Galactic Archaeology with RAVE
RAVE is an observational program in the field of Near Field Cosmology which exploits our position inside the Milky Way to study its formation and evolution in greater detail than is possible for other galaxies. RAVE focuses on obtaining stellar radial velocities to study the motions of stars in the Milky Way’s thin and thick disk and stellar halo. The vast majority of stars in our Galaxy have no velocity measurements, and particularly no time-consuming radial velocity measurements. RAVE utilizes fiber optics to perform multiple, simultaneous spectroscopy on up to 150 stars in a single observation. In this way it can obtain a representative sample of the nearby stars in our Galaxy which are all around, over a wide area of the sky.
Most of the stars observed by RAVE are between 1500 and 13000 light years from the sun and so RAVE surveys halfway to the Galactic center and halfway to the edge of the Galaxy’s disk. Left: Fly by movie showing the distribution of RAVE stars compared to a model of the Milky Way disk. In blue are dwarf stars, in red the much brighter giant stars.
The radial velocity is the speed with which a star moves towards us or away from us. Any motion in space consists of two parts: the velocity on the celestial sphere which is perpendicular to the line of sight, and the radial velocity which is parallel to the line of sight. The radial velocity is being measured by RAVE for up to 1,000,000 stars.
Astrometric space missions like HIPPARCOS (finished) and GAIA (planned) provide the exact positions of a star on the celestial sphere, the distance from the Earth and the motion on the celestial sphere. The missing component – the radial velocity – is measured by RAVE.
The motion projected onto the celestial sphere is called “proper motion” of a star. While the measurement of this needs precise positions for several years, the radial velocity requires a very precise spectrum of the star. RAVE uses the Doppler effect on spectral lines for the determination of the velocity along the line of sight. The spectral range investigated is 840-875 nm, where the Doppler shift of Ca triplet lines is measured. This spectral range is also favored by the GAIA mission.
The survey: observations & reductions
Since the end of the 6dF Galaxy Survey in 2004, the UK Schmidt telescope at Siding Spring has been dedicated to the RAVE survey. This followed the first year of the pilot phase of the survey (April 2003 – April 2004), where RAVE observed around the full moon period. RAVE exploits the wide field-of-view of the UK Schmidt (6 degrees on the sky) as well as the capabilities of the UK Schmidt’s 6dF instrument. The multi-object spectrograph 6dF (6 degree Field) utilizes interchangeable field plates with robot-positional optical fibers, with 150 fibers each. Each fiber is attached to a `button’ which is positioned by the robot (to a precision of 10 microns) so as to capture the light from one target star.
Once the field plate is configured it matches the pattern of stars on the sky that it is observing. Three field plates are available, which means that one plate can be configured by the robot while another is being used in the telescope. From the telescope the light is sent down the optical fibers to the spectrograph unit to be dispersed through a diffraction grating and then recorded on a 1056 X 1027 CCD chip.
An initial quality check of the data is performed at Macquarie University in Australia and the results are sent to the University of Padova, Italy for data reduction. The reduced data is then sent to the Leibniz-Institut für Astrophysik Potsdam in Germany for final extraction of the radial velocities and the other stellar parameters.
Studies from RAVE either concentrate on peculiar stars and objects or overall trends for the different components of our Galaxy, with a main focus of the structure and formation of the Milky Way.
RAVE is suited to searching for stellar streams, some of which are remnants of dwarf galaxies that merged with the Milky Way during galaxy formation. A new stream, dubbed the Aquarius stream as it lies in the constellation of Aquarius, is an example of such remnants that can be found with RAVE (Williams et at. 2011). Another kind of stellar stream, known as moving groups, are born inside our Galaxy. New members of nearby moving groups have been found in RAVE (Kiss et al. 2010) and the survey promises to reveal more in the future. Lastly, Kunder et al. 2015 was the first to use RAVE to successful search for streams, tidal debris and extra-tidal stars around globular clusters; RAVE stars have been used to show extended tidal debris around the globular clusters NGC 3201 (Anguiano et al. 2016) and Omega Cen (Fernández-Trincado et al. 2015).
With RAVE we can efficiently search for the very first stars (Fulbright et al. 2010) or detect interesting trends in the motions of the stars in the region around the sun (Siebert et al. 2011). The survey is also a great tool to study our Galaxy’s thick disk, which sandwiches the thin disk in which we reside. Two recent studies from RAVE, Wilson et al. 2010 and Ruchti et al. 2010, have focussed on uncovering the thick disk’s origin, which is still poorly understood by astronomers.