Entire galaxies are being hurled from their clusters, reaching escape velocity as they race for the outermost reaches of space. In the process of discovering this, astronomers have settled a debate about how a class of galaxies are formed.
Humans have accelerated spacecrafts to such speeds that they escape the gravitational grip of our sun. For this to happen naturally, however, it usually requires an interaction with two larger bodies such that the smaller object is hurled into interstellar space. The same process can occur when stars, or even clusters, are expelled from galaxies.
Unsurprisingly, the same dynamics occur within galactic clusters. Galaxies interact with each other frequently, often growing by absorbing smaller objects or stealing less tightly held stars. Nevertheless, the speed it takes for a galaxy to escape its cluster is immense, roughly 3,000 km/s (almost 7 million mph).
Dr. Igor Chilingarian of the Harvard-Smithsonian Center for Astrophysics reports in Science on 11 compact elliptical galaxies that are isolated from other galaxies.
Compact elliptical galaxies are small by galactic standards, around a thousandth of the mass of the Milky Way. The most famous example is M32, beloved of amateur astronomers. Their density is unusually high for something so small, but as light as they are, their mass is packed into a very tight and brightly lit space.
Their unexplained origins interested Chilingarian. Initially, compact ellipticals were thought to represent the remnants of moderate-sized galaxies whose outer reaches are stripped in an encounter. Something similar, but on a smaller scale, is thought to account for some of the globular clusters that surround galaxies like the Milky Way.
However, two years ago an isolated compact elliptical galaxy was found, followed by another. If there was no nearby galaxy that could have done the stripping, then how could these galaxies have lost their outer layers? And if they never had such extensions, why do researchers assume that other compact ellipticals form in the same way?
When Chilingarian started his research, there were 30 known compact ellipticals galaxies, but, “We recognized we could use the power of the archives to potentially unearth something interesting,” Chilingarian says. “And we did." He and his co-author found so many that had been photographed but not identified that they increased the known population to 195.
The commonalities in the sample suggested all had similar origins. "We conclude that the tidal stripping process can explain all observational manifestations of compact elliptical galaxies," the paper notes. Yet some were moving strangely fast. "We asked ourselves, what else could explain them?" says Chilingarian. “The answer was a classic three-body interaction."
However, where some survived encounters with larger galaxies, losing only their more distant stars, others had their encounter crashed by another heavyweight. These suffered the further insult of being flung away from the galaxy that stripped them, sometimes so fast that they escaped the entire cluster, or soon will.
“These galaxies are facing a lonely future, exiled from the galaxy clusters they used to live in," Chilingarian says.