Strong evidence for the ‘missing link’ of black holes

10 Jul. () –

Newly identified fast-moving stars in the Omega Centauri star cluster provide strong evidence that there is a central black hole in the cluster.

With at least 8,200 solar massesis the best candidate for a class of black holes that astronomers have long believed exist: intermediate-mass black holes, formed in the early stages of galaxy evolution. At 18,000 light-years, it is the closest massive black hole to Earth.

The discovery reinforces the idea that Omega Centauri is the central region of a galaxy that was swallowed by the Milky Way billions of years ago. Stripped of its outer stars, that galactic core has remained “frozen in time” ever since. The study has been published in the journal Nature.

Omega Centauri is a spectacular collection of about 10 million stars, visible as a speck in the night sky from southern latitudes. Through a small telescope, it looks no different from other so-called globular clusters: a spherical collection of stars, so dense towards the center that it becomes impossible to distinguish individual stars.

But now a new study, led by Maximilian Häberle (Max Planck Institute for Astronomy), confirms what astronomers have long suspected: Omega Centauri contains a central black hole.

The black hole appears to be the “missing link” between its stellar and supermassive relatives: stuck in an intermediate stage of evolution, it is considerably less massive than typical black holes at the centres of galaxies. Omega Centauri appears to be the core of a small, separate galaxy whose evolution was interrupted when the Milky Way swallowed it.

Maximilian Häberle, a PhD student at the Max-Planck Institute for Astronomy, led the work of creating a huge catalogue of the motions of the stars in Omega Centauri, measuring the velocities of 1.4 million stars by studying more than 500 Hubble images of the cluster.

Fast-moving stars are so because of the presence of a concentrated mass nearby. In the case of a single star, it would be impossible to determine whether it is fast because the central mass is large or because the star is very close to the central mass, or whether the star is simply flying in a straight line. with no mass in sight.

But seven such stars, with different speeds and directions of motion, allowed Häberle and his colleagues to tease apart the different effects and determine that there is a central mass in Omega Centauri, with a mass of at least 8,200 suns. The images do not indicate any visible objects at the inferred location of that central mass, as you would expect from a black hole.

The broader analysis not only allowed Häberle to precisely determine the velocities of his seven high-velocity stars, but also narrowed down the exact location of where the three-light-month-wide (in the images, three arcseconds) central region lies within Omega Centauri.

Furthermore, the analysis provided statistical confirmation: a single high-velocity star in the image might not even belong to Omega Centauri. It could be a star outside the cluster passing just behind or in front of Omega Centauri’s center by chance. Observations of seven such stars, on the other hand, They cannot be pure coincidence and leave no room for explanations other than a black hole.

18,000 LIGHT YEARS AWAY

“Previous studies had led to critical questions like ‘Where are the high-velocity stars?’ We now have an answer to that and confirmation that Omega Centauri contains an intermediate-mass black hole. At a distance of about 18,000 light years, this is the closest known example of a massive black hole.“says co-author Maximilian Neumayer.

The supermassive black hole at the center of the Milky Way is about 27,000 light-years away. This detection not only promises to settle the decade-long debate about an intermediate-mass black hole at Omega Centauri. It is also the best candidate so far for the detection of an intermediate-mass black hole in general.

In light of their findings, Neumayer, Häberle and their colleagues now plan to examine the centre of Omega Centauri in more detail. They already have approval to measure the star’s high-speed motion towards or away from Earth (line-of-sight velocity) using the James Webb Space Telescope, and there are future instruments (GRAVITY+ on ESO’s VLT, MICADO on the Extremely Large Telescope). that could determine stellar positions more accurately than Hubble.

The long-term goal is to determine how stars accelerate — how their orbits curve. But following those stars once around their full orbit, as in the Nobel Prize-winning observations near the black hole at the center of the Milky Way, is a project for future generations of astronomers.

The lower mass of the Omega Centauri black hole means timescales ten times longer than for the Milky Way: orbital periods of more than a hundred years.