In this episode of Space Sparks, ESA / Hubble summarizes their findings.
Sometimes in science, when you set out to find something you had predicted, you will find something quite different. This was the case recently when astronomers using the Hubble Space Telescope examined the core of a globular cluster, a “ball” of densely packaged old stars called NGC 6397, looking for a central black hole of intermediate mass. Instead, they unexpectedly 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 black hole of stellar mass 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 it is believed. exist in the center of each large galaxy and contains the mass of many millions of stars. Apart from these two types, astronomers believe that there should also be a black hole in the middle of the road, the black hole of intermediate mass, with a mass 100 to 100,000 times the mass of our sun, and there there are several candidates for these, but only a few confirmed cases.
The 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 in the center that astronomers were looking for. Instead, they made the first detection of a collection of black holes in the center of a globular cluster.
The globular cluster NGC 6397, which contained hundreds of thousands of stars, was imagined by the Hubble Space Telescope. The blue stars in the cluster are warmer and near the end of their lives, while the oranges are expanded red giant stars. White stars include stars similar to our sun. Image via NASA / ESA / T. Brown and S. Casertano (STScI).
Astronomers went to this particular globular cluster, NGC 6397, because 7,800 light-years away it is one of the closest globular clusters to Earth. More importantly, scientists think 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 ancient, sometimes almost as old as the universe itself. A collapsed globular cluster, such as NGC 6397, is large enough because the most massive stars have gravitated toward the center of the cluster and the youngest stars have traveled to its outskirts. This gives the globular cluster a very dense nucleus.
Because these black holes cannot be observed directly, astronomers measured how the stars in the cluster (their velocities) move to find the mass distribution in the cluster. The places where stars were found to move faster are areas where more concentration is concentrated. However, the distribution of these stars was not limited to a central location resembling a point in the core, as was expected from the presence of an intermediate-sized black hole. In contrast, the mass appears to be more randomly distributed and extends up to a percentage of the cluster size.
Therefore, based on stellar evolution, astronomers concluded that the remains of stars in the form of black holes of stellar mass populate the inner regions of the globular cluster. NGC 6397 could accommodate more than 20, the type of “lighter” black holes.
The concept of this artist shows the concentration of black holes in the center of the globular cluster NGC 6397. In reality, the black holes are too small for direct observation of any existing telescope. This globular cluster can contain more than 20 black holes. Image via ESA / Hubble / N. Bartmann.
Like many scientific discoveries, the finding of a concentration of black holes in a globular cluster raises more questions, being one of the main ones: could the fusion of these black holes pressed in nearby neighborhoods could cause gravitational waves? Maybe scientists will find out if the colliding black holes produce the ripples of gravity that some instruments have recently seen, and maybe they’ll discover something they didn’t even plan.
Conclusion: Astronomers looked 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 NGC 6397 contain a black hole of intermediate mass or a more diffuse internal subcluster?
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Via ESA
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