Galaxies such as our own Milky Way are formed when smaller galaxies are ‘engulfed’ by larger ones. This process leaves a ‘footprint’ in terms of the chemical composition and movement of stars. “By detecting and analysing these features, we can derive the history of our galaxy,” explains Gal-HD project coordinator Xavier Luri, professor at the Department of Quantum Physics and Astrophysics, University of Barcelona in Spain. A great deal, however, remains unknown. The European Space Agency’s Gaia space observatory was launched into orbit in 2013 to shed new light on our galaxy’s evolution. This was achieved by measuring the positions, distances and motions of stars with unprecedented precision. The aim was to develop the largest, most precise three-dimensional map of our galaxy.
Rethinking galactic evolution
The Gal-HD project, undertaken with the support of the Marie Skłodowska-Curie Actions programme, sought to exploit this data in order to better understand several key evolutionary galactic processes. These included the assemblage of the halo, (the external, mostly spherical region of a galaxy), as well as the processes of star formation within the galactic ‘disc’. This was done by using the Gaia ‘star map’ to explore the imprint that these processes left in terms of the distribution of stars and their velocity. This enabled the team to find clues and put forward new ideas as to how the Milky Way evolved. “Our work showed a clear connection between Milky Way disc dynamics and the Sagittarius dwarf galaxy, a small galaxy in the process of being tidally disrupted by the gravitational pull of our own,” adds Marie Skłodowska-Curie fellow, Teresa Antoja also at University of Barcelona. “This process has left a clear imprint in the dynamics of the disc.” Antoja and Luri were also able to show that our galactic disc is not as stable as previously thought. This challenges conventional wisdom that the disc is in equilibrium. “Perturbations, like the one caused by the passage of Sagittarius, are probably the cause of different substructures that lie in the outermost parts of the Milky Way disc,” notes Antoja.
Mapping the Milky Way
The project’s discoveries, such as the impact of the Sagittarius dwarf galaxy on the Milky Way, have opened up new conversations within the field of astrophysics. “Our findings represent a change of paradigm in our understanding of galactic evolution,” says Antoja. “Galactic models that apply the assumption of disc equilibrium for example need to be rethought.” Sagittarius has also been shown to have an impact on the inner part of the disc, further complicating our understanding of disc dynamics. “All this will certainly be investigated in the near future,” explains Antoja. In December 2020, a new release of Gaia data was published, providing scientists with even more stars and precision with which to work. “We have already started to enrich our work with this new data,” says Antoja. “We aim to combine it with our existing work, in order to deepen our understanding of the Milky Way.” Luri believes that the Gal-HD project represents an important milestone in our understanding of galactic evolution and structure. “It has opened entire new areas of research and has changed existing paradigms,” he adds. “Together with the work of the larger community exploiting Gaia data, this project will lead to a much more complete and detailed understanding of the evolution of galaxies, and in particular of the Milky Way.”
Gal-HD, astrophysics, quantum, space, galaxy, Gaia, Milky Way, Sagittarius