Cosmic Beasts and Where to Find Them

I did a science! Our new research paper has been published this week in the Monthly Notices of the Royal Astronomical Society journal!

We’ve discovered two GIANT radio galaxies. This was possible thanks to the super duper capabilities of South Africa’s new MeerKAT radio telescope. The discovery gives us insight into the mysteries of galaxy evolution and is tantalizing evidence that there might be many more out there!

Here I’ll try to explain in detail and in plain English what we found and why it’s so cool.

What are giant radio galaxies?

Galaxies are big collections of stars, dust, gas and dark matter. Most of them, like our own Milky Way, have a supermassive black hole at the centre, which can be millions of times the mass of our Sun.

In radio galaxies, “stuff” is falling into the supermassive black hole. It’s gorging itself on gas and dust and generally wreaking havoc. As this happens, enormous amounts of energy are released and charged particles, such as electrons, are accelerated to nearly the speed of light.

The charged particles get trapped in the strong, twisted magnetic fields near the supermassive black hole and get funneled out into space in huge jets either side of the galaxy. As they do so, they form a plasma which “glows”.

The jets glow not in optical light, which we can see with our own eyes, but in radio light, which can be seen with a radio telescope. This is why these systems are called radio galaxies.

Think of a water jet from the blowhole of a whale. It comes out in a narrow column then turns into a cloudy plume. That’s more or less what the radio jets look like.

Here’s an example of a nearby radio galaxy. It’s called Centaurus A and is basically the “poster child” for radio galaxies because it’s so pretty.

The cloudy bits at the ends of the jets are called ‘lobes’. They are caused by the fast moving particles slamming into gas clouds in the intergalactic medium (the gas that exists between galaxies).

The radio jets can extend out to enormous distances from the centre of the galaxy. When the jets are bigger than 700 kiloparsecs from end-to-end, these systems are called ‘giant radio galaxies’.

Side note 1: Astronomers tend to love acronyms and aren’t fond of repeating lots of words. So we shorten ‘giant radio galaxy’ to GRG.

How big is 700 kiloparsecs? Really, really, really big. This is about 22 times the diameter of the Milky Way (as seen in optical starlight). These are the largest single objects in the Universe.

Side note 2: I say “single” objects, because clusters of galaxies are technically “objects” which are bigger than GRGs. But they consist of more than one galaxy so they are cheating in the size Olympics. Anyhow, I digress…

GRGs are thought to actually be quite rare. Astronomers have found hundreds of thousands — even millions — of radio galaxies. But only around 800 of these are giants.

What did we find?

We found two WHOPPING HUGE giant radio galaxies! They are some of the biggest of the biggest. They are larger than 93% of all known GRGs.

They are about 2 Megaparsecs across. This is about 62 times the diameter of the Milky Way! It would take light 6.5 million years to travel from the end of one jet to the end of the other.

No, I didn’t get to name the galaxies 😦 They have to have serious catalogue names. They are called MGTC J095959.63+024608.6 and MGTC J100016.84+015133.0. These names are too long (see side note 1!) so I will give them nicknames. However, I am pretty bad at naming things, so I will unimaginatively call them GRG1 and GRG2.

Let me introduce you to my new space pets, GRG1 (left) and GRG2 (right). I am a proud galaxy-mum and think they are so beautiful that they can compete with the likes of “poster child” Centaurus A any day.

Maybe at this point you are thinking something like “Oooh shiny!!! But, erm, what exactly am I looking at?” A valid question.

In the background is an optical image. The data was taken by the Hyper Suprime-Cam mega digital camera on the Subaru telescope in Hawaii. In the optical picture you can see many, many galaxies.

The huge bright spots on the right are stars within our own Milky Way, which are photobombing the picture. The stars are so bright they saturate the camera and cause those weird, stripy artefacts. But just ignore those pesky attention-seeking stars.

The red represents the radio light. We usually call it radio ’emission’ because the charged particles are ’emitting’ radio light. More details on the features of the radio emission in a moment.

The ‘host’ galaxy — where all the stars and the central supermassive black hole live — is the round bit in between the two jets. In the above image, they are mostly hidden behind the red radio emission. So let’s look at the same image without the red bits.

Now the radio emission is represented by the yellow and purple contour lines. Pretty much all the contour lines inside the purple line represent radio emission being produced by our friendly GRGs. The contour lines outside the purple ones are radio emission from different galaxies and stuff, so can be ignored for now.

The point is that you can now take a good look at the ‘host’ galaxies of these giants. The inset images show a zoom-in of the host galaxies. GRG1 is much closer to us than GRG2, so its host galaxy looks a lot bigger.

The host galaxies of GRG1 and GRG2 are both ‘ellipticals’. They are elliptical in shape like a ball, and unlike spiral galaxies such as our own Milky Way. They are particularly big for elliptical galaxies, each with a mass of about 100 billion suns!

They’re also kind of yellowish in colour, because they contain mostly old stars and aren’t forming any bluer new ones. These galaxies may look all innocent in the optical, but hidden in their centres are the supermassive black holes which are causing all the fuss in the radio.

Ok, now back to the radio emission where all the fun is. These next images don’t show the optical light at all, only the radio. No more red or purple or yellow, we’re going with a straight-up black and white image this time. The darker bits are where there is more radio emission and the lighter bits are where there’s less.

Ok, I lied a bit. The magenta contour lines are still there to guide the eye. The brightest blob in the middle of each image, labelled ‘core’, is where the host galaxy is. Moving outwards from that in either direction are the radio jets. And the fuzzy cloudy bits at the end of the jets are the lobes.

Within three of the four lobes there are bright patches called ‘hot-spots’. These are caused by the aforementioned charged particles slamming into the intergalactic gas, compressing the gas and getting really hot.

The little blue circles indicate other small galaxies which happen to live along the line of sight between us and the GRGs. So they aren’t part of the GRGs themselves.

The ‘z’ numbers in the labels are the ‘redshifts’ of the GRGs and are a way of measuring the distance to them. But it will make more sense if we talk about these distances in light years. (Which is of course the distance that light can travel in a year.)

GRG1 is 2.1 billion light years away and GRG2 is 3.8 billion light years away. So the light we are seeing now left GRG2 just after the Earth was formed and waaaay before the dinosaurs ever roamed the land. Even so, it’s not really all that far away compared to other radio galaxies we routinely study.

How and where did we find them?

We spotted GRG1 and GRG2 in new radio maps of the sky made with the MeerKAT telescope.

MeerKAT is a new telescope in the Karoo semi-arid region of South Africa. It officially commenced science observations in June, 2018 (just before I moved to South Africa). It is a crazy good telescope.

MeerKAT is an ‘interferometer’ which consists of 64 individual radio dishes, each with a diameter of 13 metres. The signals are collected by these dishes and combined together via a supercomputer called a ‘correlator’.

One of the very large survey programs being conducted with the MeerKAT telescope is called MIGHTEE. MIGHTEE stands for… [checks notes again]… the MeerKAT International GigaHertz Extragalactic Exploration survey. (As I said, astronomers love their acronyms and not saying lots of words!)

The main goal of MIGHTEE is to understand how galaxies have changed, or ‘evolved’, over the history of the Universe. The MIGHTEE survey team is made up of dozens of professional astronomers and graduate students around the world, including myself.

In total, MIGHTEE will take roughly 1900 hours to complete. This is obviously a big investment of time with the MeerKAT telescope. So it was important to do a test run to see if the survey plan would work.

As a kind of “beta test” or ‘Early-Science observations’ of MIGHTEE, MeerKAT was pointed to a particular patch of sky called COSMOS. COSMOS had already been looked at by many other telescopes at all different wavelengths of light, so there was lots of other data we could compare with to make sure everything looked alright.

Well… I think it’s quite safe to say that the “beta test” was a roaring success. With less than 24 hours of observations with MeerKAT, members of the MIGHTEE team created a spectacular image of the radio sky that was one of the best of its kind ever made!

We team members couldn’t wait to sink our teeth into this delicious new data to see what we could find.

One of the first things that jumped out at us were large streaks in the radio map which were almost certainly the jets and lobes of radio galaxies. The first one was spotted by Dr Ian Heywood from the University of Oxford, and the second by Dr Matthew Prescott from the University of the Western Cape.

One of my tasks was to figure out how far away these galaxies were. Only then could we tell whether they could be classified as ‘giants’. They could have been quite small radio galaxies but just very close to us, making them look big.

But, as you already know, this wasn’t the case. They were several billion light years away, meaning they were some of the most giant of the giants!

I set out to discover more about them.

Why is this discovery important?

The discovery of GRG1 and GRG2 is important for several reasons.

Reason Number One is simply that they had not, and could not, have been discovered before.

In my previous job I worked as part of a team to conduct what was, at the time, the most advanced radio survey of the COSMOS field. You can read more about it here and also on my research page.

We conducted our survey with the Very Large Array (VLA) radio telescope in the USA. We needed around 400 hours of observations with the VLA to make our radio map. It was a very high quality map in which you could see fainter and smaller things than in any previous maps.

We could see the host galaxies of GRG1 and GRG2 in our VLA map, but only the very inner parts. So we knew they were radio galaxies. But we couldn’t see the large-scale jets and lobes, so we had no idea that they were giants.

Here’s what GRG1 looks like in the VLA image (black and white background image). Again, the darker regions are where the VLA saw brighter radio light.

Overlaid over the top are the same contour lines from MeerKAT that we already saw above. The trusty purple line is there to remind you where the jets and lobes are.

The inset shows a zoom-in of the ‘core’ or ‘host galaxy’ region. Here we can see a bright detection of an elongated structure. It is probably the inner most part of the jets — a part much closer to the supermassive black hole than we can see with MeerKAT.

We can see these small details in the VLA image because the VLA telescope has a better ‘angular resolution’ than MeerKAT. This means it can pick up smaller, finer details like these inner parts of the jets.

You can also see that the VLA picked up a little bit of the emission from the upper hot-spot. The rest of the jets and the lobes seem to be completely invisible though!

And here is the VLA image of GRG2 with its MeerKAT contour lines overlaid.

In this image, the VLA has detected the central galaxy (the small black dot seen in the inset) and maybe a bit of the upper hot-spot (if you kind of squint a bit). But again, the jets and lobes are invisible!

What is going on here? Why are the radio jets seen in the MeerKAT image but not in the VLA image?

The answer is that the MeerKAT telescope is the first to be able to “see” very faint radio emission which is distributed across a large area of sky. The VLA can detect faint emission, but only if it is concentrated in a fairly small region.

The reason for this is pretty complicated and involves words like ‘Fourier transform’ and ‘uv plane’. Far be it from me to be able to explain these things coherently. But in a nutshell, the individual MeerKAT dishes are spread out in such a way that allows MeerKAT to detect large-scale, ‘diffuse’ (faint and fuzzy) radio emission.

Therefore, MeerKAT can detect things that are simultaneously fainter and bigger than any other radio telescope. In technical terms, we say that MeerKAT has a good ‘surface brightness sensitivity’. This is why it could detect the huge but faint jets and lobes of GRG1 and GRG2 and why we now know that these objects are giant radio galaxies.

And that brings us to Reason Number Two of why this discovery is important.

GRG1 and GRG2 are fainter than any other giant radio galaxies of roughly the same size. So if we consider both size and luminosity (or brightness), no other giants are quite like GRG1 and GRG2. This makes them a little bit special. (Proud galaxy mum!)

We did expect galaxies like these to exist, though. There have been several ideas to explain why some radio galaxies become giants and others do not. But the idea that is currently winning is age.

We think that GRGs are simply the older radio galaxies, which have had enough time for their jets to grow outwards to these enormous sizes. As the radio galaxies age, they usually also fade.

If this is the case, then it totally makes sense for very large but very faint GRGs to exist. And in fact, there should be more of them out there than we have already seen.

And that brings us to Reason Number Three why this discovery is important.

We should not have found TWO giant radio galaxies in the COSMOS field! In fact, we shouldn’t have even found one!

This is because, as far as studies of giant radio galaxies go, the COSMOS patch of sky we observed with MeerKAT is actually pretty small. It has an area of about 1 square degree. To give you an idea of how big this would appear if you looked up at the night sky, four full moons could fit inside this area.

We know, mostly from observations with the Low Frequency Array (LOFAR) telescope in the Netherlands, that there are only about 800 giant radio galaxies spread across a large part of the sky. This means that the probability of finding one in only 1 square degrees of the sky is actually pretty small.

Finding TWO GRGs with such enormous sizes and at (relatively) small distances from us is extremely unlikely. In fact, the probability is only 0.0003%!

So either we have been stupidly, ridiculously lucky, or GRGs are much more common than we previously thought! And this is the big zinger of a result from our work.

How does this help us understand the Universe?

The finding that GRGs could be more common than thought lends weight to the ‘aging’ explanation for why giant radio galaxies exist. Galaxy evolution models predict that these faint, extended giants should exist, so we should be able to see more of them.

It’s also important to have a good understanding of these aged radio galaxies because they are perfect laboratories to examine how the radio jets might influence the host galaxies.

The general idea is that the longer the supermassive black hole-induced radio jets exist, the more havoc they wreak on their host galaxies. They probably heat up and blow out the gas from the galaxy. This prevents new stars from forming and so essentially “kills” the galaxy, leaving it “red and dead”.

It’s also relevant to point out again just how super cool it is that we now have a telescope that is capable of finding all these extra GRGs! It demonstrates how powerful MeerKAT is and that it is able to detect new and unique objects out there in the cosmos.

When completed, the MIGHTEE survey will map 20 square degrees of the sky. This is 20 times bigger than the map of COSMOS we already made during the Early Science observations. Hopefully this means we will find more of these huge and faint GRGs in the rest of the MIGHTEE data. (I’ll certainly be looking out for them!)

If MeerKAT can detect these fascinating new galaxies, then what else will it find? And for that matter, what is the Square Kilometre Array going to find??

MeerKAT is one of the precursor telescopes to the almighty Square Kilometre Array (SKA) telescope. The SKA will be a behemoth trans-continental radio telescope built across southern Africa and Australia.

Construction of the SKA will commence as early as this year and continue until about 2027. When complete, we think it will revolutionise our understanding of radio galaxies and of galaxy evolution! (Goosebumps!)

I cannot wait to find out what else MeerKAT and the SKA will discover.

Related links

Leave a comment

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: