The position of the Sun in the Milky Way makes it almost impossible to study (at optical wavelengths, at least) the structure of the Galaxy along its disk, since it is filled with gas and dust. The study of the halo, nevertheless, still harbours a large amount of information about the formation of the Galaxy and the environment in which a giant spiral galaxy evolves.
All sky surveys, like SDSS , VST-ATLAS or J-PLUS, are ideally suited for the study of the structure of the Halo of the Milky Way. In fact, SDSS has discovered several star streams (see figure) which are interpreted as the relics of the formation of the Galaxy.
The filter set of J-PLUS, specifically aimed at the characterization of stars in our Galaxy, will play a fundamental role in this scientific niche. Being able to fit the spectra of all the stars observed in the halo of the Milky Way, J-PLUS will create a unique catalogue (to an unprecedented combination of depth and survey area) of stars with their physical parameters (effective temperature, gravity and metallicity).
Some types of stars will particularly benefit from the characteristics of J-PLUS. RR Lyrae stars will be selected via their peculiar colours and, through their variability J-PLUS will be able to tell their distance. This will provide not only a study of the three-dimensional distribution of this type of stars (which can be used as tracers of stellar populations in the halo of the Milky Way) but also of the interstellar material along the line of sight. Cataclysmic variable stars (CVs) will also be identified via their colours. These stars will provide insights on the origin of explosive phenomena which are at the basis of the chemical evolution of the Universe.
White dwarfs are the most common stellar remnant, the remaining degenerate core of hydrogen-burning stars with a mass below eight solar masses. J-PLUS photometry is able to improve the discrimination between the most typical DA white dwarfs, presenting hydrogen lines, and other types (DB, DC, DZ, etc.).
Cataclysmic Variables are binary stars made of a white dwarf which is accreting mass from a less evolved companion which is loosing mass through Roche-lobe overflow. The filters of J-PLUS are ideally suited for the identification of Cataclysmic Variables, hence reducing the probability to mix these objects with quasars.
The improvement of our knowledge on the present structure of the Milky Way will provide constraints on the Galaxy formation theories and simulations (see figure right), hence the importance of having a complete picture of it. Several satellite galaxies tidal streams and debris have been discovered in the last years that are offering clues on the Milky Way formation history, as well as on that of galaxies in general.