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A billion years ago, in a small galaxy far away, the stars began to disappear. Fatally attracted by much larger neighboring galaxies, the small Hickson Compact Group 98 stretches and twists when its stars disappear. Now a team of astronomers in Israel, the United States and Russia, blessed with luck, advanced technology and patience, have discovered the ghostly remains of this disturbed galaxy, which looks like to a tadpole, with head and tail.
What began as a dwarf galaxy about 300 million light years from Earth is now a cosmic frog baby that is really hard to see even though it spans over a million years. 39, light years from one end to the other. Its tail alone is 500,000 light-years.
In total, this space relic is 10 times longer than the Milky Way, says Dr. Noah Brosch of the Florence Observatory and George Wise of the School of Physics and Astronomy at Tel Aviv University, which has directed the research work.
How far is 300 million light years away? It takes tens of millions of years for its light to reach us, it is far. "When the light left this place, we had the precursors of the dinosaurs on Earth, not even the dinosaurs," says Brosch.
Not alone at home
Our story begins, as we say, a billion years ago by a dwarf galaxy. But wee HCG98 was not alone.
After the Big Bang, the universe was not created equal. Some areas of the primordial universe – and of the universe today – have more material than others (we are not discussing dark matter or energy here, but the type of material we can recognize).
"In astronomy, where do we find a galaxy in the universe? Next to another galaxy, "says Brosch. "They formed in denser areas of the universe and tend to form more or less in the same place. And if there is a large concentration of matter, it will attract more matter. That's how we get a group of galaxies. "
So, even as the universe spreads, galaxies tend to regroup and get closer together than ever before.
In the case of the small HCG98, it seems to have attracted over the eternities to a compact group of perhaps four galaxies, rather close to each other, which exerted a colossal attraction. The gravitational force of two visible galaxies has attracted the stars of this small, vulnerable galaxy.
The head of the tadpole is the area of the dwarf galaxy closest to the two poachers; the stars trailing in the victim galaxy formed the tail.
What we see today is the extended HCG98 process "falling" into this mini-cluster of galaxies, with which it will eventually merge, summarizes Brosch.
"Finally" means in hundreds of millions of years, he clarifies.
When all is over, we will no longer have Hickson Compact Group 98: it will have merged with other neighboring galaxies.
It's quite a feat to notice a galaxy scattering its stars while everything is so pale and far away. To do this, scientists have collected dozens of target images, each exposed through a filter that selects red light while virtually eliminating any light pollution.
When galaxies meet
Billiards is the best help to understand the galactic disruption process.
"When you start a game of billiards, you place the balls in a well-defined triangle. Then you remove that frame and shoot a bullet at the carefully placed balls. Bullets go everywhere. That's the disruption, "says Brosch.
"In our case, the stars of the little galaxy devoured by the two big ones are like the balls in the triangle. They meet the stars and materials of other galaxies and scatter everywhere, "he says.
This means that when the galaxies are disturbed, their stars are either incorporated into the most massive galaxies, or ejected into an intergalactic space.
The name of the tadpole was not more attractive, it was apparently a kind of non-galactic entity. "It was a small galaxy, with just not very bright stars. Now that the stars are dispersing, there is very little left of what is left, "says Brosch.
"The extragalactic tadpole contains a system of two very close" normal "galaxy discs, each about 40,000 light-years away," he adds. "Together with other nearby galaxies, galaxies form a compact group."
The compact groups were identified in 1982 by astronomer Paul Hickson, who published a catalog of 100 of these groups. Our tadpole belongs to the group n ° 98.
In the end, in a billion years, all over there in HCG98 will merge into one galaxy.
Brosch collaborated on the study with Professor R. Michael Rich of UCLA, Dr. Alexandr Mosenkov of the University of St. Petersburg and Dr. Shuki Koriski of Florence Observatory and George Wise of TAU and School of physics and astronomy. The results were published in the Monthly Notices of the Royal Astronomical Society of Oxford University.
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