‘Benjamin Button’ effect allows astronomers to better calculate ‘last big crash’
TROY, NY — An amazing new study may overturn everything we know about our cosmic home – the Milky Way galaxy. According to researchers at Rensselaer Polytechnic Institute, our galaxy may have collided with another galaxy billions of years later than scientists previously thought. In fact, according to the study, Earth had already formed when the Milky Way’s last collision with another star cluster. What a light show it must have been!
Finding, in a nutshell
In their groundbreaking study published in Monthly Notices of the Royal Astronomical Society, a team of astronomers has uncovered compelling evidence that the Milky Way galaxy experienced a massive merger with a dwarf galaxy about six billion years later than previously thought. The discovery challenges the long-held theory that the last major merger, known as the Gaia-Sausage/Enceladus (GSE), occurred a staggering eight to 11 billion years ago. Instead, new research suggests that the debris we see in the Milky Way’s stellar halo—the diffuse sphere of stars surrounding the galaxy’s disk—is the result of a collision that occurred just one to two billion years ago, a cosmic blink. eye in astronomical terms.
Researchers Heidi Jo Newberg and Tom Donlon focused on the “wrinkles” in our galaxy that form when other galaxies collide with the Milky Way.
“As we age, our wrinkles diminish, but our work reveals that the opposite is true for the Milky Way. It’s a kind of cosmic Benjamin Button that shrinks over time,” says Donlon, lead author of the new Gaia study, in a press release. “By looking at how these wrinkles dissipate over time, we can trace when the Milky Way experienced its last big crash – and it turns out that it happened billions of years later than we thought.”
“For the wrinkles of the stars to be as obvious as they appear in the Gaia data, they must have joined us no less than three billion years ago – at least five billion years later than previously thought,” adds Newberg. “New star wrinkles are formed every time the stars swing back and forth through the center of the Milky Way. 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 anchor points
To unravel this mystery, researchers used a variety of cutting-edge techniques. First, they developed a semi-analytical model that relates the number of “corrosives” (wrinkles or folds in the phase-space distribution of stars) to the time since the merger. By analyzing data from the Gaia space observatory, the team identified several caustics in the local stellar halo and used their model to estimate the time of this collision.
However, he did not stop there. They delved deeper into the dynamics of these caustics 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 the simulated dwarf galaxy as it collided with the host galaxy at different times.
To make the comparison as accurate as possible, the researchers introduced a new metric called “causticity,” which calculates the degree of unevenness in the phase-space distribution of stars. A high value of caustiality indicates that the stars are not yet fully phase-mixed, indicating a more recent collision.
Key results: Cosmic collision in recent history
The results of this analysis were nothing short of astounding. The observed data from the Gaia observatory showed a high caustic value, which revealed the presence of pronounced, asymmetric caustics. When compared to the simulated data, the observed causticity best matched the simulated merger fragments at around one to two billion years after the collision.
This finding is in stark contrast to the widely accepted scenario of the GSE merger occurring between eight and 11 billion years ago – long before Earth was formed. The researchers found that during these ancient cosmic ages, the simulated data showed much lower causticity, indicating a higher degree of phase mixing than is observed in the Milky Way’s stellar halo today.
Study restrictions
While the study presents compelling evidence for a recent merger, the researchers acknowledge several limitations and issues. One key limitation is the reliance on a single cosmological simulation, which may not fully capture the complexity of the Milky Way’s formation history.
In addition, the observed data are limited to the local stellar halo within five kiloparsecs (about 16,000 light-years) from the Sun. It is possible that the phase-space distribution of stars at greater distances could reveal a different picture.
Another challenge lies in the up-sampling process used to increase the resolution of the simulated data. Although necessary for a meaningful comparison, this process could introduce bias or underestimate the true degree of phase mixing.
Takeaway food
Despite these limitations, the researchers say their findings are robust and consistent with other lines of galactic evidence. For example, the observed stellar shells and substructures in the Milky Way halo are better explained by a recent collision rather than an ancient one, as older debris would have had more time to phase mix and be less pronounced.
Moreover, the study provides a compelling alternative to the GSE scenario, which has faced increasing scrutiny in recent years. Some researchers have argued that the chemical and kinematic signatures attributed to GSEs can be explained by other processes, such as secular evolution or multiple minor mergers.
If confirmed, the results of this study could profoundly affect our understanding of the Milky Way’s formation history and the role of mergers in galaxy formation. They could 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 study, Doctors Newberg and Donlon have made a surprising discovery about the history of the Milky Way galaxy,” says Curt Breneman, Ph.D., dean of Rensselaer’s School of Science.