Milky Way collision may change everything we know about our galaxy

Our understanding of the Milky Way might be facing a cosmic shakeup. A recent study by researchers from the Rensselaer Polytechnic Institute suggests that our Milky Way experienced a major collision much more recently than previously thought.

This discovery challenges long-held beliefs and offers a fresh perspective on the Milky Way’s formation and evolution.

Mystery of Milky Way’s collision

Scientists have believed for years that the Milky Way’s last significant merger happened between eight and eleven billion years ago.

Known as the Gaia-Sausage/Enceladus (GSE) event, this collision was thought to have shaped much of the galaxy we see today.

However, new research reveals that a significant merger might have occurred just one to two billion years ago. This recent collision would place it well after the formation of Earth.

“Wrinkles” in the Milky Way due to collision

Researchers Heidi Jo Newberg and Tom Donlon spearheaded this study, focusing on the “wrinkles” in our galaxy. These wrinkles form when other galaxies smash into the Milky Way.

“We get wrinklier as we age, but our work reveals that the opposite is true for the Milky Way. It’s a sort of cosmic Benjamin Button, getting less wrinkly over time,” noted Donlon.

By studying how these wrinkles dissipate, the team traced the timeline of the last big collision.

Pinpointing the Milky Way’s last merger

The study’s innovative approach used a semi-analytical model to relate the number of “caustics” (wrinkles in the phase-space distribution of stars) to the time since a merger event.

Analyzing data from the Gaia space observatory, the researchers identified several caustics in the local stellar halo. They compared these findings with a state-of-the-art cosmological simulation, part of the FIRE-2 Latte suite, to determine the timeline of the collision.

“For the wrinkles of stars to be as obvious as they appear in Gaia data, they must have joined us no less than three billion years ago – at least five billion years later than was previously thought,” said Newberg.

New metric for Milky Way collision

The team introduced a novel metric called “causticality,” which measures the unevenness in the phase-space distribution of stars.

A high causticality value indicates a more recent collision, where the stars have not yet fully phase-mixed. Comparing their observations with simulated data, the researchers found a match at a timeline roughly one to two billion years ago.

The widely accepted GSE scenario, suggesting a merger eight to eleven billion years ago, contrasts sharply with these new findings.

The observed high causticality in the Gaia data points to a much more recent merger. This discrepancy raises questions about the previously understood formation history of our galaxy.

Limitations and challenges

Despite the compelling evidence, the study acknowledges several limitations. One key challenge is the reliance on a single cosmological simulation, which may not capture all of the complexities of the Milky Way’s history.

Additionally, the observed data is limited to the local stellar halo within 16,000 light-years of the Sun. The phase-space distribution of stars at larger distances could potentially reveal a different picture.

Moreover, the up-sampling process used to increase the resolution of the simulated data could introduce biases or underestimations in the degree of phase-mixing.

Formation of the Milky Way

If confirmed, these findings could significantly impact our understanding of the Milky Way’s formation and the role of mergers in shaping galaxies.

The study also provides a compelling alternative to the GSE scenario, which has faced increasing scrutiny. Some researchers argue that the chemical and kinematic signatures attributed to the GSE could be explained by other processes, such as secular evolution or multiple smaller mergers.

“Through this study, Doctors Newberg and Donlon have made a startling discovery about the history of the Milky Way galaxy,” noted Dr. Curt Breneman, dean of Rensselaer’s School of Science.

The Milky Way’s collision history

The study opens a new chapter in our understanding of the Milky Way and its collision history. The possibility of a significant collision occurring just one to two billion years ago reshapes our perspective on galactic evolution.

As researchers continue to explore the cosmos, we can expect more surprising revelations that challenge our understanding of the universe and our place within it.

In the grand timeline of the universe, a few billion years might seem like a blink of an eye. Yet, for us on Earth, these discoveries provide a profound connection to the vast, dynamic galaxy we call home.

The study is published in the Monthly Notices of the Royal Astronomical Society.

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