Despite extensive progress in identifying kinematic streams, the field is far from exhausted. On the contrary, this decade is likely to see a resurgence in this field, leading to unprecedented insights into the formation of our Galaxy.
From the current generation of spectroscopic surveys, we can expect significant progress in the coming years. In terms of sheer volume of spectra, the Chinese LAMOST survey is unsurpassed (Deng et al., 2012). By gathering over a million spectra each year, this spectroscopic survey has great potential. Already one new candidate kinematic overdensity has been identified (Zhao et al., 2014) and various works are analysing substructure in the local velocity distribution, for example the work of Xia et al. (2015) which is utilising the extreme deconvolution technique (Bovy Jo et al., 2011).
If we think about the accreted galaxies which built up our stellar halo, they will of course have a range of masses and accretion times. Their chemical composition will therefore vary since the amount of enrichment that can take place depends on these factors (see, for example, Lee et al. 2015). As a consequence, a detailed dissection of the accretion history of our halo will require both kinematics and chemistry. By combining dark matter simulations with semi-analytic prescriptions for the star formation and chemistry, it is possible to make predictions for what we may be able to detect and how much we can infer about our Galaxy's accretion history from a given set of of kinematic and chemical abundance data (e.g. Johnston et al., 2008).
There are a number of spectroscopic surveys that operate at resolutions sufficient to carry out detailed chemical abundance analyses, for example the SDSS project APOGEE (Holtzman et al., 2015), the GALAH survey (Freeman, 2012), or the Gaia-ESO survey (Gilmore et al., 2012, Randich et al., 2013). These detailed abundances opes up the possibility of ''chemical tagging,'' whereby abundance ratios are used to disentangle the different formation sites for groups of stars (Freeman & Bland-Hawthorn, 2002). Clearly these additional dimensions will be extremely valuable when attempting to identify kinematic substructures in the local disk, where groups may overlap if one looks at only the 6D phase-space. Although this technique is ideally suited to finding moving groups in the disk, as mentioned above chemistry will allow us to classify halo streams and understand their origins - in effect carrying out the discovery and follow-up in one step.
As the Gaia satellite begins to deliver scientific return, there is no doubt that we are on the cusp of a true revolution in this field. This mission, which is led by the European Space Agency, is collecting high precision astrometry of a billion stars in our galaxy. All stars in the sky brighter than 20th magnitude will be observed, leading to exquisite proper motions and parallaxes. The precision is so great that it will be able to measure distances (through trigonometric parallax) to less than 1 per cent for ten million stars. In addition to the astrometry, Gaia will provide detailed photometric information (from spectrophotometry) including stellar parameters and, for stars brighter than around 17th magnitude, spectroscopic information including radial velocities. A description of the science capabilities can be found in de Bruijne (2012), although continually updated performance information can be found on the Gaia webpage. The final catalogue is expected in 2022, with interim releases before then.
Clearly such an unprecedented mapping of 6D phase space will open up an entirely new view of the local velocity distribution. While we wait for the first Gaia data to appear, many authors have attempted to estimate what we might be able to see. One example of this is Gómez et al. (2010), who modelled the Milky Way halo through th e accretion of satellite galaxies, then convolved these with Gaia's observational errors. Fig. 10 shows what we may be able to detect in a solar neighbourhood realization; there are 1e5 stellar halo particles within this sphere of 4 kpc radius, plus around 20,000 stellar disk particles. Upon applying a detection algorithm to identify substructure, they confirm 12 separate accretion events, corresponding to around 50 per cent of all disrupted satellites in this volume. For some of these detections the authors find that it should be possible to directly estimate when these satellites were accreted, exploiting the fact that disrupting satellites form separate clumps in frequency space and the separation of these clumps relate to the time since accretion (McMillan & Binney, 2008 Gómez & Helmi, 2010). This remarkable feat requires a large enough sample of stars with accurate parallaxes (typically 50 or more stars with parallax error less than 2 per cent), but in this realization Gómez et al. predict that it should be attainable for at least four of their detected satellites. Being able to determine the time of accretion, together with a detailed analysis of the chemistry of these stars, will undoubtedly teach us a great deal about the evolution of star formation in these earliest galaxies.
Figure 10. An example of how Gaia might see the distribution of accreted satellites in the solar neighbourhood. The different colours correspond to different satellites in the space of energy and the vertical component of the angular momentum. Black circles denote the four satellites for which it will be possible to estimate the time of accretion. Taken from Gómez et al. (2010).
With Gaia in mind, a number of other studies have devised methods to search for substructures. One such work is that of Mateu et al. (2011), who utilise the fact that (for a spherical potential) streams will fall on great circles as viewed from the Galactic centre. This is based on an earlier study (Johnston et al., 1996), but by extending the analysis to include data such that will be available from Gaia (i.e. parallaxes and kinematics) the method has much greater efficacy. Of course this technique still requires streams to be confined to orbital planes and, as such, is ill-suited to the inner halo where the shorter dynamical times lead to significant phase mixing. However, at intermediate distances in the halo where Gaia will still be able to provide reasonable parallaxes (with distance accuracy of say 30 per cent), this technique will thrive.
Although Gaia will play a dominant role in the coming decades, it will not provide all of the answers. The on-board spectroscopy is limited to only the brightest stars and will not deliver detailed chemistry, meaning that a huge ground-based follow-up program is required. The realization that this limitation hampered the scientific return of the Hipparcos mission, led to the Gaia-ESO survey, and also provides strong motivation for future instruments, such as Subaru's Prime Focus Spectrograph (Takada et al., 2014), 4-MOST (de Jong et al., 2014), WEAVE (Dalton et al., 2014) and the Maunakea Spectroscopic Explorer (Simons et al., 2014).
It is fascinating to see how, 150 years since Mädler and his contemporaries made their first discoveries, the analysis of kinematic substructures is still playing an important role in understanding the evolution of the Galaxy. Mädler couldn't have imagined that some moving groups could be the relics of other galaxies, but today these substructures are illuminating our knowledge of the earliest galaxies and hierarchical assembly. As new surveys are undertaken, the census of substructures becomes more complete. Perhaps in 150 years accretion events such as these will still be contributing new insights.
Acknowledgements The author acknowledges financial support from the CAS One Hundred Talent Fund, NSFC grants 11173002 and 11333003, the National Key Basic Research Program of China (2014CB845700) and the Strategic Priority Research Program ''The Emergence of Cosmological Structures'' of the Chinese Academy of Sciences (XDB09000000). This work is partially supported by the Gaia Research for European Astronomy Training (GREAT-ITN) Marie Curie network, funded through the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 264895. This chapter uses data obtained through the Telescope Access Program (TAP), which has been funded by the Strategic Priority Research Program ''The Emergence of Cosmological Structures'' (Grant No. XDB09000000), National Astronomical Observatories, Chinese Academy of Sciences, and the Special Fund for Astronomy from the Ministry of Finance.