A billion years ago, in a tiny galaxy far, far away, stars started to vanish. Fatally attracted by much larger nearby galaxies, little Hickson Compact Group 98 stretched and writhed as its stars were stripped off. Now a team of astronomers in Israel, the United States and Russia, equipped with luck, cutting-edge technology and patience, have discovered the ghostly remains of this disrupted galaxy, which looks like a tadpole, complete with head and tail.
What began as a dwarf galaxy about 300 million light years away from Earth is now a cosmic baby frog that is really hard to see, though it stretches a million light years from end to end. Its tail alone is 500,000 light years long.
Altogether, this space relic is 10 times longer than the Milky Way, says Dr. Noah Brosch of the Florence and George Wise Observatory at Tel Aviv University’s School of Physics and Astronomy, who led the research for the study.
How far away is 300 million light years? It takes 300 million years for its light to reach us, that’s how far. “When the light left this place, what we had on Earth was the precursors of the dinosaurs, not even the dinosaurs themselves,” Brosch explains.
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Our story begins, as said, a billion years ago with a dwarf galaxy. But wee HCG98 was not alone.
Following the Big Bang, the universe was not created equal. Some areas of the primordial universe – and of the universe today – have more matter than others (we are not discussing dark matter or energy here, but the kind of matter we can recognize).
“In astronomy, where do we find a galaxy in the universe? Next to another galaxy,” says Brosch. “They formed in denser regions of the universe, and tend to be formed more or less in the same place. And if there is a large concentration of matter – it will attract more matter. That is how we get a cluster of galaxies.”
So even though the universe is spreading, wherever there are galaxies they tend to group and grow closer than ever.
In the case of little HCG98, it seems to have become attracted over the eons to a compact group of perhaps four galaxies, rather close to each other, that exerted a colossal pull. The gravitational force of two visible galaxies attracted stars away from this vulnerable small galaxy.
The head of the tadpole is the area of the dwarf galaxy that’s nearest to the two poachers; stars lingering in the victim galaxy formed the tail.
What we see today is the process of stretched-out HCG98 “falling” into this mini-cluster of galaxies, with which it will eventually merge, Brosch sums up.
“Eventually” means in hundreds of millions of years, he clarifies.
When it is all over, wee Hickson Compact Group 98 will be no more: it will have merged with the other galaxies next to it.
It is quite a feat to notice a galaxy scattering its stars when the whole thing is so faint and far away. The scientists did this by collecting dozens of images of the targets, each exposed through a filter that selects red light while virtually eliminating light pollution.
When galaxies meet
The best aid to understanding the process of galactic disruption is the pool table.
“When you start a game of billiards, you put the balls in a triangle, nicely arranged. Then you take away that frame and shoot a ball toward the neatly arranged balls. The balls go everywhere. That is disruption,” says Brosch.
“In our case, the stars of the small galaxy eaten by the two big ones are like the balls in the triangle. They meet the stars and material in the other galaxies and scatter everywhere,” he explains.
Which means that when galaxies are disrupted, their stars are either incorporated into the more massive galaxies or get ejected into intergalactic space.
The tadpole hasn’t been given a more attractive name because it had been something of a galactic nonentity to begin with, apparently. “It had been a small galaxy, just with stars not especially bright. Now that the stars are scattering, what remains there is very faint,” says Brosch.
“The extragalactic tadpole contains a system of two very close ‘normal’ disc galaxies, each about 40,000 light years across,” he adds. “Together with other nearby galaxies, the galaxies form a compact group.”
Compact groups were identified in 1982 by the astronomer Paul Hickson, who published a catalog of 100 such groups. Our tadpole is in group No. 98.
Ultimately, in a billion years the whole lot over there in HCG98 will merge into a single galaxy.
Brosch collaborated on the study with Prof. R. Michael Rich of UCLA, Dr. Alexandr Mosenkov of St. Petersburg University and Dr. Shuki Koriski of TAU’s Florence and George Wise Observatory and School of Physics and Astronomy. The results were published in Oxford University’s Monthly Notices of the Royal Astronomical Society.