| Abstract: |
The concordance cosmological model of LambdaCDM predicts that structure grew through hierarchical merging, with large galaxies like the Milky Way evolving through accretion and assimilation of smaller systems. While extremely successful at matching large-scale structure, the predictions of LambdaCDM have been shown to be in tension with observations on the scales of individual galaxies. Whether this reflects the physics of dark matter or of baryons is a topic of much debate. I will present a previously unrecognized tension, related to chemical enrichment at early epochs. The inner regions of the Milky Way contain a significant population of old, metal-rich stars, consistent with having formed in an early starburst within a deep potential well. Simulations of Milky Way-like galaxies in standard LambdaCDM models fail to reach such high metallicities at early epochs, plausibly reflecting the low-mass sub-structures in which the ancient stars are predicted to form. In my talk I will quantify this comparison between observations and predictions, using ages and metallicities for stars in our Galaxy and public datasets from a range of simulations of Milky Way mass galaxies. I will then interpret the results in the context of other indications of the rapid growth of Milky Way-like galaxies and their associated dark matter haloes, and resulting tensions with the predictions of CDM on galactic scales. |
