The Cosmic Horseshoe

Astronomers have found a truly huge black hole! It’s in the massive galaxy in the center here, called the Cosmic Horseshoe. The blue ring is light from a galaxy behind the Cosmic Horseshoe, severely bent by gravity.

This black hole is 36 billion times the mass of the Sun. It’s not just ‘supermassive’: any black hole over 10 billion times the Sun’s mass is considered ‘ultramassive’. Not many have been found. T

To me the coolest part is that the Cosmic Horseshoe has swallowed all the other galaxies in its group: it’s part of something called a ‘fossil group’, which I hadn’t heard about until today.

Our galaxy, the Milky Way, is part of a group too: the Local Group. Many smaller galaxies have already fallen into ours. Eventually the Milky Way and Andromeda may collide and form a single bigger galaxy. This will be an ‘elliptical galaxy’—too disorganized to have spiral arms. So it’s not surprising that if you wait long enough, galaxy groups form a single big elliptical galaxy which eventually eats the rest.

Some ancient galaxy groups have already done this, and they’re called `fossil groups’. The Cosmic Horseshoe is the big bully in this particular fossil group: it’s 100 times heavier than the Milky Way. It’s surrounded by a halo of very hot gas, 10 million degrees. But most of its mass can’t be explained by stars, gas and dust, so we say 90% is dark matter. All this is completely typical of a fossil group, except the Cosmic Horseshoe is bigger than average.

Fossil groups show us what the future will be like. Big galaxies will eat the rest, and big black holes at the center of these galaxies will eventually eat most of the matter. Dark matter—whatever that is—takes longer to fall in. But there’s plenty of time.

Here’s the new paper about this ultramassive black hole. Luckily, it’s free to read:

• Carlos R. Melo-Carneiro, Thomas E. Collett, Lindsay J. Oldham, Wolfgang Enzi, Cristina Furlanetto, Ana L. Chies-Santos and Tian Li, Unveiling a 36 billion solar mass black hole at the centre of the Cosmic Horseshoe gravitational lens, Monthly Notices of the Royal Astronomical Society 541 (2025), 2853-–2871.

Below is a picture from this paper. The blue blob in a box at left is called the ‘counter-image’ of the galaxy that’s making the Einstein ring. When light from behind a massive body is bent by gravity, it often forms a ring, called an ‘Einstein ring’, together with a counter-image.