Probing the Milky Way galaxy through thick and thin (discs and halo) with the Correlation Radial Velocities (CORAVEL) and Radial Velocity Experiment (RAVE) surveys

Seabroke, George (2008). Probing the Milky Way galaxy through thick and thin (discs and halo) with the Correlation Radial Velocities (CORAVEL) and Radial Velocity Experiment (RAVE) surveys. The Observatory: A review of astronomy, 128 pp. 520–521.



Within the past decade it is being increasingly recognized that many of the clues to the fundamental problem of galaxy formation in the early Universe are contained in the motions and chemical composition of long-lived stars in our Milky Way galaxy. The growing awareness of the importance of the 'fossil record' in the Milky Way Galaxy in constraining galaxy formation theory is reflected by the increasing number of surveys and missions designed to unravel the formation history of the Galaxy. The RAdial Velocity Experiment (RAVE) is a spectroscopic survey measuring the radial velocities (RVs) and stellar atmosphere parameters (temperature, metallicity and surface gravity) using the Six Degree Field on the UK Schmidt Telescope at the Anglo-Australian Observatory in Australia. The RAVE programme started in 2003, obtaining medium resolution spectra (median R~7500) in the calcium triplet (Ca II) region (8410-8795 \AA). To date, RAVE has observed >200 000 stars. This thesis has made major technical contributions to the RAVE project and scientifically exploited its data.

The thesis describes how I built the currently in use RAVE input catalogue from DENIS and 2MASS. This was a critical contribution to RAVE because the original input catalogue was almost exhausted. My new input catalogue contains 959,057 stars and has increased RAVE's observing rate to ~90 000 stars per annum. Funding permitting, RAVE has enough target stars to run until ~2016.

I helped to discover two RV errors: a systematic fibre-to-fibre RV offset and a variable RV zero-point offset. I established a correlation between RV zero-point shift and spectrograph temperature change. This discovery and consequent zero-point recovery reduced the variable systematic RV error from ~|5| to ~|1| km/s, increasing the reliability of the 25 274 publicly released RVs in the first public RAVE data release and all future releases.

Accurate distances to RAVE stars have not yet been derived. Instead, RAVE RVs can be selected to approximate Galactic space velocities towards the Galactic cardinal directions. In order to interpret these, I analysed actual Galactic space velocities (U, V, W) from the CORAVEL survey. I revisited one of the fundamental aspects of disc galaxy evolution with the CORAVEL dwarfs: the age-velocity dispersion relation. Previous fitting of the age-$\sigma_{U,V,W}$ relations with power laws has led to the interpretation of these as evidence for continuous heating of the disc in all directions throughout its lifetime. I used the CORAVEL giants to show that structure in the local velocity distribution function distorts the in-plane (U and V) velocity distributions away from Gaussian so that a dispersion is not an adequate parametrization of their functions. My new result is that a power law is not required by the data for the age-$\sigma_{W}$ relation: disc heating models that saturate after ~4.5 Gyr are equally consistent with observations.

Efficient vertical phase-mixing near the Galactic plane produces apparently symmetric CORAVEL W velocity distributions. I searched for in-falling stellar streams on to the local Milky Way disc in the CORAVEL W distributions and RAVE RVs that are sensitive to W (b < -45 degrees). I found that the local volumes of the solar neighbourhood sampled by the CORAVEL and RAVE surveys are devoid of any vertically coherent streams containing hundreds of stars. This is sufficiently sensitive to allow my RAVE sample to rule out the passing of the tidal stream of the disrupting Sagittarius dwarf galaxy through the solar neighbourhood. There are no vertical streams in the CORAVEL giants and RAVE samples with stellar densities >16 000 and 1500 stars per cubic kpc respectively and therefore no evidence for locally enhanced dark matter.

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