Chasing a Galactic Starburst: Clues from the Milky Way’s High Proper-Motion Stars

When galaxies collide, they don’t just leave behind scattered debris – they can ignite dramatic bursts of star formation. In this study, Deokkeun An and collaborators explore whether a long-ago collision between our own Milky Way (MW) and a smaller galaxy, Gaia-Sausage/Enceladus (GSE), sparked such a starburst. By focusing on a special group of stars moving unusually fast across the sky – the High Proper-Motion Sequence (HPMS) – the team uncovers evidence that could link these stars to one of the most important events in our galaxy’s past.

Setting the Scene: A Turbulent Past for the Milky Way

Roughly 8 to 10 billion years ago, the MW merged with GSE, a smaller galaxy rich in gas. While the stars from GSE were absorbed and now move on elongated orbits in the MW’s halo, this event likely also stirred up existing stars and compressed gas in the MW's disk. The result? New stars forming in chaotic, high-pressure environments. The authors aim to find these possible “merger-triggered” stars by looking for distinct chemical signatures and orbital patterns in stars currently zooming through the sky.

Following the High Proper-Motion Trail

The authors concentrate on stars with high proper motions, meaning they move quickly across the sky as seen from Earth. These are ideal tracers of dynamic galactic events. By analyzing stellar data from several major surveys (like GALAH, APOGEE, SDSS, and LAMOST), the authors examine these stars' chemical compositions and orbits. They identify a distinctive structure – the HPMS – that contains a mix of previously known populations: ancient GSE stars, heated stars from the MW’s disk, and potentially, a new group born from the merger event.

The Chemical Fingerprint of a Starburst

Among these fast-moving stars, the researchers discover something unusual. Some stars have unexpectedly high levels of sodium (Na) and aluminum (Al), but lower “alpha” elements (like magnesium and silicon). This combination is different from typical stars that were either pulled in from other galaxies or stirred up from the MW’s original disk. These “low-alpha, high-Na” (LAHN) stars seem to represent a new chemical fingerprint. Their presence, especially in a narrow metallicity range, suggests they may have formed during a short-lived, intense star-forming episode fueled by gas from the GSE merger.

Orbiting in Evidence

The orbits of LAHN stars support their unique origins. Compared to GSE debris, LAHN stars follow tighter, more compact paths and rarely stray far above or below the Galactic plane. This suggests their gas clouds may have been funneled inward during the merger, forming stars in a more centralized region of the MW. Their orbits are eccentric like GSE stars, but they don’t wander as far, reinforcing the idea that they formed during the GSE event but in a different environment.

Simulations Support the Story

To test their theory, the team uses detailed computer simulations of MW-like galaxies. These models show that after a major merger, galaxies can experience bursts of in-situ star formation, producing stars with similar orbits and chemical features to those seen in the LAHN group. This connection between theory and observation strengthens the idea that these stars are fossil evidence of an ancient galactic starburst.

How Many Are Out There?

The study estimates that LAHN stars make up about 5% of the local stars in the HPMS region, with another 20% likely being stars from the MW’s disk that were dynamically heated during the merger. The majority—about 75%—still appear to be debris from the GSE galaxy itself. While not a dominant group, the LAHN stars provide key insights into how the MW responded to one of its biggest past collisions.

Implications and the Road Ahead

This research offers a new lens for looking at galaxy formation. By tracing back the orbits and chemical “birthmarks” of today’s stars, astronomers can reconstruct the MW’s dynamic history. The discovery of the LAHN stars adds weight to the idea that the GSE merger didn’t just shake up the MW – it helped build it, triggering new stars and shaping the structure we see today. As future surveys gather even more data, astronomers will be better equipped to uncover the hidden stories written in the stars.

Source: An