Megan's Blog

Spitzer spies a starburst

This picture is a new view of M82, the galaxy I spend most of my time studying. M82 is a nearby galaxy which has had a recent encouter with its neighbour M81. The interaction caused all sorts of gravitational instabilities in the large gas clouds in the centre. These instabilities caused the clouds to start to collapse, getting denser and hotter as they did so. Eventually the conditions in the centres of these collapsing clouds became so extreme that nuclear fusion could begin and, voila! A star is born.

We know how much "stuff" (mainly hydrogen gas) is needed to make a star, and we know how much of it is in these galaxies because we can measure it using radio telescopes. Because the interaction causes the collapse of large amounts of gas, it also causes large amounts of star formation. We can measure how fast stars are being born by several methods, including the rate at which they die - the more supernovae explosions we see, the more stars had to have formed in the first place. Once we know these numbers we can use them to estimate how long the star formation could last with the amount of gas that is available. It turns out that the star formation rate in galactic mergers and interactions is so high that if the rate had been the same since the galaxy formed it would have long ago run out of gas and the star formation would have stopped. This is why they are called starbursts, the period of vigorous star formation happens over a timescale much shorter than the lifetime of the galaxy as a whole.

The starburst activity results in winds of material streaming out of the disk of the galaxy, powered by powerful supernova explosions. Some of this is gas, but a lot of it is dust particles (like the smoke from a campfire) and it is the dust that Spitzer has detected in this image. The dust itself is warm, having absorbed photons from the stars in the plane of the galaxy (the blue regions in this image) it heats up and re-radiates some of the energy as lower energy infra-red photons (the red stuff). The clouds seen here stretch out a massive 20,000 light years from the disk in both directions, that's pretty big. We already new this wind existed, but this is by far the best image of it yet. Well done Spitzer!

Thanks to Ant who spotted it first!