In this episode of Space Sparks, ESA / Hubble summarizes their findings.
Sometimes, in science, when you set out to find something you predicted, you find something completely different instead. This was the case recently when astronomers using the Hubble Space Telescope examined the core of a globular cluster, a densely packed “ball” of old stars called NGC 6397, in search of a central black hole with an intermediate mass. Instead, they suddenly found a whole swarm of small black holes, as reported by NASA and ESA on February 11, 2021.
There are two well-established types of black holes: the stellar black hole that forms when a large star runs out of fuel and collapses and weighs only a few times the mass of our sun, and the supermassive black hole that is believed to exist in the center of each large galaxies and contains the mass of many millions of stars. In addition to these two types, astronomers believe that there should be a type of black hole in the middle of the road – the black hole with intermediate mass, with a mass 100 to 100,000 times larger than the mass of our sun – and there are more many candidates for them, but only a few confirmed cases.
Analysis by these researchers of data from the Hubble Space Telescope and the Gaia Space Observatory on this globular cluster did not provide evidence of a medium-sized black hole at its center that astronomers were looking for. Instead, they first detected a collection of black holes in the center of a globular cluster.
The NGC 6397 globular cluster, which contains hundreds of thousands of stars, was imagined by the Hubble Space Telescope. The blue stars of the cluster are hotter and near the end of their life, while the orange ones are giant red stars. White stars include sun-like stars. Image by NASA / ESA / T. Brown and S. Casertano (STScI).
Astronomers have targeted this globular cluster, NGC 6397, because 7,800 light-years away, it is one of the closest globular groups to Earth. More importantly, scientists believe that a globular cluster is an ideal place to find medium-sized black holes due to the dense collection of stars in their nuclei.
Globular clusters are large spherical collections of stars that orbit the periphery of galaxies. Clusters are old, sometimes almost as old as the universe itself. A collapsed globular cluster, such as NGC 6397, is old enough that more massive stars have gravitated toward the center of the cluster, and younger stars have traveled to its periphery. This gives the globular group a very dense nucleus.
Because these black holes are not directly observable, astronomers have measured how the stars in the cluster move – their speed – to find the mass distribution in the cluster. The locations where the stars have been found to move faster are areas where more mass is concentrated. However, the distribution of these stars was not limited to a central location similar to a point in the core, as would be expected from the presence of an intermediate-sized black hole. Instead, the mass appears to be more randomly distributed, expanding to a few percent of the cluster size.
Therefore, based on stellar evolution, astronomers have concluded that the remnants of stars in the form of black holes with stellar mass populate the inner regions of the globular cluster. NGC 6397 could house more than 20 of them, the “lightest” type of black holes.
This artist’s concept shows the concentration of black holes in the center of the globular group NGC 6397. In reality, black holes are far too small for direct observation by any existing telescopes. This globular cluster can contain more than 20 black holes. Image by ESA / Hubble / N. Bartmann.
Like many scientific discoveries, finding a concentration of black holes in a globular cluster raises several questions, one of which is: can the fusion of these black holes pressed into nearby spaces lead to gravitational waves? Maybe scientists will find out if the black holes that collapse together produce the gravitational waves recently seen by some instruments and maybe they will discover something they didn’t even plan.
Bottom line: Astronomers searched for a medium-sized black hole in the center of a globular cluster and instead found a whole swarm of smaller black holes.
Source: Does the NGC 6397 contain a black hole with intermediate mass or a more diffuse inner subcluster?
Through NASA
Via ESA
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