Densest galaxy is jam-packed with stars

This image shows M60 and the region around it, based on data from NASA's Chandra X-ray Observatory, which show the presence of a bright X-ray source in its center. One explanation for this source is a giant black hole weighing in at some 10 million times the mass of the Sun. (Credit: NASA/CXC/MSU/J.Strader et al)

Astronomers have discovered the densest galaxy in the nearby universe. The stars are about 25 times closer than those in the Milky Way.

Imagine the distance between the sun and the star nearest to it—a star called Alpha Centauri. That’s a distance of about 4 light years. Now, imagine as many as 10,000 of our suns crammed into that relatively small space.

The inset is a close-up view of M60-UCD1 in an Hubble Space Telescope image. (Credit: X-ray: NASA/CXC/MSU/J.Strader et al, Optical: NASA/STScI_
The inset is a close-up view of M60-UCD1 in a Hubble Space Telescope image. (Credit: X-ray: NASA/CXC/MSU/J.Strader et al, Optical: NASA/STScI)
Hubble's view of Galaxy M60 and M60-UCD1. (Credit: NASA, ESA, CXC, and J. Strader/Michigan State)
Hubble’s view of Galaxy M60 and M60-UCD1. (Credit: NASA, ESA, CXC, and J. Strader/Michigan State)

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“This galaxy is more massive than any ultra-compact drawfs of comparable size,” says Jay Strader, assistant professor of physics and astronomy at Michigan State University, “and is arguably the densest galaxy known in the local universe.”

As detailed in the recent edition of the publication Astrophysical Journal Letters, the ultra-compact dwarf galaxy was found in near the massive elliptical galaxy NGC 4649, also called M60, about 54 million light years from Earth.

What makes this galaxy, dubbed M60-UCD1, so remarkable is that about half of its mass is found within a radius of only about 80 light years. This would make the density of stars about 15,000 times greater than that found in Earth’s neighborhood in the Milky Way.

“Traveling from one star to another would be a lot easier in M60-UCD1 than it is in our galaxy,” Strader says. “Since the stars are so much closer in this galaxy, it would take just a fraction of the time.”

The discovery of ultra-compact galaxies is relatively new—only within the past 10 years or so. Until then, astronomers could see these “things” way off in the distance but assumed they were either single stars or very-distant galaxies.

Another intriguing aspect of this galaxy is the presence of a bright X-ray source in its center. One explanation for this is a giant black hole weighing in at some 10 million times the mass of our sun.

Astronomers are trying to determine if M60-UCD1 and other ultra-compact dwarf galaxies are either born as really jam-packed star clusters or if they are galaxies that get smaller because they have stars ripped away from them.

The possible massive black hole, combined with the high galaxy mass and sun-like levels of elements found in the stars, favor the latter idea.

A giant black hole at the center of M60-UCD1 helps tip the scales against the scenario where this galaxy was once a star cluster, since such large black holes are not found in these types of objects.

The galaxy was discovered using NASA’s Hubble Space Telescope. Follow-up observations were done with NASA’s Chandra X-ray Observatory and ground-based optical telescopes, including the Keck 10-meter telescope in Hawaii.

“Twenty years ago we couldn’t have done this,” Strader says. “We didn’t have Hubble or Chandra. This is one of those projects where you bring together the full force of NASA’s great observatories, plus ground-based resources.”

Source: Michigan State University