The Milky Way still under the effect of a galactic encounter

Our galaxy, the Milky Way, is still suffering the effects of a disturbance caused by the passage of another galaxy in its vicinity that has created the movement of millions of stars, a bit like the ripples of a pebble on a pond.

Gaia, the star mapping mission of the European Space Agency (ESA), has shown that this celestial collision probably took place over the last 300 to 900 million years. Astrophysicist Teresa Antoja, from the University of Barcelona in Spain, and her European colleagues come to this conclusion after analyzing the movement of the stars in the Milky Way disk – one of the main components of the galaxy.

Many studies have already shown that the appearance of the Milky Way was the result of celestial collisions. For example, in 2011, American astronomer Chris Purcell and his colleagues at the University of California claimed that the spiral arms of the galaxy would have deployed following two impacts with the dwarf galaxy Sagittarius.

The model revealed by the Gaia probe not only establishes the positions of 1.3 billion stars, but it also accurately measures their velocity in the celestial plane. For a subset of a few million stars, Gaia even provided an estimate of full three-dimensional velocities, allowing stellar motion to be analyzed by combining position and velocity, a concept known as phase space.

This European satellite, launched in December 2013, has been scanning the light sources of our galaxy since 2014. Positioned 1.5 million kilometers from the Earth, it executes no less than 500 million measurements per day. Its catalog contains the positions and brilliance of 1.7 billion stars. Thus, in the phase space, stellar motions revealed an interesting and totally unexpected pattern of stars’ positions as a function of their velocity.

Teresa Antoja could not believe her eyes when she saw them for the first time on her computer screen. One form in particular caught her attention. It was a snail-shell pattern that showed the altitude of the stars above or below the galaxy plane as a function of their speed in the same direction. This had never been observed before.

“I was a bit surprised and thought there might be a problem with the data because the shapes are very clear,” she says.

It should be noted that the data collected by Gaia have undergone multiple validation tests by the Gaia Data Processing and Analysis Consortium teams. Ms. Antoja and her collaborators also did some tests to try to understand the errors that could be behind such shapes. However, no matter what they checked, the only conclusion they could draw is that these features are indeed real.

The existence of this structure being confirmed, the authors of the study published in the journal Nature wanted to understand where it came from. Our Sun is located on one of the spiral arms, about 27,000 light-years away (1 AL = 9460 billion km) from the center of the Milky Way. It’s like throwing a rock into a pond, which moves the water like waves and waves.

However, unlike the water molecules, which settle again, the stars retain a kind of memory of their disturbance. This memory can therefore be observed in their movements. Better still, after millions of years, even though the ripple is no longer easily visible in the distribution of stars, it is still noticeable when analyzing the speed of stars.

Astrophysicists have long since established that our neighbor, the Andromeda galaxy, is heading straight for the Milky Way and that a collision between the two celestial monsters will occur in about 4 billion years. Computer simulations developed using Hubble data suggest that it will take 2 billion years for the merger to be complete.

Carl Frantz

Polyglot, humanitarian, Carl was born in Germany but raised in the USA. He writes mostly on tech, science and culture.