The Oldest Stars in the Galaxy – Searching for Metal-Poor Stars in the Galactic Bulge

Abstract

The nature of the first generations of stars that formed after the Big Bang is one of the major topics in contemporary astrophysics and cosmology. Such stars formed out of gas that was relatively free of elements heavier than helium, during a time before generations of supernovae had polluted the environment. Previous hunts have found many of these ‘old’ stars in the Galactic halo. However, not all parts of the Milky Way evolved at the same rate. Cosmological models of galaxy formation have drawn the conclusion that, due to the "inside-out" formation of galaxies like the Milky Way, the oldest stars should today preferentially reside close to the centre of the Galaxy – in or near the Galactic bulge. This thesis documents the initial results of the EMBLA Survey, the first successful search for ancient and metal-poor stars in the bulge of the Milky Way. By utilising the metallicity indicator provided by SkyMapper photometry, we were able to search through more than five million bulge stars, looking for those rare objects with extremely low iron abundances. The AAOmega spectrograph on the AAT gave us the opportunity to obtain intermediate- spectroscopy of approximately 350 stars simultaneously; during 24 nights of observing spread over three years, we observed more than 14,000 candidate metal-poor stars to spectroscopically confirm the photometric metallicity estimates. We found that 49% of the stars observed with the AAT had metallicities below [Fe/H]= 1.0, and around 1,000 stars with [Fe/H]< 2.0. This is a remarkable achievement given that previous searches had only found a total of 21 stars with metallicities that low. Thirty-seven of the most metal-poor stars were then observed with high-resolution spectro- scopy using 8 m telescopes like Magellan and the Very Large Telescope (VLT), to determine their detailed chemical compositions. As part of the Gaia-ESO Survey, four stars were observed in 2012 on the VLT, with metallicities of 2.72[Fe/H] 2.48. We then observed a further ten stars with the MIKE spectrograph at Magellan in 2012, and in 2014 we observed a final 23 targets. We have found the first EMP stars in the bulge; nine of our stars have [Fe/H]< 3.0, and one has [Fe/H]= 4.0. We compared the abundances found in our sample with stars of the same metallicities found in the Galactic halo, and found in general similar trends. Unexpectedly, however, we only found one carbon-enhanced metal-poor (CEMP) star (3%) while 20% of halo stars with [Fe/H]< 2.0 are CEMP stars. In order to verify the predicted old ages of our stars, we also investigated their kinematics. We found that half the stars examined have tightly bound orbits; remaining within the inner Galaxy rather than being merely halo stars passing through the bulge region. This is crucial, as the oldest stars are predicted to have the lowest binding energies. The two most metal-poor stars in our sample have binding energies low enough that there is a 50% chance they formed at redshifts of z > 12, which would make them the oldest known objects in the Universe.