes, in order to obtain the radial velocity and detailed chemical abundances for fainter stars.”
The recent efforts by the GREAT network and in particular its GREAT Chemical Tagging Working Group (WGA3), and subsequent discussions with a number of other GREAT WGs (e.g. WGA1) has resulted in the proposal to form an inclusive consortium to undertake an ambitious Gaia Chemo-Dynamical Survey (GCDS) of the Milky Way. The kick-off meeting took place in Paris on the 27th of April 2010, and efforts are ongoing to lay the foundations of this very large collaborative effort. It is now time to open the discussions to a wider community, and the proposed workshop will bring together the galactic archaeology, galactic dynamics, stellar physics communities to lay out and discuss the main science cases for the GCDS, and their best implementation across the various existing and planned instrumentation available to Europeans.
Particular emphasis will be given to answers to the call for Large Public spectroscopic Survey announced by ESO.
The workshop will be organised in sessions, following the four Science Experiments (SEs) that have been identified :
• SE1: Mass Distribution of the Galaxy
• SE2: How did galaxies form as traced by chemistry
• SE3: How clusters and stars formed and evolved
• SE4: Additional Science (includes various science topics that will benefit from the survey, mainly in stellar physics -variable stars, hot stars, …)