Over the past two years, astronomers have rewritten the history of our galaxy
A digital simulation of the collision of a large galaxy, similar to the Milky Way, with a dwarf galaxy. Astronomers believe it was involved in at least one such collision during the formation of the Milky Way.
Representatives of the Khoisan peoples from Black Africa , observing the winding strip of stars and dust separating the night sky, saw the coals of a fire in it. Polynesian sailors saw a shark in the sky devouring clouds. The ancient Greeks saw the flow of milk, and called this path "milky" - galaxias - from where the term " galaxy " later came from .
In the 20th century, astronomers discovered that our silvery river is just part of a huge island of stars, after which they wrote their own history of the origin of the Galaxy [when it became clear that the Milky Way is not the only galaxy, the word "Galaxy" with a capital letter was left as another proper name for the Milky Way / approx. per.]. In a nutshell, the Milky Way was created about 14 billion years ago by the merging of huge clouds of gas and dust under the influence of gravity. Over time, two characteristic structures appeared - first a huge spherical "halo", and then - a dense bright disk. Billions of years passed, and our own solar system appeared inside the disk. And now, looking at the sky at night [in an area with relatively low light pollution / approx. per.] we see spilled milk - or a disc stretching in the sky from the side of the rib.
Yet, over the past two years, researchers have rewritten almost all major chapters of the history of the Galaxy. What happened? They got better quality data.
On April 25, 2018, the European Gaia spacecraft released a staggering amount of information about the sky. Most importantly, the data collected over the year by the apparatus contained a detailed description of the motion of about a billion stars. In previous studies, only a few thousand were marked. This data revived a previously static galaxy. “Gaia launched a new revolution,” said Federico Sestito , an astronomer at the Strasbourg Astronomical Observatory in France.
Astronomers rushed to download a dynamic map of the stars, after which a whole waterfall of discoveries appeared. It has been found, for example, that some parts of the disc are impossibly old. Evidence has been found for epic collisions that shaped the hot youth of the Milky Way, as well as indications that the galaxy continues to mix in unexpected ways to this day.
The Gaia satellite revolutionized the understanding of the Milky Way since its launch in December 2013.
All these results together constitute a new story of the turbulent past of our Galaxy and its continuous development in the future. “Our understanding of the Milky Way changed very quickly,” said Michael Petersen , an astronomer at the University of Edinburgh. “It is now believed that the Milky Way is not a static object. Everything is changing very quickly throughout its scope. "
The earliest stars
To get a glimpse into the earliest days of the Galaxy, astronomers are looking for the stars that existed at that time. They consisted only of hydrogen and helium, the most primordial materials in the universe. Fortunately, the smallest stars of that generation also burn much slower than the rest, which is why many still glow today.
After decades of observation, the researchers compiled a catalog of 42 old-timers known as ultra-metal-poor stars (astronomers call "metal" anything heavier than helium). According to the standard history of the development of the Milky Way, these stars should hang around the entire volume of the halo - the part of the Galaxy that appeared in the first place. And the disk - which, according to calculations, should have taken about a billion years to spin up and flatten - should be populated with stars with heavier elements, such as carbon or oxygen.
At the end of 2017, Sestito decided to study the movement of this metal-poor swarm, writing a program that analyzes the results obtained in the Gaia project. Perhaps their trajectories lying on the sphere could suggest the history of the halo.
He pulled information on the trajectories of 42 ancient stars from a complete dataset. It turned out that most of them, as predicted, moved within the halo. However, some - about 1 in every 4 - moved differently. They seemed to be stuck in the plane of the disk - the youngest section of the Milky Way. “What the hell,” Sestito wondered, although at that moment he used a slightly different word. - What is going on?"
Subsequent research confirmed that these stars have long been in the disk, and are not just tourists passing through it. Based on two recent observations, Sestito and colleagues have assembled a library of 5,000 metal-poor stars. Several dozen of them definitely turned out to be permanent residents of the disk.... Another group of scientists combed another 500 stars found during another observation, and found that every tenth of them orbits lie in the plane of the disk. The third group of scientists studied several stars of different metallicities (and, therefore, different ages), orbiting inside a flat disk. “It was something completely new,” said lead author Paola di Matteo, an astronomer at the Paris Observatory.
How did these anachronisms get there? Sestito theorized that individual clusters of ancient gas somehow managed to avoid all the metals erupted by supernovae for extended periods of time, after which deceptively old stars formed from this gas. Or the disk began to take shape at the same time as the halo - 1 billion years ahead of schedule.
To find out which of these assumptions is more likely, he contacted Tobias Buck , a researcher at the Potsdam Astrophysical Institute in Germany who specializes in creating digital simulations of galaxies. In the past, such attempts usually resulted in the halo being formed first and the disk subsequently forming, as expected. However, these attempts had a relatively low resolution.
In these digital simulations, a galaxy similar to our own Milky Way has been forming and evolving over 13.8 billion years, from the beginning of the universe to the present day. The left column shows the distribution of invisible dark matter. On average - gas temperature (blue - cold areas, red - hot). On the right is the density of stars. The rows are responsible for the scale: the top row is a close-up view of the galactic disk. The middle one is a little further away, overlooking the halo. The lower one is a distant view showing the galaxy's surroundings.
Buck increased the resolution of the simulations by about 10 times. Each run of such a simulation required significant computational resources. And although he had access to the supercomputer center. Leibniz in Germany, one simulation took about three months of computing time. And he repeated this six times.
Five times out of six, it turned out to be a twin of the Milky Way. In two of these simulations, the disk contained significant amounts of metal-poor stars.
Where did these ancient stars come from in the disk? In other words, they were star immigrants. Some of them were born in clouds that appeared before the Milky Way. Then these stars from the clouds simply ended up in such orbits that in the future will form part of the galactic disk. Other stars originate from dwarf galaxies that collided with the Milky Way and entered the plane of the forming disk.
Results publishedresearchers in November 2020, say that the classical models of the formation of the Galaxy were incomplete. Gas clouds actually collapse into spherical halos. But stars arriving at suitable angles can trigger disk formation at the same time. “The theorists weren't wrong,” Buck said. "They just missed part of the big picture."
Stormy youth
The difficulties did not end there. Gaia helped uncover direct evidence of catastrophic clashes. Astronomers already assumed that the Milky Way had an active life, but Helmer Koppelman , who works at the Princeton Institute for Advanced Research, using data from Gaia, was able to find specific remnants of one of the largest mergers.
Koppelman recalls that Gaia's data was published in April 2018 on Wednesday, and due to the frenzied download race, the project's website practically stopped. On Thursday he processed the data, and by Friday he knew that he had stumbled upon something big. In all directions, there were large numbers of stars bouncing to and fro in the center of the Milky Way in a similar and strange way. This was evidence that they appeared in a dwarf galaxy. By Sunday, Koppelman and his team had a short scientific paper ready , and a more detailed analysis was ready by June.
Debris from colliding galaxies could be found anywhere. Up to half of the stars in the inner halo 60,000 light-years in diameter (the halo itself has a radius of hundreds of thousands of light-years) could have appeared as a result of this one collision. It could increase the total mass of the Milky Way by 10%. “This changes everything,” Koppelman said. "I expected there would be many different small objects."
Simulation showing the formation and development of a galaxy like the Milky Way over 10 billion years. Many small, dwarf galaxies fall into the main disk, and often become part of it.
The group named the galaxy that collided with ours, Gaia Enceladus, after the Greek goddess Gaia and her son by Titan, Enceladus. Another team independently discovered the same galaxy around the same time, and named it "Sausage" after its shape on some orbital maps.
After the collision of the Milky Way and Gaia-Enceladus, which happened probably about 10 billion years ago, massive damage has spread across the fragile disk of the Milky Way. Astronomers argue about why the disk of our Galaxy consists of two - a thin disk, and a thicker one, in which the stars orbiting the center of the Galaxy, jump up and down. Today , di Matteo's research suggests that Gaia Enceladus simply ripped open a large portion of the disc, making it thicker during the impact. “The first ancient disk formed pretty quickly, and then, we believe, Gaia Enceladus just destroyed it,” said Koppelman.
In globular star clusters, hints of other mergers can also be found. Diederik Kruissen , an astronomer at the University of Heidelberg in Germany, used galaxy simulations to train a neural network that carefully explored globular clusters. He launched a network to study the age, composition and orbits of the clusters. Based on the data obtained, the neural network was able to recreate the collisions that formed galaxies. And then he applied this information to real data from the Milky Way. The program reconstructed both well-known events, such as the collision with Gaia-Enceladus, and an older and more significant confluence, which the group of scientists dubbed the Kraken.
In August, the Kruissen group published a list of mergersThe Milky Way and the dwarf galaxies that formed it. Scientists also predicted 10 more collisions that occurred in the past, which they hoped to obtain from independent observers. “We haven’t found these ten collisions yet,” Cruissen said, “but we will.”
All of these mergers have led some astronomers to believe that the halo may be almost entirely composed of immigrant stars. Models from the 60s and 70s predicted that most of the stars in the Milky Way would form in place. But over time, more and more stars turn out to be aliens. Probably, astronomers no longer need to assume that many, or even any stars were born on the spot, says di Matteo.
A quietly growing galaxy
Recently, the history of the Milky Way has been rather quiet, but newcomers continue to strive for it. Astronomy lovers from the southern hemisphere can see a couple of dwarf galaxies, the Large and Small Magellanic Clouds, with the naked eye . Astronomers have long considered these two loyal satellites of our Galaxy, something like the moons of the Milky Way.
But then several observations by the Hubble telescope between 2006 and 2013 showed that they looked more like meteorites flying towards us. Nitya Kallivayalil , an astronomer at the University of Virginia, calculated that they are heading straight towards us at a speed of about 300 km / s - almost twice as fast as previously thought.
The rise of the Large and Small Magellanic Clouds over Mount Bromo - an active volcano in the Java Island National Park in Indonesia
When a team led by Astronomer Jorge Penarrubia at the Royal Edinburgh Observatory, processed the data a few years later, they concluded that these velocity clouds must be very massive - probably 10 times harder than previously thought.
“Surprise after surprise,” said Peñarrubia.
Various groups have predicted that unexpectedly heavy dwarfs could lug around parts of the Milky Way, and this year Peñarrubia teamed up with Petersen to find evidence of this.
The problem with studying the motion of galaxies is that the Milky Way is a furious blizzard of stars, and astronomers are trying to look out from one of the snowflakes. Therefore, Peñarrubia and Petersen worked out for most of the quarantine how to neutralize the movements of the Earth and the Sun, as well as to average the motion of the stellar halo so that its outer boundary could serve as a stationary background.
After calibrating the data in this way, they found that the Earth, the Sun, and the rest of the disk they were in were all tilting in the same direction. But they are not moving to the current location of the Large Magellanic Cloud, but to the place where it was a billion years ago (Petersen explained that the Galaxy is a clumsy beast with slow reflexes). They recently detailedtheir discoveries in the journal Nature Astronomy.
The motion of the disk relative to the halo destroys the fundamental assumption that there is a balance in the Milky Way. It can spin and slide through space, but most astronomers assumed that after billions of years, the adult disk and halo had formed a stable configuration.
The analysis of Peñarrubia and Petersen proves that this assumption is false. Even after 14 billion years, mergers continue to shape the overall shape of the galaxy. This is the newest change in our understanding of the processes taking place in the giant milky stream hanging in our sky.
“Now we need a new model to describe the future and history of the Milky Way, which we thought was already known,” said Petersen.