A team of astronomers led by Dr Nelson Caldwell from the Harvard-Smithsonian Center for Astrophysics has discovered a globular star cluster thrown out of its home galaxy Messier 87 (M87 for short).
Globular clusters are relics of the early Universe. These objects usually contain thousands of stars crammed into a ball a few dozen light-years across.
The Milky Way galaxy is home to about 150 globular clusters. The giant elliptical galaxy M87, in contrast, holds thousands.
Dr Caldwell and his co-authors have spent years studying the space around the supergiant elliptical galaxy M87, also known as NGC 4486.
They first sorted targets by color to separate stars and galaxies from globular clusters. Then they used the Hectospec instrument on the MMT Telescope in Arizona to examine hundreds of globular clusters in detail.
A computer automatically analyzed the data and calculated the speed of every cluster. Any oddities were examined by hand.
Most of those turned out to be glitches, but one cluster named HVGC-1 (hypervelocity globular cluster) was something unexpected.
“Astronomers have found runaway stars before, but this is the first time we’ve found a runaway star cluster,” said Dr Caldwell, who is the first author of the paper accepted for publication in the Astrophysical Journal Letters (arXiv.org version).
HVGC-1 now drifts between the galaxies with a speed of about 3,200,000 km per hour.
“We didn’t expect to find anything moving that fast,” said study co-author Dr Jay Strader of Michigan State University.
The discovery of HVGC-1 suggests that the core of M87 holds not one but two super massive black holes.
This must be the result of a long-ago collision between two galaxies, which merged to form a single giant galaxy.