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| Our galaxy, the Milky Way, is surrounded by about fifty dwarf galaxies. (Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO) |
According to the data collected, ESA's Gaia project provides new evidence for forming our galaxy, the Milky Way. What had historically been thought of as satellite galaxies to the Milky Way are now primarily newcomers to our galactic environment.
A dwarf galaxy is a grouping of between thousand and several billion stars. For decades it has been commonly thought that the dwarf galaxies surrounding the Milky Way are satellites, meaning that they are stuck in orbit around our universe and have been our constant companions for many billions of years. The movements of these dwarf galaxies have been estimated with remarkable accuracy owing to data from Gaia's early third data release, and the findings are startling.
François Hammer, Observatoire de Paris—Université Paris Sciences et Lettres, France, and colleagues from throughout Europe and China, utilized the Gaia data to compute the motions of 40 dwarf galaxies around the Milky Way. They achieved this by estimating a set of numbers known as the three-dimensional velocities for each galaxy. They then used them to compute the galaxy's orbital energy and the angular (rotational) momentum.
They determined that these galaxies are travelling far faster than the big stars and star clusters that are known to be circling the Milky Way. So rapidly that they couldn't be in orbit yet around the Milky Way, where interactions with our galaxy and its contents would have drained their orbital energy and angular momentum.
Our galaxy has cannibalized a lot of dwarf galaxies in its history. For example, 8-10 billion years ago, the Milky Way engulfed a dwarf galaxy dubbed Gaia-Enceladus. Its stars can be detected in Gaia data because of the eccentric orbits and variety of energy they contain.
The Sagittarius dwarf galaxy was just grabbed by the Milky Way 4-5 billion years ago and is rapidly being torn apart and absorbed. The energy of its stars is more significant than those of Gaia-Enceladus, reflecting the shorter period that they have been exposed to the Milky Way's influence.
In the current research's case of the dwarf galaxies, which comprises the bulk of the dwarf galaxies encircling the Milky Way, their energy is incredible. This definitely shows that they have just recently arrived in our area in the past few billion years. Billions of years.
The finding replicates one made concerning the Large Magellanic Cloud (LMC), a more giant dwarf galaxy near the Milky Way that is seen as a smudge of light in the night sky from the southern hemisphere. The LMC was assumed to be a satellite galaxy of the Milky Way until the 2000s when scientists studied its velocity and concluded that it was moving too rapidly 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 dwarf galaxies too.
So will these arrivals settle into orbit or just pass us by? "Some of them will be grabbed by the Milky Way and will become satellites," adds François.
But saying precisely which ones is tricky since it relies on the exact mass of the Milky Way, and that is a figure that is impossible for astronomers to quantify with any absolute certainty. Estimates differ by a factor of two.
Because it requires us to reexamine the nature of dwarf galaxies as a whole, finding dwarf galaxy energies is crucial.
As a dwarf galaxy circle, the Milky Way's gravitational pull will attempt to break it apart. In physics, this is known as a tidal force. "The Milky Way is a large galaxy, so its tidal power is just tremendous and it's extremely simple to annihilate a dwarf galaxy after maybe one or two crossings," explains François.
In other words, being a partner to the Milky Way is a death sentence for dwarf galaxies. The only thing that could defy our galaxy's devastating grasp is if the dwarf possessed a considerable mass of dark matter. Astronomers believe in the existence of a mysterious element known as dark matter, which they think is responsible for the additional gravity that holds galaxies together.
And thus, under the old idea that the Milky Way's dwarfs were satellite galaxies that had been in orbit for many billions of years, it was expected that they must be dominated by dark matter to balance the Milky Way's tidal pull and maintain them intact. Given that Gaia has shown that most dwarf galaxies are now orbiting the Milky Way for the first time, we must reevaluate whether these systems are in balance or are undergoing annihilation, given that dark matter is not required for this to occur.
"Since Gaia was launched, it has become clear that the Milky Way's past is significantly more complex than previously thought. We want to learn more about the intriguing history of our galaxy by delving further into these enticing hints "Timo Prusti, Gaia Project Scientist, ESA, states thus.
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