Episode 32: The MeerKAT Fornax Survey
with Dr Paolo Serra
In this episode we will be discussing some more exciting work being conducted with the MeerKAT radio telescope. We’re joined by Dr Paolo Serra from the Cagliari Astronomical Observatory in Italy. He is the principal investigator of the MeerKAT Fornax Survey.
The Fornax Cluster is a nearby galaxy cluster containing about 60 large galaxies and a similar number of dwarf galaxies. Astronomers have estimated that the centre of the Fornax Cluster is in the region 65 million light-years from Earth. It is one of the closest of such clusters beyond our Local Group of galaxies.
Paolo and his team are using the MeerKAT telescope astronomers to study the physics of gas that is accreting onto and being stripped off galaxies as they fall into the Fornax cluster.
They have already used this data to discover hydrogen gas getting stripped off a big radio galaxy, called Fornax A, at the cluster centre. This solves the mystery surrounding the whereabouts of the gas missing from Fornax A.
This week’s guest
- Press release: South Africa’s MeerKAT discovers missing gas in distant galaxy
- The MeerKAT Fornax Survey: https://sites.google.com/inaf.it/meerkatfornaxsurvey
- MeerKAT: https://science.ska.ac.za/
Transcript by Sumari Hattingh.
Dan: [00:00:00] Welcome to The Cosmic Savannah with Dr. Daniel. Cunnama
Jacinta: [00:00:07] And Dr. Jacinta Delhaize. Each episode, we’ll be giving you a behind the scenes look at world-class astronomy and astrophysics happening under African skies.
Dan: [00:00:18] Let us introduce you to the people involved, the technology we use and the exciting work we do. And the fascinating discoveries we make.
Jacinta: [00:00:26] Sit back and relax as we take you on a safari through the skies.
Dan: [00:00:36] Right. Welcome to episode 32.
Jacinta: [00:00:38] Today we are talking about galaxy clusters. Our guest is Dr. Paolo Serra from the Cagliari Astronomical Observatory which is in Italy.
Dan: [00:00:50] And Paulo will be talking about a different galaxy survey. Last week, we spoke about the MIGHTEE Galaxy Survey with Jacinta. Jacinta, how did your press release go?
Jacinta: [00:01:01] It actually went really well. It was really amazing. There was a huge pickup from the media with dozens and dozens of articles from many different countries. There were radio interviews and TV interviews. There were ones in CNN and Forbes. So that was really cool. It was really exhausting though, I was in interviews one day from 5:00 AM until 11:00 PM. I’m not used to that. Dan, you do that much more often than I do.
Dan: [00:01:26] Yeah. It’s pretty exhausting. But I mean you did amazingly, I think you were on sort of every radio station you can think of and TV and news and you kept popping up all over the place.
Jacinta: [00:01:36] Yeah, it was exciting. I was really pleased to see that the general public and the media were really interested in the results in the work. And that everyone was really proud that South Africa’s MeerKAT telescope was doing such awesome things.
Dan: [00:01:51] Yeah, that’s awesome. I mean, I think we’ve said it many times that it is something really to be proud of and it’s great to see the public and the media getting on board with that and really enjoying what we’re putting out.
Jacinta: [00:02:01] Yeah, as you said, I was talking about the MIGHTEE Survey on MeerKAT last week. And today we’re going to hear about the Fornax Survey with MeerKAT that’s led by Dr. Paolo Serra, who is our guest today. He’s based in Italy where he has a research group and they use MeerKAT to look at a particular galaxy cluster cold Fornax. Dan, do you want to go through what a galaxy cluster is?
Dan: [00:02:29] We all know about stars; stars like our Sun and stars don’t come uniformly distributed across the Universe. They form in galaxies. So we have galaxies which consist of billions, hundreds of billions, of stars and those galaxies also aren’t uniformly distributed across the Universe. They form sometimes in voids and they’re very, very sparsely distributed, and sometimes they form in clusters. So in this case, we’ll be talking about galaxy clusters, which is a very dense region where there are many galaxies relatively close together.
Jacinta: [00:03:06] Yeah, exactly. And in this Fornax cluster at the very center, there’s a radio galaxy. Last week, we spoke all about radio galaxies and the discovery we made of two giant radio galaxies. This one in Fornax is called Fornax A, it’s not as big as the ones that we found, but it looks bigger on the sky because it’s a lot closer. Paolo is going to tell us about how his team used MeerKAT to look at the cluster and to look at this radio galaxy and what they hope to find with it.
Dan: [00:03:40] Yeah, sounds good.
Jacinta: [00:03:42] Shall we hear from Paolo?
Dan: [00:03:43] We shall.
Jacinta: [00:03:44] Hello with us today is Dr. Paolo Serra. Welcome Paolo.
Paolo: [00:03:54] Thank you.
Jacinta: [00:03:54] Can you tell us who you are and where you’re from and how you’re involved with Africa?
Paolo: [00:03:58] Sure. So indeed, I’m Paolo Serra. I’m an astronomer and I’ve been a professional astronomer for about 10, 15 years now. I’m originally from Italy though I did part of my studies in the Netherlands and then worked in Australia. I got involved a lot with South African astronomy and recently moved back to Italy where I work at the moment. My involvement with South African astronomy dates back from about 10 years ago when I started mostly working with collaborators based in South Africa. And eventually I think a big turning point for my connection with South Africa has been when the decision was taken to build a big new radio telescope in South Africa, in the Karoo desert. At that time, there was a call for projects from the international community. So I had an idea, submitted an idea for a project which was approved. And from that point on my engagement has increased a lot because I’m now leading a project, which is going to take data with the South African telescope called MeerKAT. And so I’m now from Italy and stay very closely connected and in fact, travel quite often to South Africa for this particular work.
Jacinta: [00:05:04] And how do you like your time in South Africa?
Paolo: [00:05:06] It’s wonderful. I wish I had more time to travel around there. I’ve been mostly to Cape Town to the big towns, say Cape Town, Joburg, Pretoria. Luckily once in a while we have meetings in some nice locations like Port Alfred – a few years ago. So that gives me a chance to do some road trips across the country, which is beautiful. And probably a highlight recently was a visit to the MeerKAT site itself in the Karoo desert.
Jacinta: [00:05:31] Oh wow, you got to go there?
Paolo: [00:05:32] Yeah. We flew there with a small plane from Cape Town and that was absolutely fantastic.
Jacinta: [00:05:37] So can you tell us a bit more about the survey that you are leading? What’s it called?
Paolo: [00:05:41] So the survey is called the MeerKAT Fornax Survey. It’s a survey of a particular system – astrophysical system – called the Fornax galaxy cluster. And hence, the name is done with the MeerKAT telescope. So the name just comes from the telescope and the system itself. The Fornax cluster is indeed a cluster of galaxies. And so I can explain that very briefly. You know that our Solar System is part of a galaxy, which is a collection of many tens of billions of stars. There are lots of galaxies in the Universe around us, you know, billions, tens of billions, hundreds of billions. And the key point of this project is that galaxies are not distributed uniformly across the universe. In other words, they are not all the same distance from one another, from the nearest neighbour. They can be very far from one another, in some regions of the Universe, which we call voids. They can be tens or hundreds of millions of light years from one another, but they can also in some regions of the Universe, be very close to one another in these places where lots of galaxies live pack together and in small volumes, relatively small volumes – are called galaxy clusters. In galaxy clusters you can have from hundreds to thousands of galaxies, all living again, as I said, very close to one another. And when they do so they interact with one another, quite a lot. There’s mashing to one another. Essentially they can destroy, disturb, deform one another, eventually also merge and make bigger galaxies by just merging with one another. And the other thing that they do is that they interact with the gas that lives in between them – the Universe is not empty. It’s filled with a very tenuous, but nevertheless, existing gas. In galaxy clusters this gas is relatively dense and hot. And it can disturb galaxies. And so this is a major way in which galaxies during the 15 or so billion years of the life of the Universe, change their appearance, change their properties, their composition, by interacting with one another and by interacting with the gas in between them. And in galaxy clusters, this happens at a very fast rate. It happens with particularly violent or dramatic events. And, and so studying a cluster like Fornax, which is very well visible from the MeerKAT site and is very close to us. So you can look at it in details – it’s very exciting and useful to understand the astrophysics that drives this galaxy evolution.
Jacinta: [00:08:04] Okay. So Fornax is a cluster of galaxies and you’ve just explained in wonderful detail what a cluster is. So Fornax is nearby.
Paolo: [00:08:12] Yeah.
Jacinta: [00:08:12] How close was it?
Paolo: [00:08:13] It’s about 60 million light years away.
Jacinta: [00:08:17] Okay.
Paolo: [00:08:18] Which on the scale of the Universe is quite close.
Jacinta: [00:08:20] Yeah, it’s basically next door.
Paolo: [00:08:21] It’s basically next door.
Jacinta: [00:08:23] And what’s so special about, this cluster? Why did you choose to look at it with MeerKAT?
Paolo: [00:08:27] The small distance from us is an important thing. The other important thing for this cluster is that clusters come in all kinds of shapes and sizes and masses. And what we know, what I’ve described earlier about the way galaxies evolve in clusters, how they interact with one another and with the gas in between – has been particularly well-studied for big clusters. But big clusters are only one type of clusters. Well, what we need to understand is, to understand how galaxies evolve across our entire range of cluster masses. And so Fornax is important here because it’s a relatively small cluster. And so there are two key aspects to it, which we’ve mentioned: the distance it’s close, so we can look at it in big details and really understand how galaxies evolve and it’s smaller than the typical classes that have been studied so far. So it’s a very important compliment to the understanding we have now of galaxy evolution in clusters.
Jacinta: [00:09:23] And what do you hope to find with your data looking at Fornax?
Paolo: [00:09:27] Yeah, there are a few key goals we have. One of them is to indeed study in more detail, how much galaxies interact with the gas in between them in a cluster of this size. Now this interaction depends on a number of properties of galaxies and the cluster itself. I mentioned that in clusters this gas is a bit dense, denser than in emptier regions of the Universe, how much this gas is able to disturb galaxies depends on this density. And it depends also by how fast galaxies move through this gas. It’s like this gas exerts a pressure, a wind, if you want. The same way that if you put your hand outside of your car window, when the car is driving, you feel this wind, the same thing happens to galaxies as they move through this gas. What is unclear for a cluster of this mass, like Fornax, is whether actually this wind is any relevant, whether it can do any damage to the cluster, we just don’t know. We know it can do lots of damage in bigger clusters but in a cluster, like Fornax, we don’t know. So it’s an important missing ingredient for our understanding of how galaxies evolve. These observations will be the first ones to actually answer that question for a cluster like Fornax. A bit of a longer shot, a more ambitious goal if you want, but still within reach probably, is that our observations we MeerKAT will be so sensitive. Now we might be able to detect cold gas, which is what MeerKAT can see for us at radio frequencies that is streaming from the intergalactic medium into galaxies in various regions of the Fornax cluster. And in that respect, our observation is an advantage because it’s very sensitive, but it also covers a relatively large area of the sky, which means we can actually trace this flow of gas, if it is there, over large distances, which is key to actually detect it.
Jacinta: [00:11:17] Is this like some sort of waterfall of gas back down onto the galaxies?
Paolo: [00:11:21] It’s essentially it is, it’s a combination of lots of different processes. There is some gas that flows back onto galaxies, like you said, that used to be in them and has been thrown out of galaxies through explosions of dying stars and then it can flow back, but it is gas that has never been inside a galaxy throughout the entire history of the Universe and is now for the first time flowing into them. And so it’s this kind of observation that’ll actually be able to tell us how much the relative importance of these two different components gas flowing back towards galaxies or gas that is for the first time flowing in.
Jacinta: [00:11:57] And why is it flowing in?
Paolo: [00:11:59] It’s gravity. Galaxies essentially have a strong mass. They are the most massive things in their surrounding, and that’s why they’re formed in the first place. And so they keep pulling matter towards them. And some of this matter is in the form of hydrogen gas that we detect with radio telescopes.
Jacinta: [00:12:15] And why do we need to know whether the gas is falling into the galaxy?
Paolo: [00:12:19] It’s because it will tell us something about the rate at which they grow. Also, gas is the material from which new stars form. So in a sense, a galaxy that stops creating new gas is like, you know, like a zombie, it will stop creating new material from which will form new stars. And so it will stop forming new stars and it means its own stars – the one that it already has – will keep on aging and aging and the galaxy will essentially stop being active and being rejuvenated. When a galaxy does create new gas, it can continue forming new generations of stars and keep being young and the shining blue in the sky while the old ones will shine bright.
Jacinta: [00:13:02] So the gas keeps the galaxy young. Great. And are there any particularly special galaxies in the Fornax cluster?
Paolo: [00:13:11] There is a few special galaxies, quite well-known to astronomers and probably also amateur astronomers around the world because, indeed the cluster is nearby, so you can actually observe its brightest galaxies, even with an amateur telescope and take nice pictures of them. There’s a few of them. One is, you know – not probably very exciting name – NGC 1365. It’s a beautiful spiral galaxy with a big bar connecting its spiral arms. It’s truly wonderful to see in the sky with dust lanes, stretching across the stellar body and large regions of star formation. Another very famous one is called Fornax A – the name comes again from the name of the cluster, which actually comes from the name of the constellation. And again, what you see is this cluster in this galaxy and the sky, it’s a constellation of Fornax. The galaxy itself is called Fornax A, because it’s a very powerful radio source. And that’s what astronomers have called “powerful radio sources”. You know, the name typically would come from the constellation where they are and then we add a letter; alphabetical letter. There’s only a few of them on the sky Centaurus A, Fornax A, Virgo A – just a few of them. Fornax A is one of the brightest radio sources in the sky and beautiful to see, but to see that you can’t use your naked eyes in that case, you need a radio telescope.
Jacinta: [00:14:29] Because our eyes can’t see radio light, right?
Paolo: [00:14:31] Exactly.
Jacinta: [00:14:32] And so we need radio telescopes to be our eyes.
Paolo: [00:14:34] Precisely.
Jacinta: [00:14:35] Great. And what’s going on in this galaxy?
Paolo: [00:14:37] In this galaxy, there’s lots of things going on. It looks beautiful. As in optical light, it looks very disturbed, very distorted. It doesn’t look like a galaxy that has been living on its own for a long time. So what we think has happened and what our colleagues think happened to this galaxy, is that it’s formed through a merger between two galaxies – two big galaxies roughly the same size – and this gives rise to a beautiful system where the central part has now relaxed, but the outer regions are still in the process of coming back into / towards the center of the galaxy and settling into the gravitational field of the galaxy itself. So in the optical light, that’s what you see. And so we think that this merger happened about 3 billion years ago. Ever since then, lots of small galaxies, other satellite galaxies, have been falling into a – what we call a merger remnant. The galaxy formed by the merger of two galaxies – two big galaxies – and all these galaxies bringing their own stars and their own gas and they sink towards the center of the big galaxy, Fornax A, itself. So they do a lot of things when they do that, they fuel new star formation in a sense, they bring in also themselves, new gas into this big galaxy. Some of this gas may actually make its way all the way to the central black hole of the galaxy itself. And this black hole, as it accretes matter, it attracts extremely fast, relativistic particles, electrons, for example. That is electrons that start spiraling in the magnetic field of the galaxy and they emit radiation at radio frequencies. And so these can dump energy again, back into the intergalactic medium. So we see this relativistic electrons at radio frequency – we see these two giant clouds, if you will, or what we call lobes; radio lobes of relativistic electrons, which are an indication indeed of a very active supermassive black hole at the centre of the galaxy.
Jacinta: [00:16:36] Cool. So you’ve got this huge cluster of galaxies – groups and groups of galaxies – there, and they’re all kind of moving through this gas and creating this wind and it’s messing up with the galaxies and then they’re all kind of crashing into each other and interacting and triggering supermassive black holes. So you’ve got a lot of crazy activity going on here in this cluster. But of course, when you look at it with the telescope, everything just looks like it’s staying still. Why?
Paolo: [00:17:07] Yeah, that is because all this fun processes that happen and that affect their life, actually have a very long timescale. They happen very slowly. I mentioned earlier that Fornax A formed through a merger between two big galaxies, about 3 billion years ago, and we see still now, 3 billion years later, parts of the galaxy still settling in after the merger happened. And so all these things take a long time. So whenever we look at a system like the Fornax cluster or NGC 1365 or Fornax A, we’re essentially seeing a snapshot of their life. And if we wanted to see some action, some real action – we would have to observe these systems for hundreds of millions of years, at least.
Jacinta: [00:17:50] Really? So even if we keep looking at this system for an entire lifetime, a human lifetime, we won’t see any change?
Paolo: [00:17:56] Absolutely. Well on some scales – maybe – in the very central engine here, these black holes, for example, they do vary over relatively fast timescales and that you can see with telescopes, but on the large-scale, you know, of the entire size of a galaxy or the entire size of a galaxy cluster, no, you won’t see a change, certainly not in a lifetime.
Jacinta: [00:18:17] Why not?
Paolo: [00:18:18] It’s just the speed at which things move. You know, the individual stars, if you will, it seems very fast to us. They move at hundreds of kilometers per second, but they need to cover distances of millions of light years. And that will take a long time. And that’s why we don’t see really these processes in action. And that’s why we need to observe as many galaxies as possible because each of them gives us a snapshot of their life. It’s like, if you wanted to get an idea of the evolution or the aging of a human by just observing a human for 10 seconds, of course you don’t get that. If you observe a few hundreds of them, you get kids, you get old people, you get teenagers, you get people in their forties, you get all that. Then you can try to piece that together into what might look like if you were able to see a single human aging from their birth to their death. That’s what we try to do.
Jacinta: [00:19:09] Yeah, that’s a really great analogy. So instead of watching one galaxy evolve, we just look at many different galaxies, which might be at different evolutionary stages and we try and come up with what the expected pathway is. Right?
Paolo: [00:19:20] Exactly.
Jacinta: [00:19:21] Yeah. Great. And so why do you want to use the MeerKAT telescope in particular to do this study?
Paolo: [00:19:27] So MeerKAT, as we mentioned before, it’s a radio telescope and we want a radio telescope in this case, because the radio telescope allows us to see hydrogen gas. Hydrogen is the most abundant element in the universe. It’s just hydrogen atoms, essentially free floating in a universe and interacting with one another through gravity and hydrodynamics. And they emit a single spectrum line of radiation. They emit electromagnetic radiation at thay specific frequency, which happens to fall in the radio radio frequencies.
It’s about 1,400 megahertz.
Jacinta: [00:20:05] About 21 centimeters wavelength, right? We can imagine what that is. Kind of the span of your big hand, maybe?
Paolo: [00:20:12] We can see that. Yeah. And so you need a radio telescope to see this radiation and MeerKAT is one of them. Now there is a few radio telescope in the world. Radio telescopes have been actually with us for several decades now.
And of course, as with everything else in human technology and research, they’re getting better and better with time. Now MeerKAT is one of the most recent and shiniest radio telescopes and its unique property is that it’s extremely, extremely sensitive. It is the most sensitive telescope we have radio telescope we have right now. It’s sitting there in the Karoo desert for us.
And so if you want to perform extremely sensitive observations of a relatively small region of the sky, MeerKAT is a telescope to use now, and it will be for several years in the future. And so that’s a, that’s exactly the kind of observations we wanted to take. Take extremely sensitive images of the hydrogen gas in the Fornax cluster, which is not too big on the sky. And the MeerKAT was definitely the telescope to use.
Jacinta: [00:21:16] And what do you think of the timescales involved in this survey?
Paolo: [00:21:19] Well you have to be patient when you start a project of this size. And that is something you need to be aware of.
As I mentioned, this entire thing started in about 2010, if I remember correctly. When we as the international community were invited to submit ideas for scientific projects on this telescope. And so that’s already almost 10 years ago. And you do that, and then the telescope needs still to be built, to be tested.
There is all kind of really cutting edge technology that goes into this cutting edge data processing, data handling, the big data that we all talk about now. Astronomy is deep into it. And so you need to understand that this is difficult, that it requires the work of large teams of people doing all kinds of different things, being coordinated by very capable people. And that they just take some time.
And you need to be good in the meanwhile to stay on top of the science we want to do adaptative it if that is necessary, but also enjoy the ride because it’s beautiful to see the progress and see how far you go from an initial idea of a few people maybe, to a giant system that involves the work of hundreds in the end.
Jacinta: [00:22:34] We wish you a lot of luck for your project and thank you for speaking to us today. We hope you’ll come back and speak to us and tell us what you’re discovering.
Paolo: [00:22:41] My pleasure. Thank you.
Jacinta: [00:22:51] So since I spoke with Paolo, their team have actually put out a few new, exciting papers with the results from there Fornax survey. One of the papers in particular was really big news. They found hydrogen gas in Fornax A, in that central radio galaxy, and it had been thought for a long time that this hydrogen gas should be there, but it had never been seen and no one understood why.
There was a lot of dust that had been detected, but none of the hydrogen gas and it should have been there. Because the thought is that Fornax A was formed by the collision of two galaxies, one of which was kind of similar to the Milky Way, which had a lot of dust, and also some of this hydrogen gas.
So the question was, where is this gas? What happened to it? It turns out it was there, but we just couldn’t see it before. And now with the MeerKAT telescope Paolo and his team found the gas in two huge tails coming out from the center of the galaxy.
Dan: [00:23:55] Was it coming out or was it going?
Jacinta: [00:23:56] In coming out, I think. In these
two big title tails, where the dust and the stars had been detected already. They’d been pulled off the main galaxies during the collision and formed these two big tails which often form when galaxies collide. This gas seemed to have been pulled off the galaxy. This wasn’t a gas that Paolo was looking for, the infalling gas. This was a bit different.
Dan: [00:24:22] Oh right. So when the galaxy is merging, it’s quite a violent collision and these galaxies kind of fling each other around and in that process sometimes the gas and even stars get flung out of the galaxy forming these tidal tails.
Jacinta: [00:24:38] Yeah, exactly. This is all part of the exciting life cycle of galaxies.
Dan: [00:24:43] Yeah, I think Paolo’s analogy was a brilliant one, comparing it to a human life. We really want to know where galaxies started, how they’ve evolved over the billions of years that they’ve been around and then what their end point is. And from that, we can learn about the things that make them up, the stars and the stellar populations and all the way down to the elements that make up you.
And the way we can do this is by looking at many many galaxies and the more we can look at and the deeper we can look with things like MeerKAT and future telescopes, the more we can learn, the better picture we have of each point.
Jacinta: [00:25:17] So much MeerKAT stuff, coming out. So much stuff, so exciting!
Dan: [00:25:23] Yeah and I think it’s just going to carry on, right?
There’s going to be more and more coming out of MeerKAT. It’s taking data at such a rate that it’s kind of hard to keep up.
Jacinta: [00:25:33] Yeah and then there’s going to be the SKA. That’s going to be next level. So much data.
Dan: [00:25:39] We’re going to have a lot of exciting news to come. You probably going to hear a lot more podcasts. We’ve had two in a row now, you’re probably going to hear a lot more podcasts from MeerKAT data as more and more discoveries get made, but we’ll try and bring you other news too, because the frontiers of astronomy are getting pushed.
Jacinta: [00:25:56] To keep gushing about meerKAT.
Dan: [00:25:59] Yeah. I mean, I think we can sort of bask in the honeymoon phase of MeerKAT, right?
Jacinta: [00:26:05] Well, I think that’s it for today. Thanks very much for listening and we hope you’ll join us again for the next episode of The Cosmic Savannah.
Dan: [00:26:13] As always, you can visit our website thecosmicsavannah.com were we will have the transcript, links and other stuff related to today’s episode. You can also follow us on Twitter, Facebook, Instagram @ cosmicsavannah that’s “savannah’ spelled S A V A N N A H.
Jacinta: [00:26:29] Special thanks today to Dr. Paolo Serra for speaking with us.
Dan: [00:26:33] Thanks to our social media manager, Sumari Hattingh and all The Cosmic Savannah volunteers. Also to Mark Allnutt for the music production, Janus Brink and Michal Lyzcek for photography and Lana Ceraj for the graphic design.
Jacinta: [00:26:46] We gratefully acknowledge support from the South African National Research Foundation, the South African Astronomical Observatory and the University of Cape Town Astronomy Department to help keep the podcast running.
Dan: [00:26:58] You can subscribe on Apple podcasts, Spotify, or wherever you get your podcasts. And if you’d like to help us out, please rate and review us and recommend us to a friend.
Jacinta: [00:27:06] We’ll speak to you next time on The Cosmic Savannah.