It can be pretty tough when your community throws you out, but it helps if your closest neighbors are banished with you. This is the case for a star cluster that has been found to have been hurled from the warm embrace of the enormous M87 galaxy.
M87 is a seriously big galaxy, classified as a supergiant. It is the second brightest of the more than 1000 galaxies in the Virgo Supercluster, and one of the largest in the part of the universe we can easily see. A galaxy that big doesn't do things by halves, and that includes ejecting stars.
“Astronomers have found runaway stars before, but this is the first time we've found a runaway star cluster,” said Dr Nelson Caldwell of the Harvard-Smithsonian Center for Astrophysics. In The Astrophysical Journal Letters Caldwell describes hypervelocity globular cluster one (HVGC-1).
The name is no exaggeration. HVGC-1 is coming towards the Earth at at 1025km/s, the fastest approach ever measured from an object not in orbit around something else. If that's not fast enough (four times the speed the sun orbits the center of the galaxy) HVGC-1's speed compared to the galaxy that didn't want it is over 2300km/s. “We didn't expect to find anything moving that fast,” said Michigan State University's Assistant Professor Jay Strader, a co-author on the paper. The velocity is well off the curve of anything else measured.
Caldwell and Strader were examining the globular clusters that surround M87 like islands around a continent, each containing hundreds of thousands or millions of stars. Globulars are common features of substantial galaxies, but their numbers scale much more dramatically than the size of the galaxy itself. The Milky way has between 150 and 200 dotted around its margins. M87, on the other hand, has about 13,000.
After eliminating foreground stars and background galaxies from the globular clusters around M87, Caldwell and Strader set a computer to analyze the color of each cluster, seeking those that are Doppler shifted by significantly different amounts than the galaxy itself. While a few initially produced extreme results, only HVGC-1 turned out to be more than a misreading.
The researchers admit they don't know how this cluster came to be moving so fast, but think by far the most likely explanation is a close interaction with a third object. They speculate that M87 may host, or may once have hosted, a second supermassive blackhole, along with the one at its heart. This is not unlikely. Large galaxies get so big by swallowing smaller ones, and sometimes the black hole at the center of the digested galaxy remains independent, at least for a while. A cluster of stars that got caught in the gravitational whirlpool of two interacting objects of such enormous mass could find itself on a fast trip to the great beyond, just as small stars sometimes do when they pass too close to a close binary star system.
At this distance the instruments used to observe HVGC-1 were not powerful enough to get a clear look at it – in fact the images were so limited that several tests were required to make sure the cluster was not a star.
Whatever it was that happened to HVGC-1 it appears to have stripped off the outlying stars, leaving just a densely packed core. This is not unusual. Many globular clusters are reduced to their core by interactions with other objects. It is thought that at least some globulars are the cores of dwarf galaxies stripped of their outer stars during close encounters with their parent galaxy, and there is debate as to whether this is the origin of a large proportion of globulars.
The paper notes that studies with the Hubble Telescope might answer some of the remaining questions about HVGC-1, including confirming their conclusion that it's radial velocity is high enough to see it escape M87 forever.