Starship Asterisk*

Hubble Uncovers Concentration of Small Black Holes
ESA Hubble Science Release | 2021 Feb 11

Image Credit: NASA, ESA, T. Brown, S. Casertano, and J. Anderson (STScI) Science Credit: NASA, ESA, and E. Vitral and G. Mamon (IAP)

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Scientists were expecting to find an intermediate-mass black hole at the heart of the globular cluster NGC 6397, but instead they found evidence of a concentration of smaller black holes lurking there. New data from the NASA/ESA Hubble Space Telescope have led to the first measurement of the extent of a collection of black holes in a core-collapsed globular cluster.

Globular clusters are extremely dense stellar systems, in which stars are packed closely together. They are also typically very old — the globular cluster that is the focus of this study, NGC 6397, is almost as old as the Universe itself. It resides 7800 light-years away, making it one of the closest globular clusters to Earth. Because of its very dense nucleus, it is known as a core-collapsed cluster.

When Eduardo Vitral and Gary A. Mamon of the Institut d’Astrophysique de Paris (IAP) set out to study the core of NGC 6397, they expected to find evidence for an “intermediate-mass” black hole (IMBH). These are smaller than the supermassive black holes that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of massive stars. IMBH are the long-sought “missing link” in black hole evolution and their mere existence is hotly debated, although a few candidates have been found ...

Hubble Uncovers Concentration of Small Black Holes
NASA | GSFC | STScI | HubbleSite | 2021 Feb 11

Does NGC 6397 Contain an Intermediate-Mass Black Hole
or a More Diffuse Inner Subcluster? ~ Eduardo Vitral, Gary A. Mamon

• Astronomy & Astrophysics 646:A63 (Feb 2021) DOI: 10.1051/0004-6361/202039650
• arXiv.org > astro-ph > arXiv:2010.05532 > 12 Oct 2020 (v1), 08 Feb 2021 (v3)

I recently wrote a post about all the blue stragglers in the center of NGC 6397, and now ESA/Hubble has written about all the stellar-mass black holes in the center of of NGC 6397.

It should come as no surprise that there are stellar-mass black holes in the center of NGC 6397. We know that massive stars can leave black holes behind when they explode as supernovas, and it is reasonable to think that a rich globular cluster like NGC 6397 must have contained a lot of very massive stars in its earliest youth. Therefore, it is reasonable to think that at least some of those massive stars ended their lives as stellar-mass black holes.

And it's quite logical that many of the stellar-mass black holes of NGC 6397 have sunk to the center of their globular, because they are more massive than almost any surviving star in the cluster. Just like the blue straggler stars.

So why haven't the stellar-mass black holes merged into a single medium-mass black hole? Well... what are the requirements for such a merger to take place?

Have astronomers even demonstrated that it can happen?

Ann

These small black holes in NGC 6397 like what they respect
Each other, and do not devour each other

I found an interesting video about the black holes in NGC 6397.


Ann