Astronomy is quite notorious for being full of things we don’t entirely understand. Sometimes it really does feel as if the closer we look at the Universe, the less it makes sense. One thing in particular which seems to constantly evade our understanding is the way in which galaxies work. A lot of very smart people spend a lot of time taking telescope observations and creating computer simulations to try and understand how exactly a galaxy can form and evolve, and every now and again someone will discover something which doesn’t seem to fit with what they were expecting. Occasionally we find something like that which is, at least in cosmic terms, right in our back yard.
The Andromeda galaxy is practically a twin sister to our own Milky Way. Slightly larger than us but slightly less massive, Andromeda lies around 2.5 million light years away, and between them Andromeda and the Milky Way dominate the local group of galaxies. But Andromeda is not without fanciful tastes – it wears a skirt over a million light years in diameter, made up of dwarf galaxies.
A recent study headed by Rodrigo Ibata at the Strasbourg Astronomical Observatory, France, and Geraint Lewis at the University of Sydney, Australia, found a host of new galaxies in the local group. The image below gives you an idea of the scale involved, but it doesn’t show the full story – There are actually over 54 galaxies in the Local Group. Andromeda is surrounded by a small swarm of 27 dwarf galaxies, and 13 of those dwarf galaxies orbit in the same plane, the same way the planets orbit the Sun. This means that Andromeda is surrounded by a disk-like shape, the largest cohesive structure in the local group. And it’s still very much a mystery as to why it exists.
Even here inside the largest galaxy for millions of light years, space is mostly empty, but the vast expanses of intergalactic space are so devoid of anything that it’s difficult to fully appreciate (to get even more perspective on this, click here and look at the full image!). But even in the face of this terrifying emptiness, galaxies live out their lives. They pull on each other and interract. They form and coalesce. Large galaxies devour smaller ones whole, and every so often, large galaxies smash together and tear each other apart. But none of the theories we have today quite explain Andromeda’s skirt.
Those 13 dwarfs orbit Andromeda once every 5.5 billion years or so, and Ibata, with his team of researchers, has suggested a couple of explanations for the disk. Firstly is that they formed in place as they are, and have been slowly twirling around Andromeda since before the Sun was born. They may have been created during a merger between two ancient galaxies, from a streamer of gas which was spun off. Or possibly, these galaxies are as old as Andromeda itself, forming at the same time amidst all of the dark matter attracted by Andromeda’s huge bulk. This would fit with the fact that those dwarf galaxies are made up of ancient stars, implying that this structure could be truly ancient.
Or perhaps it isn’t a disk at all. Perhaps we’re seeing a slew of galaxies recently pulled into Andromeda’s gravitational grip, and it’s purely by chance that they appear to be arranged into a disk shape. It’s entirely possible, and only further research will show if the disk structure is real or not. Combined with the recently discovered halo of gas surrounding the Milky Way, it seems there may be a lot lurking out there in intergalactic space that we don’t yet understand.
But either way, both of these hypotheses have problems with them. Neither is a perfect fit. In an interview, Nicolas Martin at the Strasbourg Astronomical Observatory explained that the fact that we don’t know why these galaxies are arranged the way they are is what makes this discovery so exciting;
“The presence of this thin, rotating disk of dwarf galaxies around Andromeda suggests a strong connection between the host galaxy Andromeda and its satellites. There is currently no satisfactory scenario that can explain all the properties of the satellites in the disk, but they all require a strong interplay between Andromeda and the satellites themselves.”
Top – Robert Gendler
Middle – Andrew Z. Colvin/Wikimedia Commons
Bottom – Rodrigo Ibata/PAndAS team