Our first story looks deep into the heart of the nearby globular cluster NGC 6397. Observable from the southern hemisphere, this knot of thousands of stars is the second closest globular cluster to our solar system and is close enough for the Gaia and Hubble space telescopes and larger ground scopes to readily resolve individual stars in all but the densest part of the system’s core. 

With Gaia and Hubble, high-resolution images taken over time make it possible to actually measure how stars’ positions on the sky change as the stars orbit. The Very Large Telescope’s MUSE spectrograph also allowed motions along the line of sight, that direction in and out of the sky, to also be measured. Put together, these three-dimensional orbital motions make it possible to map out the distribution of unseen mass in the cluster. 

In a new paper in Astronomy & Astrophysics, Eduardo Vitral and Gary Mamon discuss how these motions could be explained by a central intermediate-mass black hole with a mass of 500-650 solar masses, but more could be better explained with a population of myriad compact dead stars: white dwarfs, neutron stars, and stellar-mass black holes that together have a mass of 1000-2000 solar masses.

Put differently, one more place where we expected to find an intermediate-mass black hole did not obviously have one. We thought, apparently falsely, that black holes created during the death knells of the largest stars — the so-called stellar-mass black holes — would merge together into bigger and bigger black holes – the intermediate-mass black holes we keep just not finding.

And the question is why. Why do we keep not finding intermediate-mass black holes?

In the case of NGC 6397, the answer may be dynamics. According to the paper, the merger process can give the resulting object a kick that can accelerate the resulting object up to escape velocities. This globular cluster has an escape velocity of around 50 km/s, and models of mergers in this system indicate that 70 – 85% of black hole mergers would result in the intermediate-mass black hole flying out of the globular cluster. This raises the disturbing possibility that the halo of the Milky Way and other galaxies are full of intermediate-mass black holes that are doing nothing more than hanging out and avoiding detection.

For now, this is largely a statistical argument: a myriad of dead stars fit the data better than a single large intermediate-mass black hole. The real proof could come from detecting the merger of stellar black holes (and the flinging away of the resulting intermediate-mass black hole).

More Information

Hubble press release

“Does NGC 6397 contain an intermediate-mass black hole or a more diffuse inner subcluster?“, Eduardo Vitral and Gary A. Mamon, 2021 February 11, Astronomy & Astrophysics