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Tiny, but still densest known galaxies

Smaller in width than our Milky Way, their stars are packed 10,000 to a million times more densely than in our sun’s neighborhood. Imagine the night sky!

View larger. | Two undergraduate students in astronomy have discovered two ultra-dense galaxies (insets).  The galaxies are orbiting larger host galaxies. The compact systems are thought to be the remnants of once normal galaxies that were swallowed by the host, a process that removed the fluffy outer parts of the systems, leaving the dense centers behind. Image via A. Romanowsky (SJSU), Subaru, Hubble Legacy Archive

View larger. | Two ultra-dense galaxies have been found, orbiting larger host galaxies. They might be the remnants of once normal galaxies that were swallowed by the host, a process that removed the fluffy outer parts of the systems, leaving the dense centers behind. Image via A. Romanowsky (SJSU), Subaru, Hubble Legacy Archive

Two undergraduate students in astronomy at San José State University have discovered two galaxies that are now considered the densest known. The National Optical Astronomy Observatory (NOAO) made the announcement today (July 27, 2015) in conjunction with the publication of the work at arxiv.org. They said these galaxies are reminiscent of the ordinary globular star clusters that orbit the centers of our galaxy and others. But the ultra-dense galaxies are 100 to 1,000 times brighter.

The first system is called M59-UCD3 by astronomers. It’s 200 times smaller in width than our own Milky Way galaxy, but the density of its stars is 10,000 times larger than that in the neighborhood of our sun. For an observer on a planet orbiting one of the stars in the core of M59-UCD3, the night sky would be a dazzling display, lit up by a million stars.

The second system, M85-HCC1, has an even higher density: its stars are packed about a million times more tightly than in our sun’s neighborhood.

Both systems belong to the new class of galaxies known as ultracompact dwarfs.

Undergraduate students Michael Sandoval and Richard Vo discovered the two galaxies, using imaging data from the Sloan Digital Sky Survey, the Subaru Telescope, and Hubble Space Telescope, as well as spectroscopy from the the Southern Astrophysical Research Telescope (SOAR) in Chile. NOAO, which made today’s announcement, is a SOAR partner.

The SOAR spectrum was used to show that M59-UCD3 is associated with a larger host galaxy, M59, and to measure the age and elemental abundances of the galaxy’s stars. Richard Vo explained:

Ultracompact stellar systems like these are easy to find once you know what to look for. However, they were overlooked for decades because no one imagined such objects existed: they were hiding in plain sight.

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

According to the NOAO statement:

The students were motivated by the idea that all it takes to initiate a discovery is a good idea, archival data, and dedication. The last element was critical, because the students worked on the project on their own time.

So what are these ultracompact dwarf galaxies, and how did they become so small and compact? At present, no one knows. The dense galaxies may be stripped-down cores of formerly normal galaxies. Or they might be super-clusters of stars that somehow merged. Or they might be genuine compact dwarf galaxies formed by minute fluctuation in the dark matter believed to form all galaxies.

Michael Sandoval favors the stripped hypothesis. He said:

One of the best clues is that some ultracompact dwarfs host overweight supermassive black holes. This suggests they 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.

The video below shows how that might happen.

An additional line of evidence is the high abundance of heavy elements such as iron in ultracompact dwarf galaxies. Because large galaxies are more efficient factories to make these metals, a high metal content may indicate that the galaxy used to be much larger.

To test this hypothesis, the team will investigate the motions of stars in the center of M59-UCD3 to look for a supermassive black hole. They are also on the hunt for more UCDs, to understand how commonly they occur and how diverse they are.

Bottom line: Two astronomy undergraduates have discovered what are now the densest known galaxies, called ultracompact dwarfs (UCDs). One – known as M59-UCD3 – is 200 times smaller in width than our own Milky Way galaxy, but the density of its stars is 10,000 times larger than that in the neighborhood of our sun. The second system, M85-HCC1, has an even higher density of stars, a million times that of our sun’s neighborhood

Deborah Byrd

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