February 28, 2015
‘Galactic dinosaurs’ found hiding in plain sight
Astronomers have long wondered exactly what fate befell the compact massive galaxies that could be found throughout the universe during its infancy, but new research from experts at the Swinburne University of Technology may have finally discovered the answer.According to Alister Graham, a professor of astronomy at the university and lead author of a paper scheduled for publication in the Astrophysical Journal, “When our Universe was young, there were lots of compact, elliptical-shaped galaxies containing trillions of stars.”
“Due to the time it takes for light to travel across the vastness of space, we see these distant galaxies as they were in our young Universe. However in the present-day Universe very few such spheroidal stellar systems have been observed,” he added.
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Most researchers believed that mergers that took place over time would have caused these compact massive galaxies to become destroyed and transformed into larger, elliptical galaxies. However, there have been too few galactic collisions to fully account for the tremendous reduction in these compact spheroids, according to Graham and his colleagues.
In their new paper, the professor and co-authors Dr. Bililign Dullo and Ph. D. student Giulia Savorgnan explain that they have eliminated the need for theories to explain the disappearance of these galaxies – because they have located them.
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“They were hiding in plain sight,” Dr. Dullo, whose work was sponsored by the Australian Research Council, explained. “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.”
Furthermore, the number of these hidden systems is roughly equal to the number of compact galaxies that would have existed during the early Universe, the researchers noted. These galaxies did not die out, Graham said. Rather, they are “embedded in large, relatively thin, disks of stars.”
Because of the massive scale of modern-day galaxy surveys, it has become common to treat all individual galaxies as single-component entities, the study authors said. By carefully separating the components of each one, and especially the outer disk and the inner spheroid, the researchers could finally discover the whereabouts of these missing ancient galaxies.
“While the inner component is compact and massive, the full galaxy sizes are not compact,” explained Savorgnan. “This explains why they had been missed; we simply needed to better dissect the galaxies rather than consider them as single objects.”
Even our own Milky Way galaxy may be subject to this phenomenon, she and her colleagues added. Its central spheroid appears to have existed, at least partially, when the universe was very young.
Some of our galaxy’s stars are 12 billion years old, which is not much younger than the age of the universe itself. The question that remains, and can only be answered through future studies, is what percentage of the Milky Way’s bulge may have been built through other processes.