Science: most compact galaxy ever found

Ultra-compact dwarf galaxy discovered


The most crowded galaxy in the known Universe has been discovered by an international team of astronomers, including researchers from Swinburne University of Technology’s Centre for Astrophysics and Supercomputing.

The ultra-compact dwarf galaxy, known as M60-UCD1, may be the densest galaxy close to Earth, packed with an extraordinary number of stars.

Using imaging from the Hubble Space Telescope to first identify the object, a spectrum of the galaxy was obtained with the Keck II telescope in Hawaii, using the DEIMOS spectrograph. This spectrum was used to confirm its distance and to measure the internal motions of its stars.

M60-UCD1 is estimated to be about 10 billion years old, and is about 54 million light years from Earth. It is the most luminous known galaxy of its type and one of the most massive, weighing 200 million times more than our Sun, based on observations with the Keck 10-meter telescope.

What makes M60-UCD1 so remarkable is that about half of this mass is found within a radius of only about 80 light years. The density of stars is about 15,000 times greater — meaning the stars are about 25 times closer to each other — than in Earth’s neighborhood in the Milky Way.

“The nearest star to the Sun is some four light years away. But in the densest galaxy, there would be as many as 10,000 stars within a distance of four light years,” Swinburne’s Professor Duncan Forbes said.

Astronomers want to find out whether this ultra-compact dwarf galaxy was born as a jam-packed star cluster or became more compact as stars were ripped away from it.

“We think nearly all of the stars have been pulled away from the exterior of what once was a much bigger galaxy,” Professor Forbes said. “This leaves behind just the very dense nucleus of the former galaxy, and an overly massive black hole.”

If this stripping did occur, then the galaxy originally was 50 to 200 times more massive than it is now, and the mass of its black hole relative to the original mass of the galaxy would be more like that of the Milky Way and many other galaxies.

The stripping could have taken place long ago and M60-UCD1 may have been stalled at its current size for several billion years.

“Extreme objects often lead to new avenues of research. We look forward to seeing where this discovery will lead us,” Professor Forbes said.

The research team included Jay Strader (Michigan State University), Anil C Seth (University of Utah), Duncan Forbes, Vincenzo Pota and Christopher Usher (Swinburne University of Technology), Giuseppina Fabbiano and Nelson Caldwell (Harvard-Smithsonian Center for Astrophysics), Aaron J Romanowsky (San Jose State University), Jean Brodie and Jacob Arnold (University of California), Chalrie Conroy (University of California, Santa Cruz).

The results of this work have been published in the latest edition of the Astrophysical Journal Letters.

The W M Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-metre optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and a world-leading laser guide star adaptive optics system. The Observatory is a private 501(c) 3 non-profit organisation and a scientific partnership of the California Institute of Technology, the University of California and NASA.