An arrow points to the super-compact galaxy M60-UCD1, the densest galaxy yet seen, in this composite view of images from NASA's Hubble Space Telescope and Chandra X-ray Observatory. At center is the galaxy M60 and its surrounding regions Chandra X-ray Observatory views are pink, while Hubble Space Telescope data appears in red, green and blue.
Credit: NASA, ESA, CXC, and J. Strader (Michigan State University)
Astronomers using NASA's flagship space telescopes have spotted what appears to be densest nearby galaxy ever seen, with stars packed so tightly that they are likely 25 times closer to each other than the stars in our own Milky Way galaxy.
The super-crowded galaxy is called M60-UCD1 and is located about 54 million light-years away from Earth and the sun. It weighs a whopping 200 million times more than the sun, packing half of this mass within 80 light-years of its center, scientists said. Such crowded conditions make M60-UCD1 a type of ultra-compact dwarf galaxy.
Scientists discovered the galaxy using NASA's Hubble Space Telescope, making follow-up observations using the space agency's Chandra X-ray Observatory and ground-based optical telescopes, such as the Keck Observatory in Hawaii.
Inside the galaxy's heart is a bright source of X-rays that came to light in images from the Chandra X-Ray Observatory. The X-rays may be coming from a giant black hole that's about 10 million times the mass of the sun, astronomers say. And if the X-rays are indeed coming from such a massive black hole, it would mean M60-UCD1 is likely left over from a galaxy that was 50 to 200 times larger.
"Large black holes are not found in star clusters, so if the X-ray source is in fact due to a massive black hole, it was likely produced by collisions between the galaxy and one or more nearby galaxies," a NASA statement read. "The mass of the galaxy and the sun-like abundances of elements also favor the idea that the galaxy is the remnant of a much larger galaxy."
M60-UCD1's growth would have been stunted for billions of years after the collision, astronomers added. In fact, they estimate the dense galaxy has been around for more than 10 billion years.
The research is detailed in the Sept. 20 edition of Astrophysical Journal Letters.