An unexpected pattern in the motions of the stars in the Milky Way's disk points to a recent whack from another galaxy.

Two independent groups of astronomers have found a fascinating pattern that reveals itself in the motions of the Milky Way’s stars. The pattern suggests that a neighboring galaxy swept by our own within the last billion years.

Gaia’s Spiral

The Gaia satellite has provided unprecedented exquisite measurements of the distances and motions to 1 billion stars, providing a high-resolution map of our Galaxy. Since the release of the second data release in April, there has been a deluge of exciting results, ranging from the discovery of new nearby clusters and to the motions of distant galaxies. Now, astronomers are applying the data to our own galaxy.

Teresa Antoja (University of Barcelona, Spain) and colleagues first discovered an unexpected pattern when they selected 6 million nearby stars from the Gaia data set with precise distances and velocities. Looking at the stars across space, the astronomers didn’t see anything unusual. But a spiral shape shows up when they instead looked at the stars’ motions, specifically when comparing motions within the plane of the Milky Way’s disk to motions perpendicular to the disk. The spiral shape that appeared in a plot of stellar velocities is known as a phase spiral. The discovery appears in Nature.

Most stars in the galaxy are found in the disk, orbiting the galactic center. The fact that their roughly circular orbits correlate with up-and-down motions, producing the spiral shape, means that something whacked the disk out of equilibrium. The spiral represents a large-scale gravitational disruption, so something with the size and mass of a small galaxy was the most likely culprit. When our galaxy was first forming, these types of encounters were probably common, but the ensuing disturbances smoothed out gradually, over hundreds of millions of years. Seeing a phase spiral today suggests something jostled the Milky Way’s disk pretty recently.

A Recent Close Encounter

While Gaia has set a new standard for understanding the distances and motions of stars, it has yet to release detailed chemical compositions for many stars. Stars are mostly hydrogen and helium, but trace amounts of heavier elements can reveal stars’ ages and other properties. Ground-based spectroscopic surveys, such as the Galactic Archaeology with HERMES (GALAH) survey at the Australian Astronomical Observatory, have provided high-resolution measurements of these chemical properties for hundreds of thousands of stars. Combining Gaia and GALAH data, Joss Bland-Hawthorn (University of Sydney, Australia) and colleagues studied the phase spiral in a complementary way, using not only stellar motions, but also the stars’ heavy-element content to get a better understanding of the pattern. This study appeared on the astronomy preprint arXiv on September 7th.

Each successive generation of stars contains higher amounts of elements heavier than helium, which astronomers call metals (much to the chagrin of chemists everywhere). That makes metal content, or metallicity, a rough proxy for the age of stars: the more metal-rich stars are younger. Bland-Hawthorn used the GALAH and Gaia data to identify two disks in the Milky Way, one metal-rich and young, the other metal-poor and older.

The phase spiral is more apparent in the metal-rich disk, which enabled the astronomers to estimate when our galaxy encountered its intruder. Bland-Hawthorn’s team estimated an encounter roughly 500 million years ago, consistent with Antoja’s group’s estimate of between 300 and 900 million years ago. Turns out that the Sagittarius Dwarf Spheroidal Galaxy passed close by the Milky Way about 200 million to 1 billion years ago, making it a good candidate for galactic disruption. That encounter is still in the process of gravitationally tearing the dwarf itself to shreds.

The discovery of the galaxy’s phase spiral might just be the beginning. Gaia’s precise measurements provide hints of other correlated motions. — Antoja, for example, points out “snail shells and ridges” that complement the spiral. Identifying and understanding the creation of these patterns will help astronomers understand how our galaxy has evolved over time and what other recent interactions it may have had with its neighbors.