The ‘Benjamin Button’ effect allows astronomers to better calculate ‘last big crash’
TROY, N.Y. — A stunning new study could turn everything we know about our cosmic home – the Milky Way Galaxy – upside down. According to researchers at Rensselaer Polytechnic Institute, our galaxy may have collided with another galaxy billions of years later than scientists previously thought. According to the study, Earth had already formed when the Milky Way collided with another star cluster for the last time. What a light show that must have been!
The findings in a nutshell
In their groundbreaking research, published in the Monthly notices of the Royal Astronomical Society, the team of astronomers has discovered compelling evidence that the Milky Way galaxy experienced a massive merger with a dwarf galaxy about six billion years later than previously thought. This discovery challenges the long-standing theory that the last major merger, known as the Gaia Sausage/Enceladus (GSE), occurred eight to eleven billion years ago. Instead, the new research suggests that the debris we see in the Milky Way’s stellar halo — the diffuse sphere of stars surrounding the Milky Way’s disk — is the result of a collision that took place just one to two billion years ago, a cosmic wink of an eye in astronomical terms.
Researchers Heidi Jo Newberg and Tom Donlon focused on the ‘ripples’ in our galaxy, which are created when other galaxies collide with the Milky Way.
“We become more wrinkled as we age, but our research shows that the opposite is true for the Milky Way. It is a kind of cosmic Benjamin Button, becoming less wrinkled over time,” said Donlon, lead author of the new Gaia study, in a press release. “By watching these ripples disappear over time, we can figure out when the Milky Way experienced its last major crash – and it turns out this happened billions of years later than we thought.”
“For the ripples of stars to be as clearly visible as they appear in Gaia data, they would have to have joined us no less than three billion years ago – at least five billion years later than previously thought,” Newberg adds. “Every time the stars swing back and forth through the center of the Milky Way, new stellar ripples form. If they had joined us eight billion years ago, there would be so many wrinkles next to each other that we would no longer see them as separate features.”
Methodology: decoding the clues
To unravel this mystery, the researchers used a variety of advanced techniques. First, they developed a semi-analytical model that relates the number of ‘corrosives’ (ripples or folds in the phase-space distribution of stars) to the time since a merger. By analyzing data from the Gaia space observatory, the team identified several corrosives in the local stellar halo and used their model to estimate the time of this collision.
However, the team didn’t stop there. They delved deeper into the dynamics of these corrosives by comparing their observations with a state-of-the-art cosmological simulation of a Milky Way-like galaxy. This simulation, part of the FIRE-2 Latte suite, allowed them to track the evolution of a simulated dwarf galaxy as it crashed into the host galaxy at different times.
To make the comparison as accurate as possible, the researchers introduced a new metric called ‘causticality’, which calculates the degree of unevenness in the phase-space distribution of stars. A high causality value shows that the stars are not yet fully mixed in phase, indicating a more recent collision.
Main results: a cosmic collision in recent history
The results of this analysis were nothing short of astonishing. The observed data from the Gaia Observatory showed a high causality value, revealing the presence of prominent, asymmetric caustics. Compared to the simulated data, the observed causality best matched the simulated merger debris at a time approximately one to two billion years after the collision.
This finding is in stark contrast to the widely accepted scenario of the GSE merger, which is believed to have occurred between eight and eleven billion years ago – long before the Earth formed. The researchers found that the simulated data showed much lower causality at these ancient cosmic times, indicating a higher degree of phase mixing than what is observed in the Milky Way’s stellar halo today.
Study limitations
Although the study provides compelling evidence for a recent merger, the researchers acknowledge several limitations and challenges. A major limitation is the reliance on a single cosmological simulation, which may not fully capture the complexity of the Milky Way’s formation history.
Furthermore, the observed data are limited to the local stellar halo within five kiloparsecs (about 16,000 light-years) of the Sun. It is possible that the phase-space distribution of stars at greater distances shows a different picture.
Another challenge lies in the up-sampling process used to increase the resolution of the simulated data. While this process is necessary for meaningful comparison, it can introduce bias or underestimate the true degree of phase mixing.
Takeaways
Despite these limitations, the researchers claim that their findings are robust and consistent with other forms of galactic evidence. For example, the observed stellar shells and substructures in the Milky Way’s halo are better explained by a recent collision than by an ancient collision, because older debris would have had more time to phase mix and become less pronounced.
Furthermore, the study offers a compelling alternative to the GSE scenario, which has come under increasing scrutiny in recent years. Some researchers have argued that the chemical and kinematic features attributed to the GSE can be explained by other processes, such as secular evolution or multiple smaller mergers.
If confirmed, the findings of this study could have a profound impact on our understanding of the history of the Milky Way’s formation and the role of mergers in shaping galaxies. They may also have implications for our knowledge of galaxy evolution in general, as the time scales and dynamics of mergers are crucial for modeling and interpreting observations.
“Through this research, Doctors Newberg and Donlon have made a surprising discovery about the history of the Milky Way Galaxy,” said Curt Breneman, Ph.D., dean of Rensselaer’s School of Science.