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Exploring the Archaeology of Galaxies

Final Activity Report Summary - GALAXY ARCHEOLOGY (Exploring the Archeology of Galaxies)

The most detailed and direct constraints on galaxy formation and evolution come from resolving old and intermediate-age resolved stars in galaxies and studying their spatial distributions, metallicities, kinematics and ages. The primary goal of the project was to improve knowledge of this so-called 'fossil record' of galaxy formation and evolution through studies of a sample of our nearest galaxy neighbours. This was achieved through analysis of the ensemble properties of resolved stars and star clusters in ten nearby systems lying within 5 Mpc - the dwarf galaxies NGC6822, NGC147 and NGC185, the low mass disk galaxies M33, the LMC, NGC4244, NGC55 and NGC2403, and the high mass spirals M31 and M81. During the course of the project, new data was also collected (but not fully analysed) for another four systems (NGC300, NGC7793, NGC247 and Centaurus A) as well as for our own Milky Way. This unprecedented dataset has already produced many exciting new results on the structure and content of galaxies and many more will come once the full analysis is completed in the coming years. A key aim has been to construct a dataset against which one can test and refine state-of-the-art cosmological simulations of galaxy formation.

The primary focus of the observational program has been on the faintest baryonic components of galaxies - stellar halos, thick disks and outer thin disks. These components are best studied in the far outer regions of galaxies, where their light dominates, and are now recognised to contain many of the most important clues about how galaxies have formed and evolved. Their extremely low surface brightness has historically presented a formidable challenge but the advent of new wide-field and high resolution imagers on 4-m and 8-m class telescopes, as well as the Hubble Space Telescope, has allowed these enigmatic regions of galaxies to be fully explored for the first time.

A number of key results were obtained by the team concerning the structure, content and assembly history of galaxies. It was demonstrated that faint extended structural components were ubiquitous in galaxies within and beyond the Local Group although their properties (morphology, structure, stellar content, luminosity) varied significantly from system to system, consistent with a primary origin in stochastic accretions of smaller systems.

Truly panoramic surveys of our nearest large neighbours M31 and M33 were conducted and revealed a plethora of new faint tidal debris streams, dwarf galaxies and globular clusters (GCs). In-depth studies of the M31's inner halo substructure revealed that not all of it had an external origin. Indeed, a significant fraction is likely heated thin disk. Various wide-field imaging datasets were used to hunt for GCs around the Local Group galaxies M31, M33 and NGC6822. More than 100 new discoveries were made, significantly extending the known extent of the GC systems in all three galaxies.

Of particular note was the highly compelling demonstration of a spatial correlation between outer halo GCs and tidal debris streams, indicating that GCs are being accreted onto M31 along with their host galaxies. Detailed constraints on the star formation history in the far outer regions of M31 and M33 were derived from deep colour-magnitude diagram analyses. M31 was found to have formed a significant fraction of its outer disk stars before a redshift of unity and to have a significantly older outer disk than that of the lower mass M33. Stellar populations were shown to change considerably across the disk break in M33, a finding that is qualitatively but not quantitatively consistent with the effects of radial migration. Finally, intermediate-age star clusters in the Large Magellanic Cloud were shown to have age spreads of approximately 300 Myr, challenging the idea of star clusters as simple stellar populations.