Omega Centauri: A Bright Skies, But Radio Silence at Its Core
Omega Centauri stands as the Milky Way’s most luminous and massive globular cluster, a tight ball of roughly ten million stars lighting up the southern sky. Early this year, scientists noted hints that an intermediate-mass black hole sits at the cluster’s heart, inferred from seven stars that whirl around so fast they should fly apart unless something vastly more massive tethers them. Now, a radio search for the black hole itself has yielded a paradoxical result: the absence of a signal.
Why intermediate-mass black holes matter: they fill a gap in our understanding of black-hole growth. We know how stellar-moss-black holes form—when massive stars die, their cores collapse into black holes with up to a few hundred solar masses. We also know the supermassive beasts at galaxy centers weigh millions to billions of solar masses. But black holes in between—hundreds to tens of thousands of solar masses—are notoriously hard to confirm. Only a few credible candidates exist, and proving them real is tricky.
Omega Centauri (image credit: NASA)
A two-decade-long Hubble survey tracked about 1.4 million stars in Omega Centauri. In its innermost region, seven stars move so rapidly that, in any ordinary gravitational setting, they should escape. Their stubborn binding implies a substantial gravitational source: a black hole with a mass of at least about 8,200 solar masses, with estimates climbing to around 47,000 solar masses.
To test whether this black hole is actively feeding, Angiraben Mahida and colleagues turned to radio waves. When a black hole accretes gas, the inflowing material forms a hot disk that emits across the spectrum, including radio wavelengths. The team used the Australia Telescope Compact Array to image Omega Centauri’s core for roughly 170 hours, reaching a sensitivity of 1.1 microjanskys at 7.25 GHz — the most sensitive radio image of the cluster to date.
Yet there was no sign of radio emission anywhere near the cluster’s center where the black hole would reside.
The absence of a radio signal constrains how the black hole behaves. By applying the fundamental plane of black-hole activity—an empirical relation tying a black hole’s mass to its radio and X-ray brightness—the researchers deduced that any intermediate-mass black hole in Omega Centauri must be accreting with extremely low efficiency. The upper limit suggests that less than about 0.4% of the infalling rest-mass energy is converted into radiation.
This extreme quietness aligns with a broader picture of Omega Centauri. It is widely believed to be the remnants of a dwarf galaxy that the Milky Way devoured billions of years ago. In the cluster’s dense core, there may simply be too little gas to feed a black hole, leaving the supposed intermediate-mass black hole starved and electromagnetically quiet. Unlike the gas-rich environments fueling supermassive black holes in active galaxies or the stellar companions that feed stellar-mass black holes, this putative intermediate-mass black hole seems to exist in a kind of fuel desert, effectively silent across the electromagnetic spectrum.
Source: No evidence for accretion around the intermediate-mass black hole in Omega Centauri (arXiv:2512.09649)
