Medium-sized black holes are rare, and might exist only in small and large form

Washington, August 21 : A new study has determined that medium-sized black holes are quite rare, and they might exist only in small and large form.

Astronomers have long suspected that the most likely place to find a medium-mass black hole would be at the core of a miniature galaxy-like object called a globular cluster. Yet nobody has been able to find one conclusively.

Now, a team of astronomers has thoroughly examined a globular cluster called RZ2109 and determined that it cannot possess a medium black hole.

The findings suggest that the elusive objects do not lurk in globular clusters, and perhaps are very rare.

"Some theories say that small black holes in globular clusters should sink down to the center and form a medium-sized one, but our discovery suggests this isn't true," said Daniel Stern of NASA's Jet Propulsion Laboratory in Pasadena, California.

Black holes are incredibly dense points of matter, whose gravity prevents even light from escaping.

The least massive black holes known are about 10 times the mass of the sun and form when massive stars blow up in supernova explosions.

The heftiest black holes are up to billions of times the mass of the sun and lie deep in the bellies of almost all galaxies.

That leaves black holes of intermediate mass, which were thought to be buried at the cores of globular clusters.

Globular clusters are dense collections of millions of stars, which reside within galaxies containing hundreds of billions of stars.

Theorists argue that a globular cluster should have a scaled down version of a galactic black hole. Such objects would be about 1,000 to 10,000 times the mass of the sun, or medium in size on the universal scale of black holes.

According to theory, a cluster with a small black hole cannot have a medium one, too.

Medium black holes would be quite hefty with a lot of gravity, so if one did exist in a globular cluster, scientists argue that it would quickly drag any small black holes into its grasp.

"If a medium black hole existed in a cluster, it would either swallow little black holes or kick them out of the cluster," said Stern.

In other words, the small black hole in RZ2109 rules out the possibility of a medium one.

As to how the scientists figure out that the globular cluster's black hole was small in the first place, they used modeling techniques.

Zepf and his colleagues concluded that the spectrum taken by Keck reveals high-velocity flows of matter, or "winds," firing out of the black hole.

Only a small black hole could spit out these observed high winds.

ANI