From European Space Agency: “Gaia clocks new speeds for Milky Way-Andromeda collision”

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From European Space Agency

7 February 2019

Roeland P. van der Marel
Space Telescope Science Institute
Baltimore, USA
Email: marel@stsci.edu

Mark Fardal
Space Telescope Science Institute
Baltimore, USA
Email: fardal@stsci.edu

Ekta Patel
Steward Observatory
University of Arizona, USA
Email: ektapatel@email.arizona.edu

Timo Prusti
ESA Gaia Project Scientist
Email: tprusti@cosmos.esa.int

Markus Bauer
ESA Science Programme Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int

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Future motions of the Milky Way, Andromeda and Triangulum galaxies
07/02/2019
Copyright Orbits: E. Patel, G. Besla (University of Arizona), R. van der Marel (STScI); Images: ESA (Milky Way); ESA/Gaia/DPAC (Messier 31, Messier 33)

The future orbital trajectories of three spiral galaxies: our Milky Way (blue), Andromeda, also known as Messier 31 (red), and Triangulum, also known as Messier 33 (green). The circle indicates the current position of each galaxy, and their future trajectories have been calculated using data from the second release of ESA’s Gaia mission. The Milky Way is shown as an artist’s impression, while the images of Andromeda and Triangulum are based on Gaia data. Arrows along the trajectories indicate the estimated direction of each galaxy’s motion and their positions, 2.5 billion years into the future, while crosses mark their estimated position in about 4.5 billion years. Approximately 4.5 billion years from now, the Milky Way and Andromeda will make their first close passage around one another at a distance of approximately 400 000 light-years. The galaxies will then continue to move closer to one another and eventually merge to form an elliptical galaxy. The linear scale of 1 million light years refers to the galaxy trajectories; the galaxy images are not to scale.

ESA’s Gaia satellite has looked beyond our Galaxy and explored two nearby galaxies to reveal the stellar motions within them and how they will one day interact and collide with the Milky Way – with surprising results.

ESA/GAIA satellite

Our Milky Way belongs to a large gathering of galaxies known as the Local Group and, along with the Andromeda and Triangulum galaxies – also referred to as Messier 31 and Messier 33, respectively – makes up the majority of the group’s mass.

Local Group. Andrew Z. Colvin 3 March 2011

Andromeda Galaxy NASA/ESA Hubble

The VLT Survey Telescope (VST) at ESO’s Paranal Observatory in Chile has captured this beautifully detailed image of the galaxy Messier 33, often called the Triangulum Galaxy. This nearby spiral, the second closest large galaxy to our own galaxy, the Milky Way, is packed with bright star clusters, and clouds of gas and dust. This picture is amongst the most detailed wide-field views of this object ever taken and shows the many glowing red gas clouds in the spiral arms with particular clarity.

Astronomers have long suspected that Andromeda will one day collide with the Milky Way, completely reshaping our cosmic neighbourhood. However, the three-dimensional movements of the Local Group galaxies remained unclear, painting an uncertain picture of the Milky Way’s future.

Milkdromeda -Andromeda on the left-Earth’s night sky in 3.75 billion years-NASA

“We needed to explore the galaxies’ motions in 3D to uncover how they have grown and evolved, and what creates and influences their features and behaviour,” says lead author Roeland van der Marel of the Space Telescope Science Institute in Baltimore, USA.

“We were able to do this using the second package of high-quality data released by Gaia.”

Gaia is currently building the most precise 3D map of the stars in the nearby Universe, and is releasing its data in stages. The data from the second release, made in April 2018, was used in this research.

Previous studies of the Local Group have combined observations from telescopes including the NASA/ESA Hubble Space Telescope and the ground-based Very Long Baseline Array to figure out how the orbits of Andromeda and Triangulum have changed over time. The two disc-shaped spiral galaxies are located between 2.5 and 3 million light-years from us, and are close enough to one another that they may be interacting.

NASA/ESA Hubble Telescope

NRAO/VLBA

Two possibilities emerged: either Triangulum is on an incredibly long six-billion-year orbit around Andromeda but has already fallen into it in the past, or it is currently on its very first infall. Each scenario reflects a different orbital path, and thus a different formation history and future for each galaxy.

While Hubble has obtained the sharpest view ever of both Andromeda and Triangulum, Gaia measures the individual position and motion of many of their stars with unprecedented accuracy.

“We combed through the Gaia data to identify thousands of individual stars in both galaxies, and studied how these stars moved within their galactic homes,” adds co-author Mark Fardal, also of Space Telescope Science Institute.

“While Gaia primarily aims to study the Milky Way, it’s powerful enough to spot especially massive and bright stars within nearby star-forming regions – even in galaxies beyond our own.”

The stellar motions measured by Gaia not only reveal how each of the galaxies moves through space, but also how each rotates around its own spin axis.

A century ago, when astronomers were first trying to understand the nature of galaxies, these spin measurements were much sought-after, but could not be successfully completed with the telescopes available at the time.

“It took an observatory as advanced as Gaia to finally do so,” says Roeland.

“For the first time, we’ve measured how Messier 31 and Messier 33 rotate on the sky. Astronomers used to see galaxies as clustered worlds that couldn’t possibly be separate ‘islands’, but we now know otherwise.

“It has taken 100 years and Gaia to finally measure the true, tiny, rotation rate of our nearest large galactic neighbour, Messier 31. This will help us to understand more about the nature of galaxies.”

By combining existing observations with the new data release from Gaia, the researchers determined how Andromeda and Triangulum are each moving across the sky, and calculated the orbital path for each galaxy both backwards and forwards in time for billions of years.

“The velocities we found show that Messier 33 cannot be on a long orbit around Messier 31,” says co-author Ekta Patel of the University of Arizona, USA. “Our models unanimously imply that M33 must be on its first infall into Messier 31.”

While the Milky Way and Andromeda are still destined to collide and merge, both the timing and destructiveness of the interaction are also likely to be different than expected.

As Andromeda’s motion differs somewhat from previous estimates, the galaxy is likely to deliver more of a glancing blow to the Milky Way than a head-on collision. This will take place not in 3.9 billion years’ time, but in 4.5 billion – some 600 million years later than anticipated.

“This finding is crucial to our understanding of how galaxies evolve and interact,” says Timo Prusti, ESA Gaia Project Scientist.

“We see unusual features in both M31 and M33, such as warped streams and tails of gas and stars. If the galaxies haven’t come together before, these can’t have been created by the forces felt during a merger. Perhaps they formed via interactions with other galaxies, or by gas dynamics within the galaxies themselves.

“Gaia was designed primarily for mapping stars within the Milky Way — but this new study shows that the satellite is exceeding expectations, and can provide unique insights into the structure and dynamics of galaxies beyond the realm of our own. The longer Gaia watches the tiny movements of these galaxies across the sky, the more precise our measurements will become.”

ESA’s Gaia satellite was launched in 2013 to create the most precise three-dimensional map of one billion of the stars within the Milky Way. The mission has released two lots of data so far: Gaia Data Release 1 on 14 September 2016, and Gaia Data Release 2 on 25 April 2018 (the latter of which was used in this study). More releases will follow in coming years.

Science paper:
“First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of Messier 31 and Messier 33” by R. P. van der Marel et al. is published in The Astrophysical Journal.

See the full article here .


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