Black hole clusters

Two scientists at the Argelander Institute for Astronomy at Bonn University originally set out to explore the physical properties of dying star clusters. But in the course of their computer-generated research, they became aware of the likely existence of dark clusters.

"Under certain conditions exceptionally many black holes develop into a star cluster," Alexander von Humboldt fellow Sambaran Banerjee wrote in the November 1 issue of the journal Astrophysical Journal Letters. "In these cases, we propose to call them dark star clusters."

Star clusters are vast stellar constellations, bound together by gravity. They generally tend to form at the same time and live for millions if not billions of years. If they are not too old when they die, the stars go out in luminous supernova explosions, and if, when that happens, their mass is great enough, they leave gravitationally-powerful neutron stars and black holes in their wake.

Scientists say when several black holes and neutron stars form within close range of each other that these so-called "dark clusters" begin to emerge.

The Bonn-based researchers' calculations suggest there could be hundreds of them in our galaxy, and that given the right equipment, their unique characteristics would make them relatively easy to identify.

"These objects are clusters in which the stars will be moving faster than normal," Banerjee told Deutsche Welle, adding that they appear to be held together by an invisible force. "They are also more compact than usual star clusters and are typically made up of old stars."

To find or not to find

Marianne Vestergaard, an associate professor at the University of Copenhagen's Dark Cosmology Center says the dark clusters would undoubtedly provide interesting and valid information, provided that they can actually be found.

"There is a lot of talk about up and coming gravity wave detectors," Vestergaard told Deutsche Welle. "But we won't know if they will reveal signals in the sky until they actually start turning them on."

And even when the detectors are up and running, she warns against interpreting the galactic secrets they may or may not reveal as all-telling.

"If they don't see anything, that wouldn't necessarily mean the star systems don't exist, just that we need to do more detailed calculations."

High kicks, low kicks

The scientists are hoping their current calculations can be proved by means of observation within the next couple of years.

Locating dark clusters would help give them a greater understanding of gravitational wave generation, the evolution of star clusters and the physics of supernova explosions.

What they already know is that when these explosions occur, the black holes that form as a result are born with a 'kick' - some more powerful than others. As Banerjee explains, those with a kick of several hundred kilometers per second are ejected from the cluster and lost.

"What this means is that if dark clusters are found, they will mostly comprise black holes with low kicks."

According to the scientist's calculations, they will also be found within some 15,000 light years from the center of our Milky Way, because they could achieve the dark phase any further away.

Being optimistic, he says he would ultimately expect a tenth of the clusters to be located. But whether that happens or not is still written in the stars.

Author: Tamsin Walker
Editor: Cyrus Farivar