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Massive "Phoenix Cluster" Supersizes Star Creation


For the rest of the hour, some supersized stargazing. Astronomers writing in the journal Nature say they've found a new massive galaxy cluster that's spitting out stars at a record-setting pace. The cluster is about 5.7 billion light years from Earth - OK, so not around the corner - in the constellation Phoenix and aptly named the Phoenix Cluster. Scientists studying the cluster say it creates more than 700 new stars a year. That's the highest number ever observed for a galaxy cluster. And compare that to our galaxy, the Milky Way, which averages a puny one or two stars a year. More on that in a minute.

Joining me now to talk more about the galaxy cluster is Michael McDonald. He's a Hubble fellow at the Kavli Institute for Astrophysics and Space Research at MIT. Thanks for joining us today.


LICHTMAN: This is like - this is a star factory, it sounds like.

MCDONALD: Yeah, yeah. Definitely it's forming a tremendous amount of stars, much more than it should be given its environment.

LICHTMAN: What's the recipe for a star? How do you make one?

MCDONALD: Really all you need is cold gas and to be left alone essentially.


MCDONALD: If the cold gas cools enough and nothing heats it up, then it should turn into stars. And so this cluster is a perfect star factory because it has a tremendous amount of cooling happening and very little heating. And so it's going to provide just a tremendous amount of this cold gas fuel for stars.

LICHTMAN: What's the difference between a galaxy and a galaxy cluster?

MCDONALD: A galaxy cluster is just a cluster of galaxies, so it's just you need more than, say, 100 galaxies to be called a galaxy cluster. And so our Milky Way is in a group of galaxies, which is a few big galaxies and maybe 30 or 40 little galaxies. But this cluster is thousands of Milky Way-sized galaxies, all sort of bound together, so it's much, much more massive than our Milky Way, for example.

LICHTMAN: And what makes it unusual?

MCDONALD: I mean, this cluster is specifically unusual for a number of reasons. First, it's one of the most massive clusters in the universe. So it's - it just has a tremendous number of these really massive galaxies like our Milky Way. Second, it's cooling very rapidly, which is sort of this ingredient for star formation that we need. So it's the most X-ray luminous cluster in the universe, which just means it's cooling faster than any other cluster we've ever seen.

LICHTMAN: Do we know why?

MCDONALD: It seems to be just because it's not being heated. So, I mean, every cluster should cool. A cluster is just made up of essentially a lot of 100 million degree gas. And if you leave anything hot untouched, it's going to cool just like a cup of coffee on your coffee table.

LICHTMAN: What usually keeps it hot then, I guess?

MCDONALD: Right. So what usually keeps it hot is radio jets from the central black hole should heat it up. So there's this really nice balance in most clusters between the cooling and the heating from the central black hole. So just like leaving a cup of coffee on a burner, it won't be allowed to cool. That's sort of the analogy to what's happening, typically. But this cluster doesn't seem to have a very hot burner, so the central black hole isn't providing enough heat to cancel this cooling and you get sort of a runaway cooling in the core, producing this really high star formation.

LICHTMAN: You're listening to SCIENCE FRIDAY on NPR. I'm Flora Lichtman. So what's different about the black hole then? Let's just trace this back where...

MCDONALD: Right, right. Yeah, I mean, that's the obvious question to ask, and we're not really sure. I mean, the biggest difference is that it's not producing these radio jets at the rate it should be. So typically the central galaxy in a cluster is shooting out these really bright jets that you can see from here in the radio. But this one is giving off - instead of radio, it's giving off a lot of gamma ray and X-ray, and so it seems to be almost confused about what type of black hole it is. It's giving off sort of quasar-like emission rather than this sort of more quiet, more gentle emission.

LICHTMAN: Hmm, who am I, asks the black hole?


LICHTMAN: (Unintelligible). How is this cluster discovered?

MCDONALD: So it was initially discovered a few years ago by the South Pole Telescope, which is a telescope that's essentially built to find these really massive clusters. And so it looks at light from the Big Bang, which is a background radiation; it looks for shadows in that light. And these clusters are so massive that they're going to cast a shadow in this background radiation. And so that's how this cluster was first found.

And then just last year we followed it up with the Chandra X-ray Observatory, and that's when we found out how exciting it was because in the X-ray you can really see how bright this cluster is and how quickly it's cooling and how little the central black hole is doing.

LICHTMAN: You know, I've always wondered about this. When the data comes in, I assume it's sort of numbers, maybe images...


LICHTMAN: But you paint such a visual picture down the line. I mean, is there one moment, or does this sort of take a long time to bring that image together?

MCDONALD: I mean, for this cluster, it actually was a very visual moment. I mean, the cluster was found and it was essentially just a number, like how massive the cluster is, and that was two years ago. But when we got the X-ray data, I mean, I was at my desk and the image downloaded, and I had up on my screen the X-ray image of the cluster. And it looked so different than any other cluster I've looked at. So it was a very visual a-ha moment.

LICHTMAN: Hmm. I read somewhere that - I think you were quoted as saying that it's sort of - it has - the name Phoenix, and that might relate to - also to how it's risen from the dead or something like that. Can you explain that?

MCDONALD: Right. I mean, the reason it's named that is because it's in the constellation of Phoenix, so that's sort of traditionally how these types of systems are named or how they were named back in the day. But the name's appropriate because in the middle of clusters usually you have this really massive galaxy that was essentially the first one to form. So they formed billions of years ago, and now there's star formation around them. But they're pretty much dead, and they're only growing by sort of accumulating smaller galaxies or eating smaller galaxies.

But here's this new galaxy cluster, that the central galaxy is actually forming its own stars, and it's sort of coming back to life, if you will. And it's - rather than just sort of gobbling up smaller things, it's forming 740 of its own stars every year, and so it's sort of the most youthful thing in the cluster, when really it should be the oldest.

LICHTMAN: What's the lifespan on something like this? Can it go on like this forever?

MCDONALD: No, no. It definitely can't. And the argument that you sort use is it's just growing too fast, and we're seeing it as it was six billion years ago almost. So if they grew that fast for six billion years, it would be just far too big.


MCDONALD: So it probably only lasts for a measly 100 million years or so.


LICHTMAN: Where do you go from here with this work?

MCDONALD: Right. So the obvious thing we want to do is to really try to understand this star formation and to understand what's going on in the black hole. So we're going to sort of take two approaches to this. First, we want to get much more detailed imaging, like Hubble imaging of the central galaxy, where the star formation is happening, and try to sort of - I mean, there's an artist's conception image floating around in the press release, but we want to actually see what's really going and to get a high-resolution image and maybe see this cooling happening into turning - or forming stars and maybe be able to see the direct influence of the black hole.

LICHTMAN: Can't wait.

MCDONALD: Yeah. The other side is to look for more systems like this, to do a larger survey and try to find other systems, because with only one system, it's really hard to say anything about the universe, right?

LICHTMAN: Thanks, Michael McDonald, for joining us today.

MCDONALD: No problem.

LICHTMAN: Michael McDonald is the Hubble fellow at the Kavli Institute for Astrophysics and Space Research at MIT. Transcript provided by NPR, Copyright NPR.