Galaxy Genesis — Unravelling the Epoch of Dissipation in the Early Disk

Abstract How did the Galactic disk form and can the sequence of events ever be unravelled from the vast stellar inventory? This will require that some of the residual inhomogeneities from prehistory escaped the dissipative process at an early stage. Fossil hunting to date has concentrated mostly on the stellar halo, but a key source of information will be the thick disk. This is believed to be a ‘snap frozen’ relic which formed during or shortly after the last major epoch of dissipation, or it may have formed from infalling systems early in the life of the Galaxy. As part of the KAOS Galaxy Genesis project, we explore the early history of the halo and the thick disk by looking for discrete substructures, either due to infall or in situ star formation, through chemical tagging. This will require high signal-to-noise echelle spectroscopy of up to a million stars throughout the disk. Our program has a short-term and a long-term goal. The short-term goal is to quantify the size and structure of the multi-dimensional chemical abundance space (C-space) for all major components of the Galaxy. We seek to establish how many axes in (C-space) are decoupled and have large intrinsic dispersions. A critical test of chemical tagging in the short term is that stellar streams in the halo, identified from detailed phase space information, are highly localised in (C-space), or are confined to chemical tracks. These trajectories presuppose that stars form in a closed box through progressive enrichments of the gas, leading to stars dispersed along a narrow track in a complex chemical space. The long-term goal is to identify unique chemical signatures in the thick disk, originating from different formation sites, for star clusters which have long since dispersed. This will require precise chemical abundances for heavy elements such that a star can be localised to a discrete point in (C-space). If the star clusters originally formed outside the Galaxy in a bound infalling system, the stellar abundances may fall along a chemical track, rather than a discrete point in (C-space).