Gaia reveals that the majority Milky Way companion galaxies are newcomers to our nook of space

Data from ESA’s Gaia mission is re-writing the historical past of our galaxy, the Milky Way. What had historically been regarded as satellite galaxies to the Milky Way at the moment are revealed to be largely newcomers to our galactic setting.

A dwarf galaxy is a group of between thousand and several other billion stars. For a long time it has been extensively believed that the dwarf galaxies that encompass the Milky Way are satellites, that means that they’re caught in orbit round our galaxy, and have been our fixed companions for a lot of billions of years. Now the motions of those dwarf galaxies have been computed with unprecedented precision because of knowledge from Gaia’s early third knowledge launch and the outcomes are stunning.

François Hammer, Observatoire de Paris—Université Paris Sciences et Lettres, France, and colleagues from throughout Europe and China, used the Gaia knowledge to calculate the actions of 40 dwarf galaxies across the Milky Way. They did this by computing a set of portions generally known as the three-dimensional velocities for every galaxy, after which utilizing these to calculate the galaxy’s orbital vitality and the angular (rotational) momentum.

They discovered that these galaxies are transferring a lot quicker than the large stars and star clusters which are recognized to be orbiting the Milky Way. So quick, that they could not be in orbit but across the Milky Way, the place interactions with our galaxy and its contents would have sapped their orbital vitality and angular momentum.

Our galaxy has cannibalized numerous dwarf galaxies in its previous. For instance, 8-10 billion years in the past, a dwarf galaxy referred to as Gaia-Enceladus was absorbed by the Milky Way. Its stars could be recognized in Gaia knowledge due to the eccentric orbits and vary of energies they possess.

More not too long ago, 4-5 billion years in the past, the Sagittarius dwarf galaxy was captured by the Milky Way and is at present within the means of being pulled to items and assimilated. The vitality of its stars is greater than these of Gaia-Enceladus, indicating the shorter time that they’ve been topic to the Milky Way’s affect.

In the case of the dwarf galaxies within the new research, which represents nearly all of the dwarf galaxies across the Milky Way, their energies are greater nonetheless. This strongly means that they’ve solely arrived in our neighborhood in the previous few billion years.

The discovery mirrors one made in regards to the Large Magellanic Cloud (LMC), a bigger dwarf galaxy so near the Milky Way that it’s seen as a smudge of sunshine within the evening sky from the southern hemisphere. The LMC was additionally regarded as a satellite galaxy of the Milky Way till the 2000s, when astronomers measured its velocity and located that it was touring too quick to be gravitationally sure. Instead of a companion, LMC is visiting for the primary time. Now we all know that the identical is true for many of the dwarf galaxies too.

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

But saying precisely which of them is troublesome as a result of it is determined by the precise mass of the Milky Way, and that could be a amount that’s troublesome for astronomers to calculate with any actual accuracy. Estimates differ by an element of two.

The discovery of the dwarf galaxy energies is important as a result of it forces us to re-evaluate the character of the dwarf galaxies themselves.

As a dwarf galaxy orbits, the Milky Way’s gravitational pull will attempt to wrench it aside. In physics this is named a tidal drive. “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 different phrases, changing into a companion to the Milky Way is a demise sentence for dwarf galaxies. The solely factor that might resist our galaxy’s harmful grip is that if the dwarf had a major amount of dark matter. Dark matter is the mysterious substance that astronomers assume exists within the universe to supply the additional gravity to carry particular person galaxies collectively.

And so, within the conventional view that the Milky Way’s dwarfs had been satellite galaxies that had been in orbit for a lot of billions of years, it was assumed that they should be dominated by dark matter to steadiness the Milky Way’s tidal drive and hold them intact. The indisputable fact that Gaia has revealed that many of the dwarf galaxies are circling the Milky Way for the primary time signifies that they don’t essentially want to incorporate any dark matter in any respect, and we should re-assess whether or not these methods are in steadiness or somewhat within the means 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 tantalizing clues, we hope to further tease out the fascinating chapters in our galaxy’s past,” says Timo Prusti, Gaia Project Scientist, ESA.