Hubble Space Telescope image of the Andromeda galaxy M31 (credit: NASA, ESA)
A spectacular new image from the Hubble Space Telescope of the collision of three distant galaxies shows us how the Milky Way, our own galaxy, will collide with Andromeda and the Triangle Galaxy in the future
The image corresponds to Arp 195, a cluster made up of three galaxies, located 389 light years from Earth in the constellation Lynx and in which its members ‘tear’ each other in a three-way gravitational tug of war. A fate that astronomers predict will also be that of the Milky Way when, 4.5 billion years from now, when it collides with the neighboring Andromeda galaxy and its satellite galaxy. The image was taken by the veteran space telescope just two weeks after a breakdown that came close to taking it permanently out of service in late June.
The Milky Way is one of the three largest galaxies in our environment. Together with Andromeda and the Triangle Galaxy (M31 and M33), in effect, the galaxy in which we live accounts for a good part of the mass of the so-called Local Group, some thirty galaxies that travel together through space.
We have long known that due to their enormous gravity, Andromeda and the Milky Way, the two giants of the group, are destined to collide one day, something that will completely change our cosmic environment. When that happens, in about 4.5 billion years, the two galaxies will merge into a single, even larger one, which astronomers have already dubbed ‘Lactomeda’.
However, the three-dimensional motions of the galaxies within the Local Group remained unclear until recently, which did not allow us to know too many details about the future collision.
“We needed to explore the motions of galaxies in 3D to discover how they have grown and evolved, and what creates and influences their characteristics and behavior,” says Roeland van der Marel, of the Space Telescope Science Institute in Baltimore, USA and lead author of a study published in 2019 in ‘The Astrophysical Journal’-. We have now been able to do this using the second data package provided by Gaia. ‘
Gaia is a European space mission whose goal is to build the most accurate three-dimensional map yet of the stars in the nearby universe and is publishing its data in stages. For this research, those of the second information ‘package’, published in April 2018, were used.
Third in discord
The collision between Andromeda and the Milky Way, however, will have a third guest: M33, the Triangle Galaxy, smaller than the other two but still about 60,000 light years across (compared to 100,000 light years from the Milky Way ) and between 30,000 and 60,000 million stars (compared to between 200,000 and 400,000 million in our galaxy). Recent observations from the Hubble Space Telescope do indeed seem to show that M33 is, in fact, a satellite galaxy of Andromeda, although their type of ‘relationship’ is not yet too clear.
Two possibilities emerged from those observations: either M33 is in an incredibly long orbit (about 6 billion years in duration) around Andromeda but has already fallen into it in the past; or it is currently in its first ‘fall’ towards the great galaxy. Each of these scenarios reflects a different orbital trajectory and, therefore, a history and a future that are also different for each galaxy.
Hubble has provided us with, it is true, the sharpest image of Andromeda and M33 together, but Gaia, by accurately measuring the individual position and motion of millions of its stars, has provided us with information of unprecedented value.
In the words of Mark Fardal, also of the Space Telescope Science Institute and a co-author of that research: “We reviewed the Gaia data to identify thousands of individual stars in both galaxies and we studied how these stars moved within their galactic homes. And while Gaia’s primary goal is to study the Milky Way, it is powerful enough to detect especially massive and bright stars within nearby star-forming regions, even in galaxies beyond our own. ‘
The Gaia data also revealed another important piece of information: how the two galaxies each rotate around their axis of rotation. “For the first time,” says Roeland, “we can measure how M31 and M33 rotate in the sky. And this will help us to understand more about the nature of galaxies. ‘
By combining existing observations with the new Gaia data release, the researchers determined how Andromeda and Triangle move across the sky, and calculated each galaxy’s orbital path both backward and forward in time over thousands. of millions of years.
The new data revealed that when the Milky Way and Andromeda collide and merge, the timing and consequences of that interaction are likely to be different than expected.
Since Andromeda’s motion differs somewhat from previous estimates, the galaxy is likely to give the Milky Way one more hit. Something that will not happen in 3,900 million years, but in 4,500 million, about 600 million years later than expected.
And now, Hubble provides us with a ‘snapshot’ of what that moment might look like.