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These Super-Dense Galaxies Are Bursting With Stars

Life in these systems would have trouble starting, but the night skies would be spectacular to behold

An artist’s vision of what the night sky would look like from a planet at the heart of an ultracompact galaxy (NASA, ESA, G. Bacon (STScI) and P. van Dokkum (Yale University))
smithsonian.com

What would the night sky look like if you could see more than one million stars? After all, despite living in a galaxy home to 200 to 400 billion stars, we can only see about 9,000. A more intensely star-studded sky is a reality in two newly discovered galaxies, reports George Dvorsky for io9.com.

Both M59-UCD3 and M85-HCC1 are called ultracompact dwarf galaxies. They are about 10,000 and one million times more dense, respectively, than the local Milky Way neighborhood. Researchers just confirmed these super-dense star systems in a study published in Astrophysical Journal Letters

“The typical distance between the stars in M59-UCD3 is one-quarter of a light year (20 times less than in the Earth’s vicinity), and in M85-HCC1 is 1/20th of a light year (100 times less),” study co-author Aaron Romanowsky of San Jose State University in California told io9.com. If stars were as close here, our solar system would have to be much smaller — and wouldn’t have enough room to support icy, far-off regions like the Oort cloud where Pluto lives.

The research team figured out that both galaxies are relatively young, too: a youthful 9 billion and 3 billion years old, respectively. They’re also the densest known galaxies yet discovered.

Systems like these have been “hiding in plain sight" for years, says co-author Richard Vo, in a press release from the National Optical Astronomy Observatory. “When we discovered one [ultracompact dwarf] serendipitously, we realized there must be others, and we set out to find them.”

But how did they get so dense? Ultracompact dwarf systems could be the core of compact dwarf galaxies, with the less dense, outer stars somehow stripped away, explains co-author Michael Sandoval in the release. Larger, nearby galaxies could have pulled the relatively fluffy outer regions away. Many such systems do have supermassive black holes in their center that seem big for their compact size, but might indicate that they used to belong to larger galaxies.

Astronomers are interested in finding out how common these systems are. It would tell them more about the evolution of galaxies in the universe as a whole, Romanowsky told io9.com. “They could provide information about how many smaller galaxies were swallowed by the bigger ones—like someone eating cherries and leaving the pits behind," he says.

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