The Thick Disk

The oldest stars in the Milky Way are made up of at least the following contributions: any stars which formed in situ in what is now the stellar halo and thick disk; any stars which formed in situ in the thin disk, and were scattered and/or heated into what is now the thick disk; any old stars in small galaxies which later merged into the Milky Way; and the stars in the intermediate-mass galaxy whose merger created the thick disk, if this is indeed its formation mechanism. Clearly, given this probable diversity of histories, it is naive to imagine the distributions of chemical abundances and kinematics in the present halo and thick disk will be simple, smooth, or gaussian. It has proved surprisingly hard to discover robust deviations from simple, smooth distributions. Conversely, only when deviations from simple distribution functions have been quantified will it begin to be possible to measure the relative importances of these various processes.