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A Dartmouth Professor's Deep Dive Into How Galaxies Form Stars

NASA/Marshall Space Flight Center.

A Dartmouth astrophysicist is part of a team that’s been looking billions of years into the universe’s past – and they’ve found some clues that may explain why galaxies form the way they do.

Ryan Hickox is an assistant professor of physics and astronomy. The findings of his team were published in the journal Nature. Ryan Hickox joined All Things Considered with more on the findings.


Five billion light years away. That's almost half of all time in the universe. You're looking at what was at a time before there was even an Earth. Could you describe what you use to look out that far away and that far into the past, so to speak?

Sure. To see an object that far away we need to use a range of powerful telescopes, to observe this particular galaxy at a range of wavelengths, not just in visible light that you can see, but also in infrared light, which is longer wavelength light more like heat, and also in ultraviolent light and even x-ray light, which is very energetic radiation.

The particular thing we used for this study to make the discovery we did that was so interesting was actually observations done in the microwave part of the spectrum. We were looking at radiation that comes out in the same kind of radio waves that are actually used in your microwave to heat your food.

Being able to see those molecules and the radiation in actions seems to have shed some light on why galaxies are the way they are and why the stars in those galaxies form or don't form.

That's right. We were particularly interested in this galaxy, and a class of galaxies like it. They seem to represent an important phase in the overall life cycle of a galaxy. A galaxy is a big collection of stars - our Milky Way is a galaxy - and generally big galaxies have something like a hundred billion stars in them. But when we look around at the universe around us, not all galaxies are the same. Some galaxies seem to be actively forming new stars in them, and it turns out that the fuel from which stars form is actually gas - cold, diffuse gas in the galaxy that's made up of molecules. Just like air is a gas that's made of molecules, there will be hydrogen, carbon and oxygen and other kinds of gas floating around inside a galaxy. If it gets dense enough it can collapse and make stars.

But then there are other galaxies that don't have any star formation at all. All we see in them are very old stars that were formed even billions of years ago. And so the big conundrum is, why does this happen? This particular galaxy seems to be one where it's very rapidly forming stars but that process is actually blowing the gas away from the galaxy and will ultimately shut off the process of star creation.

Is it clear what causes one of these eruptions, so to speak, to start up?

That's not clear yet. That's actually one of the most interesting questions raised by this research. One of the things that can produce a huge burst of star formation in a galaxy is when two galaxies run into each other. We know galaxies are moving around in the universe and they can be attracted by each other's gravity, and when that happens, the two galaxies will come together and gravitational forces will drive a lot of the gas down to the center of the remnant galaxy that's left over. That gas will get dense, collapse and form stars very rapidly. We know that these collisions can do this, and when we looked at this particular galaxy that we studied, and also galaxies like it, they look like they have streams of stars and other kind of disturbed material around them that look like they are actually the smashed-up remnants of two galaxies crashing into each other.

That's where we think the trigger for this sort of catastrophic event happened, but we need to do more work to really confirm that.

I apologize in advance for the pun, but do people come up to you and your colleagues and ask you about your research using the phrase from Star Wars, "a long time ago in a galaxy far, far away"?

It happens every once in a while, but probably not as often as I'd like.

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