What is this dust stuff for anyway?

Anyone keeping an eye on the blog should know that we’re looking at overlapping galaxies to try and understand the nature and distribution of dust in galaxies. Prompted by very pertinent questions from Barbara, our guest for the night in the control room, it’s suddenly occurred to me that we haven’t ever told you why we care about dust in galaxies. Start with this  image of a region where stars are forming.

You can’t see them, because they’re enclosed within a cloud of dust grains which block light from outside. That’s all-important; stars form from the collapse of clumps of gas and dust, and that can only happen when the gas is cool. Warm it up much more than 20 or so degrees above absolute zero, and the gas will be moving too fast to easily collapse. Inside the cloud, the material is protected from the harsh light of interstellar space, and so the gas can cool and stars can form. So understanding what the dust is made of, and how it behaves is crucial in trying to improve our understanding of stellar nurseries such as this one. 

Where did the dust come from in the first place? A short answer is that noone knows for sure, but at least some is produced in supernovae, the explosions that mark the end of the life of massive stars. The image below, from the Spitzer Space Telescope, shows the aftermath of just such an explosion in the galaxy M74.

 This supernova was observed to explode in 2003; in mid-2004 the arrow marks the warm dust which was produced in the supernova. Just a few months later, it had vanished, indicating that the dust had cooled. So if we want to understand supernovae and the evolution of stars, we need to understand how dust is distributed and what it might be made of. 

There’s a third, more prosaic reason for studying dust distributions. Astronomers are used to ignoring dust (I was once told to blame any results which didn’t make sense – in any area of astronomy – on dust) but it has a huge effect on what we see. Infrared emission can be dominated by dust, and it blocks starlight that would otherwise hit our detectors. Unless we can quantify its effects, we have no idea how to account for them even when doing things as straightforward as measuring the stellar mass in a galaxy.