The Gaia telescope has quickly become one of the European Space Agency's most successful space observatories. Its mission to map our own galaxy has so far revealed much more than researchers could have hoped for. Now, a new discovery from its data suggests that the Milky Way has had a close encounter with another galaxy in the recent past.
As reported in Nature, the team believes that between 300 and 900 million years ago, the Sagittarius dwarf galaxy flew close enough to perturb the motion of many of our galaxy's stars. The changes caused by the flyby are small, but thanks to the huge number of stars in the Gaia data release and the high precision of the data, the subtle effect became more apparent.
The team used over 6 million stars in their analysis, and they tried to understand the cause for the perturbation using models of the Milky Way and its neighbors. The analysis gave them a possible age range for the origin of the perturbation, which matched the range's latest close encounter between the Milky Way and Sagittarius around 200 to 1,000 million years ago. It is not a foolproof link, but it’s more than just circumstantial evidence.
"The most plausible hypothesis is that the perturbation was due to the approach of the Sagittarius dwarf galaxy to the Milky Way disk," lead author Dr Teresa Antoja, from the Universitat de Barcelona, told IFLScience. "Even though we always suspected that the satellite galaxies could have had an influence on the Milky Way, especially in the disk outskirts, this is the clearest evidence so far that they have an effect that is stronger than we thought, still remaining today, and that even the stars that are today in the solar neighborhood have been disturbed."
Working on the movement of the stars in the Milky Way is extremely important, as it tells us about the changes in our galaxy. Internally, the central bar influences the shape of the disk (where the spiral arms are). Externally, we have other galaxies that get close and even sometimes eaten. A flyby is not only likely to perturb a galaxy but to increase star formation in both galaxies, and the team hopes to see evidence of this in both the Milky Way and the Sagittarius dwarf.
"The interaction between the Milky Way and Sagittarius is a reciprocal one. The Milky Way is more massive than all dwarf galaxies surrounding it, and we know both from simulations and from observations that its mass will grow even more by 'eating' these smaller galaxies," Dr Antoja explained. "The Sagittarius one has been disrupted more and more in these successive approaches and is now on the verge of being stripped and swallowed by our own galaxy. We have shown that the effects of Sagittarius on the Milky Way are absolutely non-negligible, but it seems that Sagittarius will lose the battle."
The team will now look more into this past gravitational impact to try to understand which stars were affected, and even if the Sun was among them. It might be possible to estimate the mass of Sagittarius when it perturbed the Milky Way based on the Gaia observations.
Gaia is quickly becoming a cornerstone mission. It has measured the velocity of almost 7 million stars, mapped the position of 1.3 billion stars, and included measurements of their colors. It has also spotted hundreds of transient events in distant galaxies.