It’s long been presumed that millions of ancient compact spherical galaxies, abundant in the early universe about 11 billion years ago but scarce now, collided and formed much larger clouds of stars with little form. But a new study, published in The Astrophysical Journal, suggests that these “galactic dinosaurs” didn’t go extinct after all, and have actually been hiding in plain sight this whole time.
When looking at the early universe, astronomers in 2005 found that there was an overabundance of smaller, more compact spherical galaxies, which was at odds with what they see today as the universe is now filled with large “elliptical” galaxies and spiral galaxies such as our Milky Way. Whilst most had just assumed that the smaller galaxies had smashed together to form the larger galaxies, this didn’t quite explain it. There were not enough merger events to justify the number of elliptical galaxies seen.
Researchers working at Swinburne University of Technology think that they’ve managed to solve this mystery. When the scientists looked more closely at the galaxies in the local universe, they realized that they were misidentified. What were thought to be massive 3D clusters of stars in loose association were actually flat disks with bulging spherical centers.
“They were hiding in plain sight,” explained Dr. Bililign Dullo, co-author of the research. “The spheroids are cloaked by disks of stars that were likely built from the accumulation of hydrogen gas and smaller galaxies over the intervening eons.”
The scientists went on to find that the bulging middles had the same mass and size as the galaxies seen during the early stages of the universe, accounting for the missing galaxies. They were misidentified because they were not observed edge-on to the Earth and so looked like massive 3D clusters of stars, rather than the 2D disks they turned out to be.
“While the inner component is compact and massive, the full galaxy sizes are not compact,” says Giulia Savorgnan, a PhD student involved in the discovery. “This explains why they had been missed; we simply needed to better dissect the galaxies rather than consider them as single objects.”
This could go on to explain in part our own Milky Way’s central bulge. According to the researchers, the age of some of the stars in its center could certainly point towards this being the remains of one of these spherical galaxies, though they stress that they’re unsure what proportion of its shape is due to other processes.
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