Ripples in Andromeda

Chris - In October, scientists spotted something pretty unusual in our cosmic neighbourhood. Using the Spitzer space telescope, David Block and his team have spotted a collision between the nearby Andromeda galaxy and a smaller companion galaxy and what's really exciting is that, relatively speaking, it's only just happened.

David - What we are actually reporting in Nature is something quite extraordinary, a head-on collision of one galaxy plunging through the actual disk of the Andromeda spiral. This is quite extraordinary. Normally these sorts of collisions are reserved for galaxies in our very distant Universe but, to find a head-on collision right on our doorstep, is truly riveting.

Chris - Is this something that's happened fairly recently then in cosmological time?

David - I think that this is what makes the research additionally so incredibly exciting, is that yes, dinosaurs roamed on Earth when this head-on collision actually took place. We estimate that the collision took place only 200 million years ago which, in cosmological context, is extraordinarily short.

Chris - How did you actually spot it though, David?

David - The rings were spotted using the Spitzer Space Telescope and what is very interesting is Andromeda has a very large bright pregnant bulge of stars. And these stars absolutely would masquerade any rings in the optical wavelength regime. But of course we must remember, using the Spitzer Space Telescope, we start receiving photons of lights in the near and in the mid infrared regime. And what the Spitzer images revealed were two rings, an outer ring of diameter of approximately 65,000 light years, and an inner ring. Now it's this inner ring which is totally new, which has never been really reported before. The inner ring has a dimension of around 5,000 by 3,000 light years. And the set of two rings are indeed the smoking gun evidence for a head - on collision. Perhaps if I could explain by means of an analogy. If you take a stone and you throw it into a pond of water it creates a ripple effect. And what they've gone and discovered: my team has actually found all these sets of rings point to a very violent past in our closest spiral galaxy.

Chris - How do you know it only happened 210 million years ago?

David - What has been very interesting is – and we've been very privileged to work at the Observatoire de Paris in France – and using some of France's most sophisticated computers, they've actually been able to simulate the collision. And what the simulations beautifully prove is that, if you set everything up correctly in order to get the companion galaxy to where it is today, you have to backtrack by only 200 million years for this violent head-on collision to have taken place. And so it's using very sophisticated computer codes that doctors Bournaud and Combes have actually been able to show, that this amazing collision has taken place very, very recently.

Chris - David Block from the University of Witwatersrand in South Africa who's found evidence for a headlong collision between our neighbour, the Andromeda galaxy, and a companion galaxy that formed alongside it.