In a ΛCDM hierarchical model, all galaxies are predicted to have experienced mergers, of which many should be recognizable as debris/streams that make up for a large fraction of their haloes. Haloes and their substructures thus provide a unique glimpse into the assembly history of galaxies, and can inform the models at the smallest galactic scales, where they still fall short in reproducing observations. The time is now ripe for in-depth systematic studies of the resolved stellar populations in galaxy haloes, which will dramatically increase our understanding of galaxy evolution over the next decade.
The challenges for this type of studies are of a different nature: for our own Galaxy, state-of-the-art results on its halo shape, profile and mass inevitably suffer from assumptions on underlying density models and extrapolations of the available data to radii larger than observed. The major current limitation of MW halo studies lies in observational biases due to small field-of-view samples, which preclude the identification of possible substructure contamination. Future surveys hold the promise to advance the knowledge of our Galaxy by obtaining significantly larger samples of tracers, especially in areas so far not covered. Most notably, the astrometric Gaia mission (which will provide unprecedented six-dimensional phase space information for two billion stars out to the inner MW halo) and the Large Synoptic Survey Telescope (LSST; designed to provide a southern sky counterpart to SDSS, and reaching ∼ 4 magnitudes fainter than its predecessor for a total sample of tens of billions of stars), are going to revolutionize our view of the MW. At the same time, the current and future generation of high-resolution spectrographs will follow up these surveys from the ground, providing comprehensive kinematic and chemical information to assess the origin of halo stars and characterize their birthplaces (see also Figueras, this volume).
The pioneering studies of an increasing number of haloes beyond the LG, and across a range of masses, will soon be extended by the next generation of ground-based extremely large telescopes (E-ELT, GMT, TMT), as well as space-borne missions (JWST, Euclid, WFIRST). The PAndAS survey of M31 has extensively demonstrated that only the synergy of wide-field ground-based observations, deep (but spatially limited) observations from space, and spectroscopy can return a truly global understanding of haloes made up of a complex mixture of in-situ and accreted populations. The aforementioned facilities will open new perspectives with wide-field optical and infrared imagers in concert with high-resolution spectrographs, which will allow us to systematically survey hundreds of galaxies within tens of Mpc in the next decade or two. For example, with the E-ELT/MICADO and JWST/NIRcam imagers (the former having higher resolving power and the latter a wider field-of-view), we should resolve stars down to the HB within ∼ 10 Mpc, thus identifying and characterizing the SFHs of streams and faint satellites; derive radial profiles, MDFs and stellar population gradients in haloes within 20 Mpc from the uppermost few magnitudes of the RGB; and trace the halo shape and possible overdensities down to µV ∼ 33 mag arcsec−2 from the uppermost ∼ 0.5 mag of the RGB out to 50 Mpc (Greggio et al 2016).
These observational constraints will be crucial to inform increasingly sophisticated theoretical models, and ultimately answer intriguing open questions (as well as possibly unexpected ones that will likely be raised by these observations themselves), such as:
The era of resolved populations in galaxy haloes has just begun, and it holds the promise to be a golden one.
Acknowledgements I would like to thank the organizers for a lively and stimulating conference. I am indebted to S. Pasetto for his advice and support throughout the preparation of this contribution. I acknowledge the hospitality of the Carnegie Observatories during the completion of this work.