The biggest black holes in the Universe reside at the centers of the largest galaxies. However, a new study suggests they may be proportionally even larger, compared with other galaxies. The bright cluster galaxies (BCGs) are huge galaxies found in the middle of galaxy clusters, where they grew by merging with and absorbing smaller galaxies. However, based on their X-ray and radio luminosity, their black holes may have grown much bigger—perhaps as much as ten times the mass in previous estimates. That means the largest black holes in the Universe are perhaps 60 billion times the mass of the Sun…or more.

A recent study has used an independent means of estimating black hole masses, based on their brightness in X-rays and radio light. J. Hlavacek-Larrondo, A. C. Fabian, A. C. Edge, and M. T. Hogan examined the massive central galaxies in 18 galaxy clusters and found that previous measurements could be off by as much as a factor of ten. In other words, if the luminosity-based measurements are correct, a black hole currently believed to be 6 billion times the mass of the Sun could actually be 60 billion times more massive.

That leaves two possibilities: either black holes in bright cluster galaxies behave differently by producing more light than we think they should, or the biggest black holes in the Universe might be astoundingly ultramassive. [Read more…]