From European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) GAIA Mission EU : “Gaia reveals that most Milky Way companion galaxies are newcomers to our corner of space”

From European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) GAIA Mission EU

Dwarf galaxies around the Milky Way

24/11/2021

Data from ESA’s Gaia mission is re-writing the history of our galaxy-the Milky Way. What had traditionally been thought of as satellite galaxies to the Milky Way are now revealed to be mostly newcomers to our galactic environment.

A dwarf galaxy is a collection of between thousand and several billion stars. For decades it has been widely believed that the dwarf galaxies that surround the Milky Way are satellites, meaning that they are caught in orbit around our galaxy, and have been our constant companions for many billions of years. Now the motions of these dwarf galaxies have been computed with unprecedented precision thanks to data from Gaia’s early third data release and the results are surprising.

Gaia’s new data takes us to the Milky Way’s anticentre and beyond.
03/12/2020
The stars are in constant motion. To the human eye this movement – known as proper motion – is imperceptible, but Gaia is measuring it with more and more precision. The trails on this image show how 40 000 stars, all located within 100 parsecs (326 light years) of the Solar System, will move across the sky in the next 400 thousand years. These proper motions are released as part of the Gaia Early Data Release 3 (Gaia EDR3). They are twice as precise as the proper motions released in the previous Gaia DR2. The increase in precision is because Gaia has now measured the stars more times and over a longer interval of time. This represents a major improvement in Gaia EDR3 with respect to Gaia DR2. © ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: A. Brown, S. Jordan, T. Roegiers, X. Luri, E. Masana, T. Prusti and A. Moitinho.

François Hammer, The Paris Observatory [Observatoire de Paris – PSL Centre de recherche en astronomie et astrophysique](FR) – Paris Sciences et Letters University [Université Paris Sciences et Lettres Université PSL] (FR), and colleagues from across Europe and China, used the Gaia data to calculate the movements of 40 dwarf galaxies around the Milky Way. They did this by computing a set of quantities known as the three-dimensional velocities for each galaxy, and then using those to calculate the galaxy’s orbital energy and the angular (rotational) momentum.

They found that these galaxies are moving much faster than the giant stars and star clusters that are known to be orbiting the Milky Way. So fast, that they couldn’t be in orbit yet around the Milky Way, where interactions with our galaxy and its contents would have sapped their orbital energy and angular momentum.

Our galaxy has cannibalised a number of dwarf galaxies in its past. For example, 8-10 billion years ago, a dwarf galaxy called Gaia-Enceladus was absorbed by the Milky Way. Its stars can be identified in Gaia data because of the eccentric orbits and range of energies they possess.

More recently, 4-5 billion years ago the Sagittarius dwarf galaxy was captured by the Milky Way and is currently in the process of being pulled to pieces and assimilated. The energy of its stars is higher than those of Gaia-Enceladus, indicating the shorter time that they have been subject to the Milky Way’s influence.

In the case of the dwarf galaxies in the new study, which represents the majority of the dwarf galaxies around the Milky Way, their energies are higher still. This strongly suggests that they have only arrived in our vicinity in the last few billion years.

The discovery mirrors one made about the Large Magellanic Cloud (LMC), a larger dwarf galaxy so close to the Milky Way that it is visible as a smudge of light in the night sky from the southern hemisphere.

Large Magellanic Cloud. ESO’s VISTA telescope reveals a remarkable image of the Large Magellanic Cloud.

Part of ESO’s Paranal Observatory the VLT Survey Telescope (VISTA) observes the brilliantly clear skies above the Atacama Desert of Chile. It is the largest survey telescope in the world in visible light, with an elevation of 2,635 metres (8,645 ft) above sea level.

The LMC was also thought to be a satellite galaxy of the Milky Way until the 2000s when astronomers measured its velocity and found that it was travelling too fast to be gravitationally bound. Instead of a companion, LMC is visiting for the first time. Now we know that the same is true for most of the dwarf galaxies too.

So will these newcomers settle into orbit or simply pass us by? “Some of them will be captured by the Milky Way and will become satellites,” says François.

But saying exactly which ones is difficult because it depends on the exact mass of the Milky Way, and that is a quantity that is difficult for astronomers to calculate with any real accuracy. Estimates vary by a factor of two.

The discovery of the dwarf galaxy energies is significant because it forces us to re-evaluate the nature of the dwarf galaxies themselves.

As a dwarf galaxy orbits, the Milky Way’s gravitational pull will try to wrench it apart. In physics this is known as a tidal force. “The Milky Way is a big galaxy, so its tidal force is simply gigantic and it’s very easy to destroy a dwarf galaxy after maybe one or two passages,” says François.

In other words, becoming a companion to the Milky Way is a death sentence for dwarf galaxies. The only thing that could resist our galaxy’s destructive grip is if the dwarf had a significant quantity of dark matter. Dark matter is the mysterious substance that astronomers think exists in the universe to provide the extra gravity to hold individual galaxies together.

And so, in the traditional view that the Milky Way’s dwarfs were satellite galaxies that had been in orbit for many billions of years, it was assumed that they must be dominated by Dark Matter to balance the Milky Way’s tidal force and keep them intact. The fact that Gaia has revealed that most of the dwarf galaxies are circling the Milky Way for the first time means that they do not necessarily need to include any dark matter at all, and we must re-assess whether these systems are in balance or rather in the process of destruction.

“Thanks in large part to Gaia, it is now obvious that the history of the Milky Way is far more storied than astronomers had previously understood. By investigating these tantalising clues, we hope to further tease out the fascinating chapters in our galaxy’s past,” says Timo Prusti, Gaia Project Scientist, ESA.

Science paper:
The Astrophysical Journal

See the full article here .

five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

Objective
A global space astrometry mission, Gaia will make the largest, most precise three-dimensional map of our Galaxy by surveying more than a thousand million stars.

Mission
Gaia will monitor each of its target stars about 70 times over a five-year period. It will precisely chart their positions, distances, movements, and changes in brightness. It is expected to discover hundreds of thousands of new celestial objects, such as extra-solar planets and brown dwarfs, and observe hundreds of thousands of asteroids within our own Solar System. The mission will also study about 500 000 distant quasars and will provide stringent new tests of Albert Einstein’s General Theory of Relativity.

Gaia will create an extraordinarily precise three-dimensional map of more than a thousand million stars throughout our Galaxy and beyond, mapping their motions, luminosity, temperature and composition. This huge stellar census will provide the data needed to tackle an enormous range of important problems related to the origin, structure and evolutionary history of our Galaxy.

For example, Gaia will identify which stars are relics from smaller galaxies long ago ‘swallowed’ by the Milky Way. By watching for the large-scale motion of stars in our Galaxy, it will also probe the distribution of dark matter, the invisible substance thought to hold our Galaxy together.

Gaia will achieve its goals by repeatedly measuring the positions of all objects down to magnitude 20 (about 400 000 times fainter than can be seen with the naked eye).

For all objects brighter than magnitude 15 (4000 times fainter than the naked eye limit), Gaia will measure their positions to an accuracy of 24 microarcseconds. This is comparable to measuring the diameter of a human hair at a distance of 1000 km.

It will allow the nearest stars to have their distances measured to the extraordinary accuracy of 0.001%. Even stars near the Galactic centre, some 30 000 light-years away, will have their distances measured to within an accuracy of 20%.

The vast catalogue of celestial objects expected from Gaia’s scientific haul will not only benefit studies of our own Solar System and Galaxy, but also the fundamental physics that underpins our entire Universe.

From European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) GAIA Mission (EU) via SPACE.com : “Why does the Milky Way have spiral arms? New Gaia data are helping solve the puzzle”

From European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU) GAIA Mission (EU)

via

SPACE.com

7.30.21
Tereza Pultarova

The European Gaia mission keeps uncovering the mysteries of the Milky Way.

Credit: R. Hurt/NASA JPL-Caltech(US) Milky Way The bar is visible in this image.

New data from the star-mapping Gaia satellite are helping scientists unlock the mystery of our Milky Way galaxy’s spiral arms.

Recently published studies exploring the Early Data Release 3 (EDR3), a batch of observations made available to the scientific community last December, reveal the spiral structure of our galaxy with a greater precision and detail than was possible before.

Since the 1950s astronomers have known that our galaxy-the Milky Way-looks like a spiral with several dense streams of stars and dust emanating from the galactic center winding through the galactic disc and dissolving around its edges. However, scientists have struggled to understand how many of these streams there are and what created them.

“The problem with our galaxy is that we are inside its disc and therefore it’s very difficult to understand the structure as a whole,” Eleonora Zari, a scientist at the MPG Institute for Astronomy [MPG Institut für Astronomie](DE) in Heidelberg, Germany, and author of one of the new papers, told Space.com. “It’s like being in a forest and looking around. At some point, the trees are in front of each other. Plus the forest is a bit foggy, so you really can’t see what the whole forest looks like.”

The European Space Agency’s (ESA) Gaia mission has been mapping the Milky Way since 2014, measuring the precise positions and distances from Earth of nearly two billion stars. The first two batches of data acquired by the spacecraft, which were released to the scientific community in 2016 and 2018, have revolutionized the study of our galaxy. In addition to the fixed positions, the spacecraft also measures how fast stars move in three-dimensional space, allowing astronomers to model the evolution of the Milky Way in the past as well as into the future.

The latest data release, EDR3, improves the accuracy of the previous data sets. And it’s this precision that is enabling astronomers to disentangle the spiral arms from the rest of the stars in the galactic disc with better precision.

Where are the arms?

“We derive the distance of the stars from a measure called the parallax,” Zari said. “And this parallax measurement is 20% better with the latest release. That means that stars that previously we may have seen as part of the same structure now clearly belong to different structures.”

Parallax is a star’s apparent movement against the background of more distant stars as Earth revolves around the sun. By measuring the change in the angle between the star and Earth from two opposite points in the planet’s orbit, astronomers can calculate the distance of the star using simple trigonometry.

In one new paper [Astronomy & Astrophysics], Zari and her colleagues looked at concentrations of hot bright blue stars, called the OBA-type stars, in the Milky Way’s disc. In areas where they could see a higher-than-average concentration of these stars, they could assume the existence of a spiral arm. They then compared their analysis with previously developed models of the galaxy.

“The position of the spiral arms is different and also the strength of the spiral arms, how bright they are, is different,” Zari said.

The Milky Way is known to have two main spiral arms: the “Perseus arm” and the “Scutum-Centaurus arm”. Our galaxy also possesses two less pronounced arms-or spurs-called the ‘Sagittarius arm” and the “Local Arm” (which passes close to the sun).

But in Zari’s study, the difference between the arms doesn’t seem so obvious.

“The Perseus arm seems less bright, and instead the Local arm is more prominent,” she said. “Aso the other two arms — Sagittarius and Scutum Centaurus — at least in my study, they seem to have about the same brightness.”

Zari’s colleague Eloisa Poggio looked at concentrations of 600,000 young stars to determine the precise position of the spiral arms. Young stars are especially valuable when studying the spiral arms, Poggio explained, because spiral arms, with their dense concentration of dust and gas, are believed to be where the majority of stars form.

“We calculated, for each position in the disc, whether that region was more or less populated with respect to the average,” Poggio told Space.com. “Using that approach, we were able to construct a map of the spiral arms in the region that Gaia maps, that is about 16,000 light-years around the sun.”

When the researchers compared their galaxy map to previous models they found that the Perseus arm lies further away from the center of the galaxy in the studied region. The short Local arm appeared much longer than the previous models expected.

How do the arms form?

Astronomers are also still speculating about the origin of those arms and their longevity. Some earlier theories proposed that the shape of the arms is somehow fixed and spins around the galactic center over a long period of time while individual stars, orbiting at their own velocities, move in and out of this shape.

This so-called density wave theory, however, is being disputed by the latest findings enabled by the Gaia mission. Many scientists now think that the spiral arms might not be fixed at all. Instead, they might form temporarily, as a result of the rotation of the galactic disc, and later dissolve and reform again in a different configuration.

To find which theory is correct, Alfred Castro, of the Leiden University [Universiteit Leiden] (NL), in the Netherlands, looked at so-called open clusters [On the Milky Way spiral arms from open clusters in Gaia EDR3], groups of thousands of young stars born from the same cloud of gas and dust. Due to their young age, these stars are still close to their birth place, that is within the spiral arms. If the newer theories were correct, the amount of younger open clusters in the spiral arms would be higher than the amount of older open clusters, Castro speculated. And that’s exactly what the data showed.

“I saw in the data that the spiral structure appears to contain the younger population of stars but disappears if you look at the older stars,” Castro told Space.com. “We see that the rotation rate of the shape is more or less similar to the rotation rate of the stars and varies with the radius to the galactic center. The shape and the stars can’t be decoupled, and that means we don’t have a global shape, which would be the spiral arms, and then the stars moving in and out of them as the density wave theory suggests.”

According to Castro’s analysis, the spiral arms may exist for about 80 to 100 million years, a small fraction of time in the 13-billion-year life of our galaxy.

What gave the Milky Way the spiral arms?

In the future, Poggio hopes, scientists might be able to find out why those spiral arms in the Milky Way exist in the first place. While some theories expect this swirl of stellar streams may have been born after another, smaller galaxy crashed into the Milky Way, others believe it came to existence naturally as a result of the rotation of the galactic disc.

“We expect that we would see different signatures in the motion of the stars if the spiral arms were caused by an external impact,” Poggio said. “Future Gaia data releases will give us more information about the motion of stars in a greater portion of the galactic disc, and we hope we might be able to find something there.”

The next batch of Gaia data, the full Data Release 3, is expected to be made available to scientists worldwide in about mid-2022. Gaia, one of the most productive missions in history (measured by the number of scientific papers it produces), will continue scanning the sky until 2025. The vast catalogues of stellar positions, motions and velocities it creates will keep astronomers busy for decades to come.

The papers by Poggio, Castro and Zari were published in the journal Astronomy and Astrophysics in July.