Galactic Archaeology

The main aim of my research is to unravel the origin and evolution of large spiral galaxies like our own Milky Way. This is best done by by probing and mapping in detail the age, chemical abundance, and kinematical properties of its main structural components. To do this I have utilised different tracers: dwarf stars in the solar neighbourhood, red giant stars in the inner and outer regions of the Galactic disk, microlensed dwarf stars in the central bulge region, and hot B stars in the far outskirts of the Galactic disk. For these studies I have mainly used single-slit/fibre spectroscopy to obtain high-resolution spectra of single stars. Currently I am deeply involved in large spectroscopic surveys, aiming at observing hundreds of thousands stars to truly map the structure of Milky Way’s all stellar populations. In particular, for the Gaia-ESO survey (GES), allocated 300 nights of observations with the FLAMES multi-fibre spectrograph at the Very Large Telescope, I have been responsible for the observations. I am also involved in one of the most ambitious large spectroscopic surveys, the 4MOST survey. This is a next generation survey that will be completely different from previous and ongoing surveys, in that its design and capabilities will based on pre-defined science cases. Together with Maria Bergemann I am leading the development, design, and execution for one of the surveys to be carried out with 4MOST: Milky Way Bulge and Disk High-Resolution Survey. First light is planned to 2021.

Gaia-ESO Survey

The Gaia-ESO Survey is a large public spectroscopic survey that will observe around 100 000 stars in the Milky Way and all of its main stellar constituents such as the thin and thick disks, the halo, the bulge, and open and globular clusters. The aim is to in detail map the chemical and dynamical structure of the Milky Way, also well beyond the solar neighbourhood. The project was awarded 300 nights of observing time over 5 years with the FLAMES multi-fibre spectrograph on ESO's Very Large Telescope on Paranal in Chile. With first light on New Years Eve 2011 observations will pursue into 2017. My roles within the Gaia-ESO survey are to prepare all field star observations, using FPOSS and P2PP, and to coordinate the observing team that approximately once a month go to Chile to carry out the observations.

More information about the Gaia-ESO survey can be found at www.gaia-eso.eu and publications so far based on Gaia-ESO data at the ADS

4MOST

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WEAVE

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Microlensed bulge dwarfs

Ever since the first high-resolution spectroscopic studies of bulge stars in the 1990s and 2000s, the bulge has been regarded to be an old and metal-rich population with enhanced levels of α-elements that continued to super-solar metallicities, indicative of a very fast formation history. As the bulge is approximately at a distance of 8.5 kpc from the Sun and obscured by thick layers of dust in the Galactic plane, its stars are very faint and difficult to observe. The first studies in the 1990s and 2000s were therefore confined to the most luminous giants. However, spectra of giant stars are not trivial to analyse and interpret as their cool atmospheres give rise to very strong spectral lines and a wealth of molecular lines that often blend other important diagnostic lines. Also, as it is impossible to estimate the age of an individual red giant star, the old age of the bulge was constrained from the colour-magnitude diagram of different bulge fields. Dwarf stars, on the other hand, that are easier to analyse and that usually give very robust results, are too faint to observe with high-resolution spectrographs under normal conditions in the bulge. During gravitational microlensing events they may, however, brighten by factors of several hundred making it possible to achieve high-quality high-resolution spectra during a 2 hour (or shorter) exposure. Since 2009 we have conducted an intense observing campaign, through a target-of-opportunity program with UVES at VLT, to catch these elusive events. We have been very successful and our current sample consists of 86 bulge dwarfs, and the results so far has dramatically changed the picture of the Milky Way bulge. A comparison between the microlensed bulge dwarfs and the 700 nearby dwarf stars in Bensby et al. (2014) shows for instance that the metal-poor bulge and the thick disk have very similar ages and elemental abundance trends, which might be an indication of an intricate connection between the two populations. The microlensed dwarf stars has also made it possible to determine ages for individual stars in the bulge and we find that the bulge contains a significant fraction of young and intermediate age stars – in direct contradiction with the old bulge claimed by studies of the colour magnitude diagram in different bulge fields. We further find that the metallicity distribution consists of several distinct components rather than a single one. In summary, our findings indicate that the Milky Way has no genuine bulge population, but that it rather is a mixture of the other Galactic stellar populations residing in the central region of the Galaxy.

Thin and thick disks

In the early 1980s a second disk population was discovered in the Milky Way; the thick disk. At the time of my PhD studies (2000-2001) it was unclear how this thick disk differed from the well-established thin disk in terms of kinematics, chemical composition, and age, and this became the subject of my thesis. We selected a sample of 102 nearby solar-type dwarf stars that moved on Galactic orbits characteristic of the two disk populations. A detailed abundance analysis based on high-resolution spectra revealed that the two populations showed very different abundance patterns. They also showed very different age distributions, with the thick disk being much older than the thin disk (perhaps as old as the stellar halo). These results from my thesis showed for the first time that the Milky Way most likely has two very different and distinct disk populations, a thin and a thick disk, that formed at different epochs in the history of the Milky Way. Since then we have expanded the data sample to now include more than 700 stars in order to map the properties of the two disks in more detail. Especially we aimed at constraining their lower and upper age and metallicity limits, and wether they truly are two distinct stellar populations. Figure 1 shows an example for our recent paper from 2014 based on this sample and it is clear that the solar neighbourhood contains two very different disk populations, one old and α-enhanced, and one young and less α-enhanced. We usually refer to these as the Galactic thin and thick disks. The expanded sample also includes stars from an over-density in velocity space called the Hercules stream. We showed that this stream is not distinct in neither chemical abundances nor stellar ages, but rather is a mixture of thin and thick disk stars, and is hence not disrupted cluster. Instead the most likely cause for the Hercules stream over-density is that it is dynamical resonance pattern caused by the Galactic bar.