A recent study by Brown and collaborators has examined the problem of identifying remnants of satellite galaxies in the halo of the Milky Way with Gaia data. This data, providing full phase-space information across the entire volume of the Milky Way, provides a unique opportunity to search for such remnants. The challenge is to extract the remnant signal from a very large data set, of order 1 billion objects, in the presence of observational errors and against a background population of Galactic stars.
A Monte Carlo simulation of the Gaia catalogue was performed using a realistic number of objects and yielding a catalogue of 3.5 x 108 stars (excluding regions close to the Galactic plane and centre). Within this catalogue are the hidden traces (tidal streams) of satellite dwarf galaxies. Brown and collaborators then explored the feasibility of detecting the tidal streams in the halo by examining the energy versus angular momentum plane.
The image above shows the energy-momentum space for two of the disrupted satellites in the synthetic Gaia catalogue. The panels depict from left to right: Galaxy stars only, satellite stars only, Galaxy plus satellite stars. The upper panels show the error-free E-L_z diagrams; the lower panels show the energy and angular momentum as they would appear when derived from the Gaia catalogue. Satellite remnants are easy to identify in error-free data but become more difficult to isolate when observational errors are added. Further investigation by Brown and collaborators has shown that the remnants can be recovered by restricting the search to samples of high-quality data.
For details see Detection of satellite remnants in the Galactic halo with Gaia - I. The effect of the Galactic background, observational errors and sampling, A.G.A. Brown, H.M. Velazquez & L.A. Aguilar, accepted for MNRAS.
Image courtesy of A. Brown.