"We have observed shapes with different morphologies, such as a spiral similar to a snail's shell. The existence of these substructures has been observed for the first time thanks to the unprecedented precision of the data brought by Gaia satellite, from the European Space Agency (ESA)", says Teresa Antoja, researcher at ICCUB (IEEC-UB) and first signer of the article. "These substructures -she adds- allow us to conclude that the disk of our galaxy suffered an important gravitational disturbance about 300 and 900 million years ago". This is one of the great first findings of "Galactic archaeology" following the publication of the Gaia data that should allow researchers find out about the origins and evolution of the Milky Way.
What caused this disturbance? To answer this question, the researchers compared the structure and level of twisting of the spiral with models of the dynamics of the Galaxy. As researchers noted, this allowed them to formulate the hypothesis that states that the disturbance was caused by the Sagittarius Dwarf galaxy passing near the Milky Way disk.
"The study implies, definitely, that our galaxy's disk is dynamically young, sensitive to disturbances and changing over time", says Antoja. "One of the most distinguishable forms we saw -continues the researcher- is the spiral pattern of the stars near the Sun, and which had never been seen before. Actually, the observed shapes in the graphics were that clear (unlike common cases), that we thought it could be a mistake in the data", says Antoja. In this sense, more than a hundred European engineers and scientists, among which the UB played a distinguished role, worked during months on the verification and validation tasks of the Gaia data. As part of this task, Mercè Romero Gómez, UB researcher, says that "with the simulations carried out at the UB we could also reproduce the observed spirals".
Like when a stone falls in a pound and creates waves that spread in the surface, or when a magnet gets close to iron fillings and these are alienated in a certain direction, the stars in the galactic disk ordered in a certain way after the gravitational attraction of the satellite galaxy that passed next to them. After some time, the stars maintain the effects of the disturbance that shook them in their movements, and now a spiral can be seen in the graphics.
According to Amina Helmi, researcher at the University of Groningen, "we know our Galaxy is 'cannibal' and has grown while eating other small galaxies, like it is doing now with the Sagittarius Dwarf galaxy". Nonetheless, the expert notes that "the mass of Sagittarius is still large enough to cause a notable gravitational impact". What we see now does not respond to a collision between galaxies but Sagittarius getting closer to the galactic disk.
First results of the new Gaia release
The data analysed in this study is part of the second Gaia release, which was published some months ago, on April 25, 2018. "Scientists and engineers of the UB played an essential role in making these data a reality", says Xavier Luri, director of ICCUB and coordinator of the team that built the Gaia archive. The effort of more than four hundred scientists and engineers allowed publishing positions and precise movements for more than 1,300 million objects. This second catalogue -which embraces the first twenty-two months of data gathering-, published the first spectroscopic data for some million stars in the solar surroundings, which allow researchers to measure the speed of the stars in our line of sight and obtain, therefore, the three velocity coordinates of the stars. These data have enabled the discovery that has been now published in Nature.
Now, the Gaia satellite collectsmore than 48 months of successful operations and ESA has approved of prolonging the mission until late 2020. ESA is now assessing a second two-year prorogue. According to Carme Jordi, UB researcher and member of Gaia Science Team, the scientific advisory body in ESA for this mission, "everything suggests this is only one of the first discoveries of a wide series of new findings -and surprises- hidden in Gaia data which were published on April: the tip of the iceberg in the study of the origins and the evolution of the galaxy in which we are".