A team of astrophysicists at the Strasbourg Astronomical Observatory has recently created an all-sky structural and kinematic map of the stellar stream substructures of the Milky Way halo. This was accomplished using data from Gaia mission (an ongoing astrometric space mission of the European Space Agency), which are of unprecedented quality. This initial mapping of the stellar substructures in the galactic halo is the first step to unravel the complex formation history of our Galaxy.
The formation and evolution of galaxies is one of the great outstanding problems of astrophysics. Galaxies, like the Milky Way, are quite complex physical systems that are often found to be consisting of several structural components. For example, the Milky Way itself consists of three major components – a densely populated stellar bulge that lies at the very centre of the galaxy, a flattened stellar disk lying in the plane of the galaxy, and a sparsely populated galactic halo. One of the prevailing frameworks that attempts to explain Milky Way’s complicated structure and composition, known as the hierarchical structure formation framework, suggests that the Milky Way halo was built up over an extended period through the aggregation and accretion of smaller mass systems (such as satellite galaxies). Numerical simulations based on this cosmological paradigm have shown that when low mass stellar systems orbiting the host galaxy come too close to the host’s centre, they undergo disruption and distortion due to the action of tidal forces (Figure 1). This process rips out stars from the progenitor and leads to the formation of stellar residues that are called “tidal debris”. A series of such merging and accretion events in the environment of the galaxy, collectively, gives rise to an inhomogeneous distribution of stars in the galactic halo. If this theoretically-motivated hierarchical cosmological model is indeed correct, then we expect to see some fossil residues of these past or on-going accretion events.
With the arrival of the Gaia catalogue (Gaia DR2), the team built a state-of-the-art algorithm to analyse the Gaia data for such stellar remnants. This algorithm, STREAMFINDER, is a new powerful numerical recipe that detects stellar streams in the Milky Way halo. Stellar streams (Figure 2) are a class of stellar debris that are formed when the tidal disruption acts slowly. The algorithm operates by hunting for groups of stars that share similar orbits and contain only particular types of stellar populations. As Gaia DR2 is a vast and rich dataset, containing physical parameters of ~1.6 billion stars of the Milky Way, it was no surprise that the processing of the entire catalogue would have been computationally expensive. For this, the team obtained support from the “Initiative Excellence” of the Université de Strasbourg to acquire sufficient computing processing power.
Northern sky 
Southern sky
On the very day of the data release, 25th April 2018, the STREAMFINDER was launched on the Gaia data. This study revealed, for the first time, an all-sky structural and kinematic map of the stellar streams of the Milky Way halo (Figure 3). The researchers found a rich network of criss-crossing streams, often with striking kinematic coherence. Several of these structures, which can be seen in Figure 3, were reported as discoveries. The lumpiness in these maps strongly supports the picture in which the Milky Way accumulated stars by undergoing a significant number of merging and accretion events. While these results are but a first step in the comprehensive mapping of the Milky Way’s stellar halo, they already show the promise of the Gaia mission, borne out by the rich view of the heavens it has unveiled.
Article: Ghostly Tributaries to the Milky Way: Charting the Halo’s Stellar Streams with the Gaia DR2 catalogue. Khyati Malhan, Rodrigo A. Ibata & Nicolas F. Martin
Accepted: 5th September 2018 by Monthly Notices of Royal Astronomical Society
Contact: Khyati Malhan (PhD student), kmalhan07@gmail.com, @kmalhan07
Nicolas Martin, nicolas.martin@astro.unistra.fr