One and a half decades of stellar spectroscopic surveys
Deciphering the structure and formation history of our Galaxy will provide important clues to understanding galaxy formation in a broader context. Wide-field multi-object spectroscopic surveys play a particularly important role in the analysis of the Milky Way: Spectroscopy enables us to measure the radial velocity (RV) as a key component of the six-dimensional phase space of stellar positions and velocities, which in turn allows us to study the details of Galactic dynamics. Spectroscopy also facilitates the measurement the abundance of chemical elements in a stellar atmosphere which holds important clues on the initial chemical composition of the interstellar medium and its subsequent metal enrichment.
However, despite the importance of stellar spectroscopy for Galactic Archaeology, the data situation in the early 2000s was far from satisfactory. RVs were listed for some 50,000 stars in the databases of the Centre de Données astronomiques de Strasbourg (CDS), an astonishingly small number compared to the approximately one million spectra available for galaxy redshifts listed at that time. Furthermore, these spectra comprised a very heterogeneous sample in terms of selection, resolution, epoch and Signal-to-Noise. The situation change somewhat with the advent of the Geneva Copenhagen Survey (GCS), which however covered only a sphere
of about 100 pc radius around the sun (the so-called Hipparcos sphere).
RAVE was the first systematic stellar spectroscopic survey focussed on questions of the structure and evolution of the Galaxy. Originally designed mainly to obtain radial velocities, the spectra are of high enough quality to extend the data products to include atmospheric parameters and chemical abundances. A series of 5 public data releases provided an increasing number of targets and increasingly refined data products including atmospheric parameters, and chemical abundances with the final data release, including some 500,000 spectra, being close to publication. The RAVE data releases were complemented by value-added catalogues, including spectrophotometric distances, automated spectral classification as well as catalogues of active stars and of candidates for very metal-poor stars. Furthermore, RAVE has meanwhile been complemented by surveys of similar or even larger size at lower (SEGUE, LAMOST) and higher spectral resolution (APOGEE, GaiaESO, GALAH). Future surveys like SDSS-V, WEAVE, PFS and 4MOST will further extend our census of the Milky Way, eventually reaching tens of millions of spectra.