ICCUB researchers and Gaia team members Teresa Antoja and Friedrich Anders are part of the international collaboration.
The Radial Velocity Experiment RAVE is a spectroscopic survey of stars in the southern hemisphere. As the first stellar survey using a multi-object spectrograph, it is ground-breaking in the field of Galactic Archaeology and has facilitated the activity of other ground-based surveys such as WEAVE, and influenced space missions such as Gaia.
For more than a decade RAVE, one of the first and largest systematic spectroscopic surveys, studied the motion of Milky Way stars. The RAVE collaboration now published the results for over half a million observations in its 6th and final data release. RAVE succeeded in measuring the velocities, temperatures, compositions and distances for different types of stars. The unique database enables scientists to systematically disentangle the structure and evolution history of our Galaxy.
RAVE was designed to get a representative census of the movements and the atmospheric properties of stars in the wider neighbourhood of the Sun. Using spectroscopy, the light of a star is decomposed into its rainbow colours. By analysing the spectra, the radial velocity of a star – the movement of the stars in the direction of the observer's view, can be determined. Furthermore, stellar spectra also enable scientists to determine stellar parameters like temperatures, surface gravities, and composition. To trace the structure and shape of our galaxy, RAVE successfully measured 518,387 spectra for 451,783 Milky Way stars.
Astronomers are not only used to think in long time scales – their projects also are often many-year endeavours. RAVE observed the sky for almost every clear night between 2003 and 2013 at the 1.2-metre UK Schmidt telescope of the Anglo-Australian Observatory in Siding Spring, Australia. RAVE utilized a dedicated fibre-optical setup to simultaneously take spectra of up to 150 stars in a single observation. Only with this massive multiplexing, such a large number of targets was achievable – the largest spectroscopic survey before RAVE featured only some 14000 targets. In this way, the survey obtained a representative sample of the stars around our Sun that are located roughly in a volume 15000 light-years across.
15 years of observations
Over the past 15 years, an increasing number of stars and refined data products have been released. The final RAVE data release not only provides for the first time the spectra of all-stars in the RAVE sample; the stars were also matched with stars from the DR2 catalogue of the satellite mission Gaia. Thanks to the exquisite distances and proper motions measured by Gaia, considerably improved stellar temperatures, surface gravities and the chemical composition of the stellar atmospheres could be derived.
“RAVE allowed us to study the three-dimensional velocity distribution of the Galactic disc”, says ICCUB astronomer Teresa Antoja. She worked for three years in the scientific analysis of RAVE, during her postdoctoral stay at the University of Groningen. She also supervised a thesis on the dynamics of the galactic disc, using RAVE simulations and data.
Researchers already knew about several large groups of stars moving together through the solar environment. Initially, some of these groups were thought to share a common birthplace, but this was incompatible with the ages and chemical abundances measured for these stars. The RAVE data helped to reinforce another hypothesis. “Dynamical streams and comoving groups of stars are among the most interesting objects that we observed with RAVE”, Antoja explains, “Thanks to the RAVE data we could see that far away from the Sun similar streams exist and that their motions change with their position in the Galaxy. We could find that the way these streams change is compatible with the gravitational effects of the Galaxy’s bar, which influences the motions of stars not only in the inner galaxy but also in the solar neighbourhood”. Recently, astronomers have seen that the stellar streams might also be caused by external galaxies approaching the Milky Way, or by the spiral arms of our galaxy, so they continue to gather data from distant stars to solve the puzzle.
Some of the key results of RAVE also include; the determination of the minimum speed needed for a star to escape the gravitational pull of the Milky Way; the confirmation that dark matter, an invisible component of the Universe of yet unknown nature, dominates the mass of our Galaxy. With RAVE it could be shown that the Milky Way disk is asymmetric and that the chemical element abundances of the observed stars hold important clues to the chemical composition of the interstellar medium, traced by stars of different ages and metallicities.
What comes next?
After RAVE, other surveys such as WEAVE or SDSS will continue mapping the sky from the Earth. Since it first launched in 2013, the ESA's Gaia mission has been covering not only the southern hemisphere but the full sky. Until now, it has produced two data releases (DR1 and DR2), and the third one will be published in 2021. This third data release will include millions of spectra from the Gaia/RVSinstrument that will be very similar to the ones obtained by RAVE.
The RAVE collaboration consists of researchers from over 20 institutions around the world and is coordinated by the Leibniz-Institut für Astrophysik Potsdam AIP. More than 100 refereed scientific articles based on RAVE data were published since the first data release.
Media gallery and videos at RAVE Survey
RAVE data release at the Astrophysical Journal
The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- I: Survey Description, Spectra and RadialVelocities
The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- II: Stellar Atmospheric Parameters, Chemical Abundances and Distances
Related RAVE articles
Antoja T., et al. 2014. Constraints on the Galactic bar from the Hercules stream as traced with RAVE across the Galaxy
Antoja T., et al. 2012. Kinematic groups beyond the solar neighbourhood with RAVE