These Crazy-Dense Galaxies are Packed With Stars

The most dense galaxies known to exist in our universe have been discovered by 2 undergraduate students while looking through years of archival data. The discovery of the galactic pair makes them the most dense of a new category of galaxy, called ultracompact dwarfs (UCDs), discovered to date.

To say the galaxies are “dense” is an understatement. The first galaxy, called M59-UCD3, is around 200 times smaller than our galaxy, but has a density 10,000 times the density of stars surrounding our solar system. The second galaxy found, called M85-HCC1, is even more dense; around a million times that of the solar system’s neighborhood.

For an observer standing on a hypothetical planet in the core of one of these UCDs, the “night” sky would dense with bright stars (as depicted in the artist’s impression above).

Although notable, the pair of UCDs weren’t easy to spot and the first UCD was uncovered by pure luck.

“Ultracompact stellar systems like these are easy to find once you know what to look for,” said Richard Vo, undergraduate student at San José State University. “However, they were overlooked for decades because no one imagined such objects existed: they were hiding in plain sight.”

“When we discovered one UCD serendipitously, we realized there must be others, and we set out to find them.”

Vo and fellow student Michael Sandoval scoured data from the Sloan Digital Sky Survey, the Subaru Telescope, Hubble Space Telescope, the Goodman Spectrograph on the Southern Astrophysical Research Telescope (SOAR), revealing the 2 UCDs orbiting larger, massive galaxies. Their close proximity to larger galaxies may reveal how these UCDs came to be so dense.

“One of the best clues is that some UCDs host overweight supermassive black holes, sadi Sandoval. “This suggests that UCDs were originally much bigger galaxies with normal supermassive black holes, whose fluffy outer parts were stripped away, leaving their dense centers behind. This is plausible because the known UCDs are found near massive galaxies that could have done the stripping.”

An additional clue that this may be the case is that spectroscopic analysis has revealed an abundance of heavy elements such as iron in the UCDs — a characteristic of much larger galaxies that are efficient heavy element factories. Their findings have been published in a Astrophysical Journal Letters paper.

The availability of archival data and dedication by Sandoval and Vo, who undertook this search in their spare time, helped them find these UCDs, despite not having access to astronomical facilities.

Read more at Discovery News