M: Welcome to Unsolved Mysteries of Astronomy, your number one rated astronomy podcast. This week we will be coinciding with the second highest rated astronomy podcast,
“ cooking up theories with Professor George Greenstein.” Today Professor Greenstein is with us to discuss galaxy transformation and the Butcher-Oemler Effect.
The Butcher-Oemler Effect describes how clusters at high redshifts have a larger fraction of blue, star-forming galaxies than clusters at low redshifts. One area of galaxy transformation we’re concerned with is the halting of star formation once galaxies fall into clusters. Butcher and Oemler don’t give much reason for why this happens, but could the Butcher-Oemler Effect be evidence for why star formation stops? G: Well I would put it differently. I would say that the B-O effect is telling us that star formation had better stop. So it’s not that these guys were evading the issue. It’s that B-O was telling us we gotta think of something here. B and O have no idea why this is happening. They just see that star formation isn’t going on anymore. So it’s up to us to cook up a theory. M: Sounds simple enough. Is that all there is to it? G: Well, let’s think about it. Let’s talk about it right now. Stars form out of gas. If you’re gonna stop star formation, you gotta stop the gas from turning into stars. One way is to make the gas go away. Can you think of any way that if the gas stayed in the galaxy that it would still not form stars? L: Well when gas compresses, it heats up and over time forms stars. So no compression, no hot gas, therefore no stars. So we have to stop compression somehow, right?
G: In a spiral galaxy, the spiral arms rotate relative to the stars and gas. And the spiral arm is a region of greater density. So when a gas cloud enters the spiral arm, it gets compressed. Now we got to think of some process that makes that not happen. L: Well, if the spiral arms have a lot of density...they need to somehow have less density. If a galaxy is colliding with another galaxy, and all this material is kinda just being blown away during the process, then obviously the arms would be less dense. G: Good for you! M: So less dense arms mean no compression. No compression means no heated gas, which equals no stars. So what happens to the gas in a galaxy when it falls into a cluster, or when it passes close to another galaxy?
G: So, ok. There are a whole bunch of theories that have been proposed. One of them is what you’ve described of what happens when galaxies pass close to each other. It's a little bit like if you're driving a car and took a newspaper and held it out the window and let it go; the air would simply stop the newspaper but the car would keep going. in this analogy the newspaper is the gas cloud. And the air that stopped it is gas from the other galaxy so think of two cars passing each other, and you’re holding a newspaper, and the other paper’s holding a news paper and the newspapers collide, and they fall to the ground but the cars keep going. so the newspapers are the gas and the galaxies are the cars. BONG
M: So now that we’ve talked about how galaxies are stripped of their gas and what happens when galaxies interact, the redshift aspect of the butcher-oemler effect comes into question. How does redshift come into play here? G: Let’s see. One thing that B-O tells us is that this effect was stronger at higher red shifts and not so strong at low red shifts. Can we understand that? L: That was one of the things that I wasn’t really sure how to explain. G: Good, and they aren’t either, so let’s cook up a theory. What’s different about high red shifts and low red shifts? M: High red shift is moving farther away faster. G: So something with a big red shift, we’re seeing light emitted many years ago. And this is like billions of years ago. So an object with a small red shift is close to us. The light only took a short time to reach us. An object with a big redshift is far away. The light took billions of years to reach us. So, big red shift things -- we are seeing the Universe when it was younger. You know, billions of years ago. So the Universe was younger; everything was younger. So, this is telling us that...in the past, this was a very important effect, but nowadays it’s less so. So, whatever it is that’s stopping star formation happened a lot in the past, and not so much now. L: So maybe the higher fraction of the blue galaxies at higher red shifts is due to the fact that we’re looking at the Universe at a younger age, when there were a lot of blue galaxies closer together because, as you said, everything was closer together when the Universe was younger. So, we see that higher percentage because from our viewpoint, that’s the light that we see from billions and billions of years ago. G: We’re doing archaeology. L: Yeah, yeah. Cosmic archaeology. G: Absolutely. M: Ok, great. so we’ve talked about redshift and gas stripping, and how they both relate to the Butcher-Oemler effect. But we haven’t yet talked about what galaxy clusters are made up of! So the composition of a cluster is 5% galaxies, 10% inter-cluster medium, and 85% of...something else.
Scientists call this mysterious, invisible mass “dark matter”, but we’re skeptical. Dark matter seems like a very general term. How could one thing make up 85% of a galaxy cluster? G: Well I would agree with you absolutely. There is no reason at all to say that it’s one thing. It could be lots of things. But whatever it is, it’s dark. There is no understanding of what the dark matter is. There ’s no reason to think that it’s one thing; could be 25 different kinds of things. M: Ok. And I sorta wanted to ask you, what role do you think that extra matter plays in cluster transformation? G: It’s gotta be huge. As a matter of fact, it may be the point. It may be that galaxy and galaxy collisions are irrelevant and it’s really the dark matter that’s making star formation stop happening. And another question would be why, and maybe we can cook up a theory as to why, dark matter makes star formation stop. Maybe that theory makes some prediction, which turns out to be born out by the observations. In that case we would have found out something about the dark matter through this really indirect method. So why don’t you see if you can do that, sometime before the end of classes. In your spare time.
G: Yeah, in your spare time. Then buy your ticket to Stockholm, and book a hotel for the Nobel Prize ceremony. L: Sounds like a plan. Well that concludes our discussion about the Butcher-Oemler effect and galaxy transformation.
We’d like to especially thank Professor Greenstein for taking time to talk to us about this! So now with this better understanding of galaxy clusters, a perhaps a little about dark matter, there’s a definite next step to be explored to understanding this incredible process happening throughout the Universe.
