CLASSED AS SOME OF THE MOST MASSIVE OBJECTS IN THE UNIVERSE, THESE COSMIC BEHEMOTHS ARE ALSO THE MOST MYSTERIOUS

REPORTED BY GILES SPARROW

Our galaxy, the Milky Way, is big. With a diameter of 100,000 light years, this enormous disc of stars is large for a spiral galaxy. But imagine a galaxy extending to almost 20 times the diameter of the one we call home - so big that if it were to replace our Milky Way, it would not only engulf nearby satellite galaxies such as the Magellanic Clouds, but its outer limits would reach nearly all the way to Andromeda, about 2.5 million light years away.

That’s the staggering scale of IC 1101, an enormous ball of stars more than a billion light years away that is one of the largest measured galaxies in the known universe. It’s an extreme example of a supermassive ‘cD’ galaxy - a class of objects very different from the Milky Way. cD galaxies are closely related to smaller elliptical galaxies - balls of old red and yellow stars that vary in size - from tiny dwarfs to giants with the diameter of the Milky Way - and differ in shape - from perfect spheres to elongated cigars. But they also differ in some other important ways.

“cD galaxies have an extra-large extended and diffuse luminous envelope of stars,” explains Dr Alessia Longobardi of Aix-Marseille University, France. “They are among the most luminous and massive galaxies in the universe. For example, recent studies of Messier 87 (M87), the cD galaxy at the centre of the nearby Virgo Cluster, attribute to this galaxy a total of around 1,014 solar masses. That’s roughly 100 times more massive than our own galaxy, the Milky Way.”

The term ‘cD’ comes from the Yerkes galaxy classification scheme used by many specialist astronomers. The letters don’t officially stand for anything, but astronomers have come up with a variety of ‘backronyms’. They are sometimes referred to as ‘cluster dominant’, ‘central dominant’ or ‘central diffuse’. As those names imply, cD galaxies have a very specific distribution in space that offers an important clue to their origins. While spirals, smaller ellipticals and irregular galaxies are scattered widely across space, cD galaxies are found only in massive, dense clusters - regions containing tens or even hundreds of substantial galaxies crammed into a volume of space a few million light years across. What’s more, they tend to lie close to the very centre of these clusters. In such closely packed environments collisions between galaxies are common, and it’s this process that’s thought to give rise to monster galaxies.

“The most popular scenario to describe the formation and evolution of galaxies is the hierarchical paradigm,” says Longobardi. “Structures form in a bottom-up, hierarchical manner by which smaller fragments that have been attracted gravitationally merge together to form more massive systems. A major merger occurs when the masses of the merging galaxies are comparable.

“ M87 has collided with a mid-sized galaxy within the past billion years, and it is still in the process of absorbing its stars and other material”

Right: Workers study Hubble’s main 2.4-metre (eight-foot) mirror, which has revealed unprecedented details in distant galaxies