TODAY, NASA ANNOUNCED that astronomers have found a massive galaxy cluster with astounding and unexpected properties – producing stars at a prodigious rate astronomers have not seen in other galaxy clusters.
The discovery of the Phoenix Cluster, and its central galaxy producing 740 stars per year, is prompting astronomers to re-think how galaxy clusters, among the largest structures in the universe, form and evolve over cosmic time. No nearby galaxy clusters (and therefore closer in cosmic time) produce stars in their central galaxies at such a high rate. The vigorous starburst in Phoenix is not thought to be sustainable for more than a few hundred million years because anything exceeding that would make the central galaxy in the cluster larger than anything else seen in the universe.
On the eve of the NASA announcement, Hubble Fellow Michael McDonald, a researcher at the Kavli Institute for Astrophysics and Space Research at MIT and also the lead author of a paper appearing Aug. 16 in the journal Nature, and Bradford Benson, a researcher at the Kavli Institute for Cosmological Physics at the University of Chicago and a co-author of the paper in Nature, spoke with The Kavli Foundation about the discovery..
THE KAVLI FOUNDATION (TKF): Let's start with the basics. How large is the Phoenix Cluster, where is it and how old is it?
MICHAEL MCDONALD: It's one of the most massive clusters in the universe, maybe in the top two or three – and perhaps it is the largest that we know of in the universe. It's 5.7 billion light-years away, so we're seeing it as it looked 5.7 billion years ago.
TKF: So it's much older now than it was at the time we are observing it.
MCDONALD: That's right.
BENSON: It would also be more massive now, because the central galaxy in a cluster grows through mergers over time, and clusters themselves merge with other clusters and grow bigger. But even when we’re observing it, at 5.7 billion years ago, it’s already one of the most massive clusters in the universe that we know about – even today.
BENSON: Originally this cluster was discovered by the South Pole Telescope (SPT), which is a ten-meter telescope that studies millimeter wavelength light from these objects. SPT finds clusters of galaxies in a really unique way. It detects them indirectly, by detecting the shadows these clusters make in the cosmic microwave background, which is the light that's left over from the Big Bang. Clusters of galaxies are one of the few things in the universe that are so massive, they're able to create shadows in the cosmic microwave background. Effectively, the light from the Big Bang travels 14 billion years across the entire observable universe, and these clusters of galaxies make shadows in that light. It turns out this is a very effective way to find the most massive distant clusters in the universe. The Phoenix Cluster was discovered as part of a survey by the South Pole Telescope that observed 2500 square degrees of the sky, or about 15-percent of the total sky.
BENSON: After we detected the Phoenix Cluster, we followed it up to measure its redshift -- in other words to determine its distance -- and also made several other measurements in optical, X-ray, infrared, ultraviolet and other wavelengths to learn about the properties of gas and stars in the cluster.
Mike and I were particularly interested in X-ray observations. This same hot gas that scatters a small percentage of CMB photons as they pass through the cluster also emits X-rays very brightly. The gas is on the order of 100 million degrees Kelvin, which is hotter then the interior of the Sun. So these are very bright X-ray sources on the sky. The Chandra X-ray telescope was one of the first things we used to follow up this cluster. The Phoenix Cluster stood out as having very high X-ray emission from its center, so much that it made the entire cluster the most luminous X-ray cluster ever observed.
That immediately piqued our interest, because it suggested that cool gas was condensing in the center of the cluster. So we began to get as much data as we could to try to understand what else was happening in this cluster – in particular the star formation and the central black hole. And that's where Mike came in; he was instrumental in accumulating a lot of that extra data, to figure out how this hot gas is turning into stars.
TKF: What first hints did you have that the rate of star formation was so high in Phoenix?
MCDONALD: As Brad said, when we first got these X-ray measurements from Chandra, it was pretty obvious this was a unique cluster. Specifically, the center of the cluster was very dense and cooling very rapidly. This is a process I’ve studied in other galaxy clusters, so I thought immediately that there must be vigorous star formation if we see all this cooling. I started putting together some of the data that we already had, as well as data from observatory archives and new data from both space- and ground-based telescopes. With all this data in hand, we saw that the central galaxy was very blue, indicating it was forming a lot of young stars.
TKF: And that was a surprise for a cluster so far away, correct?
MCDONALD: Right. There are a significant number of nearby clusters that are what we call “cool core”, or “cooling flow” clusters. There's been some evidence that this strong cooling is a recent phenomenon, and that it did not happen in the early universe. So this is one of the few – or one of the only – clusters that we know of at a high redshift in the early universe that is cooling very rapidly like this.
TKF: Why do we think that this didn’t happen in the early universe with the same frequency that it does today?
MCDONALD: We don't know for sure that it did not happen in the early universe. It could be that previous surveys just had trouble finding these clusters. So for example a cluster like Phoenix may be hard to detect at X-ray wavelengths initially, because the energy is so concentrated at its center that it could look like a point source – like a quasar, for example. (Quasars are the energetic cores of distant galaxies that surround a central supermassive black hole and appear as point sources of light, as opposed to the energized gas falling into the core of a galaxy cluster)
"The goal is to study more clusters to better understand their evolution. ...We need to compare other clusters to Phoenix – clusters of all ages – to see how they evolve over cosmic time and where the Phoenix Cluster fits into this evolutionary picture." – Bradford Benson
So you can misidentify extreme clusters as something else, unless you have proof that it's a cluster by indirectly detecting its presence through these shadows on the CMB, or having optical follow-up. So it could be there are in fact very few of these at high redshifts, at earlier times, or it could be we just haven't been able to find them until now.
BENSON: Originally, I started on this working with the South Pole Telescope, which was making observations of the cosmic microwave background. The primary goal is to use the cosmic microwave background to find clusters of galaxies in this manner that I talked about, with the shadows. So I started working with SPT by developing and building its microwave camera. My interest was primarily to study galaxy clusters in order to learn about dark energy.
Towards that goal, I’ve helped to identify clusters in the SPT survey, including the Phoenix Cluster, follow them up with observations at X-ray wavelengths to understand better what the gas is doing in the cluster at finer angular resolution, and also to work on determining the cluster’s mass with Mike.
MCDONALD: I joined in when the X-ray follow-up really started to get off the ground, which Brad has been leading. My job was to look at these clusters at X-ray wavelengths and try to estimate their masses, which would then get fed into Brad's cosmology work on dark energy.
So I have been looking at all the clusters that the SPT observed in the X-ray, and from time-to-time we’ve seen very interesting ones that are useful for more than just cosmology – for dark energy studies. I’m interested more in the actual physics of these clusters, and understanding how they are evolving between the early universe and now. So, the Phoenix Cluster is particularly interesting to me because I’m really interested in these cool cores, these cooling flow clusters. When we saw this, I started to get excited and began gathering all this data, including data on the star formation rate. So I’ve worked on the follow-up analysis after the original detection.
MCDONALD: I am a very glass half-full guy. I immediately thought that it was extremely exciting, and I needed people to reign me in. So right away, as soon as we looked at the first spectrum, it was just booming in oxygen emission, which is an indicator of star formation. Then we looked at the cluster in ultraviolet wavelengths, and it was again exceptionally bright. So it was obvious from every angle right away this was really unique.
However, to go from there to something that can convince the science community required several months of checking results and getting additional data. We studied all the different scenarios that could have been biasing our results. We went through everything very carefully to make sure that we weren't missing anything fundamental, and that we were doing all the corrections we needed to do to finally get a convincing, robust estimate of the rate of star formation. That took some time.
TKF: Brad, what was your initial reaction?
BENSON: When the same measurements kept coming in, and they were very consistent with a very high star formation rate - however we measured it – it was extremely exciting to see. This cluster is extremely unique. It is sort of like “the missing link” in cluster evolution. If you look at galaxy clusters today, they are very inefficient at forming stars; they form far fewer stars than you would expect from simulations. At some point in the history of galaxy clusters, you expect the star formation rate to be much higher than what we see in nearby clusters – at essentially the present time. But we really need evidence of that.
The Phoenix Cluster is the first to show extremely high rates of star formation, consistent with what we've expected. So we are seeing this cluster possibly at a stage where it is transitioning from extremely high levels of star formation to a time when the supermassive black hole at the core of its central galaxy turns on, releases more energy into the gas, and only later suppresses star formation.
TKF: Mike, why was it so important to use several different instruments to study this one cluster?
Meanwhile, the stars being formed are best observed in the ultraviolet and the infrared. You need the combination to get the bright young stars and the stars that are obscured by dust in the early stages of formation. So you need this UV and infrared combination. For the old stars that already exist there, you need more visible wavelength data. X-ray data also gives you total cluster properties. You see the cooling gas and the hot cluster halo.
So you really need this full range of data to piece everything together, and to get all the evidence of what is going on because there are so many different things at play. That is partially why I am very interested in galaxy clusters. I like to use this full range of detection methods and the different telescopes.
TKF: Is it correct to say that the supermassive black hole at the center of the central galaxy in the Phoenix Cluster – and the time we’re observing it, 5.7 billion years ago – is not yet fully mature yet? In other words, that it's not yet creating the kind of feedback energy that would slow the cooling of gas and suppress the formation of stars?
MCDONALD: I think mature is a good term to use. But it’s not that the black hole is giving off too little energy. It is actually one of the most powerful active black holes that we know about in cluster cores. Instead, it's giving off energy in the wrong way. It is giving off gamma rays and X-rays at a very large rate. Typically, these cluster-centric black holes are giving off a lot of radio emissions in the form of jets, which can sort of stir up the gas around galaxy. So it almost seems to be confused about where it is. It’s sort of acting like it’s in an ordinary spiral galaxy, whereas it is really in the center of a galaxy cluster and should be producing these radio jets. So maybe this is something that is in transition now between a quasar-like object and a radio-loud object.
TKF: And a radio jet emission would be more energetic and be able to slow this gas cooling?
MCDONALD: It would not be necessarily more energetic, but it would be the right kind of energy. It would be giving off energy that could couple to the intra-cluster gas. These radio jets can produce shocks and ripples in the intra-cluster medium that heat the gas up. It's kind of like trying to cook a turkey with a blowtorch. You're going to burn one little part of it but you won't cook the whole turkey very well. Whereas a slow gentle heat over a large area cooks the turkey much better.
TKF: And that “gentle” heating – driven by these radio jets from the central black hole – is what will slow the rate at which the gas cools and flows toward the central galaxy. And the result is a suppressed rate of star formation.
MCDONALD: Right. Exactly. Clusters in the local universe have the right amount of radio emission to offset cooling and so you get this really nice balance between radio jets coming from the central black hole heating up the gas, and the cooling of gas. The result is you don’t have enough gas left over to form stars. Whereas in the Phoenix Cluster, there is very little radio emission from the central black hole, so you get this runaway cooling of gas that turns into stars. The gamma ray and x-ray emissions coming from central black hole aren’t able to keep the gas hot enough to prevent star formation.
TKF: Let’s talk a little about what this discovery means – the big picture. Before you studied the Phoenix Cluster, what was your 30-second description of how the central galaxy in clusters formed stars over cosmic time? And what’s your new 30-second description?
MCDONALD: The picture as of a few months ago was that the central galaxies in clusters essentially formed via mergers. So you have thousands of galaxies in a cluster, and some of them are eventually going to merge with the central galaxy and you will slowly build up mass in the central galaxy. And it sort of just feeds off the other members. So that's the picture of how these supermassive galaxies form, just by accretion of smaller galaxies.
At the same time there was always another possibility that cooling should also play a role in the growth of a cluster’s central galaxy. You have this reservoir of hot gas in the cluster, that should be able to fuel star formation – if it could cool enough. Computer simulations have suggested that this happens, but this was never really observed. Astronomers saw some star formation in the cores of some clusters, but never anywhere near the rate predicted by computer simulations. So this became known as “the cooling flow problem.”
And so astronomers recently ruled out the idea that central galaxies in clusters grow by forming stars from the flow of cooling gas in the cluster toward the center.
The discovery of the Phoenix Cluster and its central galaxy throws a wrench into all this. Now we see a galaxy that appears to be forming a substantial number of its stars through this accretion of cooling material from the intra-cluster medium. Even if this goes on for just 100 million years, which is a cosmic blink of an eye, it's going to double the mass of the galaxy. So it's forming a large fraction of its stars not through mergers but through this accretion of cool gas. It's a channel predicted by simulations and by theory, but it's never really been observed until now. So maybe this is a new mechanism for how these galaxies can get so big.
BENSON: From my perspective, within the SPT collaboration, we are interested in this cluster in a slightly different way. It appears that in the Phoenix Cluster, the supermassive black hole at the core of its central galaxy has not turned on fully, at least not to the point where it has started suppressing star formation.
One of the reasons that we think supermassive black holes might be suppressing star formation in local clusters nearby is because we can see they are releasing energy into space between the cluster’s galaxies through jets and explosions started by this central black hole.
So when we see something like this – a more distant galaxy cluster where the central black hole is not suppressing star formation – we need to see if we can find more clusters like it. The big question is, when do the black holes at the center of clusters turn on with the energy needed to suppress star formation – like we see in nearby clusters? The goal is to study more clusters to better understand their evolution, and when the black holes are turning on. We need to compare other clusters to Phoenix – clusters of all ages – to see how they evolve over cosmic time and where the Phoenix Cluster fits into this evolutionary picture.
MCDONALD: First, I’ll be getting more data on this one cluster, Phoenix. The Hubble Space Telescope will allow us to see where star formation is happening – whether there are filaments of star formation or whether it's more smoothly distributed. We’ll also be able to separate the new stars from the older ones that lie closer to the core of the central galaxy.
We’ll also hopefully obtain data from the new Atacama Large Millimeter/submillimeter Array in Chile, which will tell us how much cold gas there is. And that will give us an exact estimate of how long the starburst can last. If you know how much fuel you have and you know how quickly you’re burning it, you can determine how long you can sustain the star formation.
Separately, we’re going to try to find more clusters, either like this or unlike this. The reason is that it's really hard to draw any conclusion based on one galaxy cluster. So while this is exciting, it doesn't necessarily tell us about the overall evolution of galaxies and galaxy clusters.
BENSON: We’ll be following up on many more clusters found in the SPT survey with the Chandra X-ray telescope. With that we are trying to assemble a larger statistical sample, to try to understand the percentages of how many clusters have similar properties in terms of these cooling cores, and how many of those exhibit similar signs of heightened star formation. Our goal is to better understand if the Phoenix Cluster is a unique, one-in-a-million thing, or if it’s rare simply because it’s in the midst of a short-lived period in its overall evolution that was more typical for clusters earlier in their formation.
"This discovery in the Phoenix Cluster suggests a whole new twist to that idea about how massive galaxies at the center of galaxy clusters grow. ...It makes the field more exciting from my perspective. It adds a lot of questions." – Michael McDonald
TKF: What excites you most about this discovery?
BENSON: Cosmology tries to answer some of the biggest questions about the universe, such as “How old is it?” and “How did it evolve?” I originally got involved in this from that perspective. For example, I’m trying to use clusters of galaxies to better understand dark energy, which is responsible for the mysterious force causing the universe to expand at an accelerating rate. At the same time, as you use all these astrophysical tools to try to understand cosmology better, you get more and more interested in the astrophysics, in the actual formation of these galaxies and galaxy clusters over time. We want to understand those things better so we can use them as cosmological tools. But they are also interesting by themselves. Unraveling that aspect of the mystery of how clusters form is also interesting.
MCDONALD: For me, this is exciting because in some ways it is reviving a long-held theory that never quite panned out. It has been thought over the last few years that this type of cooling-induced starburst doesn’t happen, that galaxy evolution in clusters is very much based on mergers and the assembly of smaller galaxies to make a bigger galaxy.
This discovery in the Phoenix Cluster suggests a whole new twist to that idea about how massive galaxies at the center of galaxy clusters grow, and it allows for another mechanism for the growth of these galaxies. It opens up a whole new area of research, essentially. It allows us to understand the growth of galaxies via a variety of mechanisms, rather than just assembling from mergers. So it makes the field more exciting from my perspective. It adds a lot of questions, which gives us something new to do.