At first glance the kinematics of disk stars in the solar neighbourhood might appear to be a smooth distribution, but upon closer inspection one can uncover a wealth of structure. The fact that this velocity distribution is clumpy has been known for over a century. The German astronomer J.H. von Mädler, while carrying out observations to measure the Sun's motion, noticed clumping in the distribution of proper motions (Mädler, 1846). This collection of stars moving with the same velocity, what we now refer to as a ''moving group'', consists of members of the Pleiades open cluster, including stars several degrees from the centre of the cluster. This work was build on by Proctor (1869), who found a further two moving groups - Hyades and Sirius.
The dissection of the local stellar velocity distribution has told us a great deal about star formation and Galactic structure. Figure 1 presents a contemporary analysis of the velocity distribution, showing that it is rich in substructure. These moving groups can be attributed to young open clusters which have not yet dispersed, or could be due to dynamical effects such as stars trapped at resonance with the Galactic bar or spiral arms (e.g. Dehnen & Binney, 1998, Bovy & Hogg, 2010, Sellwood, 2010, McMillan, 2011). However, neither of these will be explored in this chapter; interested readers are suggested to see Antoja et al. (2010) for a detailed and comprehensive review on the subject of moving groups. Instead we will here focus primarily on a third mechanism for creating moving groups, namely the accretion of extra-galactic systems.
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Figure 1. The local stellar velocity distribution from a compendium of observational datasets. The two velocity components correspond to the Cartesian in-plane velocities, with U increasing in the direction of the Galactic centre and V in the direction of the Sun's rotation. The approximate locations of four moving groups are denoted by the dotted lines. Figure taken from Antoja et al. (2010). |
As can be seen from the other chapters in this volume, we know of many tidal streams in the stellar halo of the Milky Way. However, more diffuse streams, which are created if the progenitor is less massive, is on a highly eccentric orbit, or was tidally disrupted in the distant past, are harder to identify as spatial over-densities on the sky. They can, however, be discovered by exploring higher-dimensional space; a stream can that has dispersed in configuration space, may still remain coherent in phase space. In this chapter the many successful efforts to find fainter and more ancient accretion events will be reviewed, starting with the solar neighbourhood (Section 2) and moving out to structures in the more distant halo (Section 3).