The Cosmic Savannah - E43: The Search for Hidden Hydrogen

Jacinta00:05

Welcome to The Cosmic Savannah with Dr. Jacinta Delhaize

Dan00:08

and Dr. Daniel Cunnama. Each episode, we'll be giving you behind-the-scenes look at world-class astronomy and astrophysics happening under African skies.

Jacinta00:16

Let us introduce you to the people involved, the technology we use, exciting work we do, and the fascinating discoveries we make.

Dan00:25

Sit back and relax as we take you on a safari through the skies.

Welcome to episode 43.

Jacinta00:41

Yes. Episode 43. And today we will be talking all about neutral hydrogen gas, which is one of my two very favourite topics. We will be speaking to Tariq Blecher, who was a PhD student at the University of Rhodes. Also to Shilpa Ranchod, who has just finished her Master's at the University of Pretoria and is now doing her PhD at the Max Planck Institute for Radio Astronomy at the University of Bonn in Germany.

Dan01:07

What is your other favourite topic?

Jacinta01:09

Active galactic nuclei.

Dan01:10

Oh, right. Okay. Hm.

Jacinta01:12

Giant radio galaxies.

Dan01:16

So yourself. But not yourself specifically... your work

Jacinta01:22

Yes. My own research, obviously. How could that not be my most favourite topic?

Dan01:26

What a blessing to be able to work in something you love.

Jacinta01:30

Do you wanna tell us how the awards night went, Dan?

Dan01:33

It was disappointing because I didn't win and neither did we. No, but it was really cool. Got dressed up in my tux and I mean, to be honest, it was a really special evening. I think that they did a very good job of making the finalists feel really honored.

And I it was an incredible achievement to be selected as a finalist, and that feeling really sat with me and, yeah, I walked away happy and smiling. I had a great evening.

Jacinta02:02

Well, you walked away with crutches, I suppose.

Dan02:05

I hopped away. Yes. So I was.

Jacinta02:08

You hopped away.

Dan02:08

I had my nice fancy tux and a moon boot.

Jacinta02:11

Yeah. Yes. So for those who haven't heard our previous episode, and don't know what we're talking about, The Cosmic Savannah was a finalist for the communication award in the NSTF-South32 Science Awards for South Africa. So kind of like the science Oscars, and Dan managed to go to the gala dinner - the awards night - in person. Looked very dapper in a custom-made tux.

Dan02:33

Oh, thank you very much.

Jacinta02:35

While I watched in my pajamas from Australia, late at night, over Zoom or YouTube and yeah, it looked like an amazing night. Exciting that these sorts of events can start to happen very slowly and socially distanced. Now with the vaccination levels getting higher.

And also congratulations to associate professor Carolina Odman, who took out the award for her amazing work that she does also with translating into different languages, astronomy and related fields.

Dan03:03

Yeah, absolutely. A worthy winner and yeah, happy to be included amongst such great company.

Jacinta03:09

So back to the episode topic of today, which is neutral hydrogen gas.

Dan03:13

Yeah. So. Jacinta you spoke to Tariq and Shilpa. Maybe because it's one of your two favourite topics, you can explain very briefly to new listeners, what neutral hydrogen is.

Jacinta03:24

Sure. Well, I worked in this area for my PhD and I ended up talking to Shilpa a little bit about that. You'll hear that later, but neutral hydrogen gas is just, I mean, it's hydrogen, which is the kind of first element on the periodic table.

It's the most abundant element in the universe, but it's also really quite hard to detect. So galaxies usually have these hydrogen gas inside them and sort of around them. And this is the raw fuel for star formation. So it's like a really fundamental building block of a galaxy, but it's actually really hard to detect.

It's very diffuse, meaning it's spread out over a large area and it's like very faint. It only glows or releases light at a very particular frequency, which is in the radio, which you can detect with a radio telescope and it's quite a weak signal. So the challenge is to be... to detect the hydrogen gas out to large distances across the universe in order to kind of study how this property of galaxies has changed over time. And so this is a really important way that we try and study galaxy evolution.

And so I was talking to Tariq and Shilpa about their work in this area, about how hydrogen gas exists within groups of galaxies or within clusters of galaxies. And this particular topic that we talk about today, which is gravitational lensing.

Dan04:41

Do you wanna have a go at what gravitational lensing is?

Jacinta04:43

Why don't you tell our listeners Dan?

Dan04:47

It's not that you don't understand. It's just that you're giving me a chance to talk.

Jacinta04:50

I did all the interviews, so let's hear our listeners wanna hear from you.

Dan04:54

All right. No, gravitational lensing is something that's super cool. I just love the picture of it and the idea of it. So when you think of a lens, you normally think of a glass lens. It's a piece of curved glass. If you had them in high school and you sort of passed light through, you could see how the rays bent as they passed through a lens. The same as for your eyes if you wear glasses.

And in this way, light can be bent and focused and magnified. And what we're talking about today is something called gravitational lenses. So we also know that space and time can bend under the influence of a lot of mass. And what that means is that the light ray appears to bend as it curves around a large galaxy or cluster of galaxies.

And that creates something like a lens. So if you're looking at a large cluster of galaxies, which is a huge amount of mass and is bending and curving space and time. Light from behind that cluster can curve around the cluster on its way to us and be curved and magnified. And in that way, you can actually look behind these massive clusters and you can get a lot of magnification from that. Which means you can actually look at something behind the lens in quite a lot more detail in some cases than you could if you're just looking at the cluster, for example.

Jacinta06:19

Yeah, exactly. And there's been different kinds of light that's been detected by gravitational lensing of galaxies that you wouldn't have been able to see otherwise. They would've been too faint. But hydrogen gas, the signal of hydrogen gas, has never been detected in this sort of gravitational lens system.

And so Tariq was trying to make the first gravitational lens detection of HI, which is the short name for a hydrogen gas. Shall we just hear from Tariq and let him explain how we tried to do that?

Dan06:47

Great. Thank you.

Jacinta06:57

Guest, who are you?

Tariq06:59

My name is Tariq Blecher. I am a PhD student, but I don't let that define me.

Jacinta07:07

Where are you from?

Tariq07:10

Born in bred in Cape Town.

Jacinta07:11

What university are you at?

Tariq07:13

Sort of a complicated arrangement. I'm registered at university currently known as Rhodes, which is in the Eastern Cape of South Africa. But I'm based at the astronomy institute in Cape Town. But I sort of drift all of the place a lot. I've got supervisors, one supervisor in Pretoria, another in Oxford, another in Australia and added to that, I'm always, I'm often like just on my own adventures.

Jacinta07:39

Okay. And can you tell us a little bit about what you do as a PhD student?

Tariq07:44

So the general field is trying to work out how... why galaxies are the way that we see them today, how they've come to be from like an early universe that is smooth to now sort of later universe that we find ourselves in, which is like filled with all these interesting shapes and forms.

So the general field is sort of how do galaxies evolve, and like tackling a little slice of the pie looking specifically at gas in galaxies.

Jacinta08:14

So as part of your talk, you mentioned gravitational lensing. Perhaps you could talk a little bit more about that.

Tariq08:20

Okay. Well, Einstein's theory of general relativity. One of the consequences of it is that mass interacts with light, and basically heavy things will change the path that light naturally takes.

And, what can happen if, say, I want to look at a distant galaxy, but in between me in this distant galaxy, there's another extremely massive object. Say another galaxy or cluster of galaxies, and now shift your focus to the galaxy that we want to see... this very distant one. You can imagine the light as rays emanating from this background galaxy. And normally the light rays would sort of... you can think of it as these lines going off in a sphere, sort of equally in all directions. But when the light rays sort of come near this massive galaxy cluster, they bend around it, because of this general relativity effect. And what can end up happening is that it sort of becomes almost like a magnifying glass in a way, or a lens where the image of the background galaxy is now magnified. So, basically trying to use this natural magnification to see more faint and distant object.

Jacinta09:39

And you spoke today about looking at hydrogen gas inside lens galaxies. Has hydrogen been seen in lens galaxies before?

Tariq09:48

No. It hasn't been seen like many other things have been seen with this lensing, typically sort of very compact objects such as the emission from black holes, or a carbon monoxide emission, which is generally quite localized. Neutral hydrogen hasn't been detected yet.

There are some good reasons for this, basically because neutral hydrogen it's the most extended galactic component, gaseous component. And this sort of dampens the effect of the lensing a bit, especially for lensing in sort of the nearby objects. Also, the hydrogen is really, really, really faint. So even if it's lensed, if it's incredibly far away, it still won't be bright enough to see.

At the moment, like I'm still sort of investigating the sort of feasibility of using lensing for neutral hydrogen. I'm still sort of trying to find the best sort of lenses. At the moment I've just used lenses that are the size of galaxies. Now I'm sort of experimenting with lenses that come in groups of galaxies or hundreds of galaxies. Because these have like more potential for giving higher magnifications. But the problem with these is that they're rarer, and so they're further away. And so the thing that they're lens is even further away, so everything becomes very faint. Yeah.

Jacinta11:18

So why do you actually want to detect neutral hydrogen in lens galaxies? What's important about neutral hydrogen?

Tariq11:25

So neutral hydrogen.... you can think of it this way. In your periodic table, hydrogen is like the... it's the simplest element. It's quite a fundamental element. Most of the what they call baryonic matter, which is basically all sort of matter that we are familiar with in our everyday lives. Most baryonic matter in the universe is a hydrogen. So it's quite an important element. There's just like a ton of science that you can do with neutral hydrogen. It's been explored, but it hasn't been explored to very far distances because it's faint.

But for instance, just one particular aspect... a neutral hydrogen search that I'm quite interested in is if you measure the amount of neutral hydrogen in a galaxy, and you measure the amount of molecular hydrogen in galaxy. So basically there's a process that... physical process that converts the neutral hydrogen into molecular hydrogen. It's based on like the pressure that the neutral hydrogen is exposed to. And this pressure is dependent on the galaxy as a whole, and it's quite strongly dependent on what's called the dark matter halo, which the galaxy resides in.

And this is quite like a mysterious sort of substance. I'm not really sure exactly what it's made of. But with the sort of molecular hydrogen to neutral hydrogen ratio, you can track the amount of dark matter in galaxies. And if you track this ratio at different points in the universe, you can see how the dark matter properties are changing.

Just one example of something you can do with HI.

Jacinta13:05

So you've actually been using a telescope in India to try and find this lensed hydrogen, haven't you?

Tariq13:11

Yeah, I've been using... it's the Giant Metrewave Radio Telescope (GMRT). Yeah, the GMRT. It's about... what is it? 30 dishes made of sort of mesh material. It's located near Pune.

Yeah, I've actually had a lot of experience sort of applying to the GMRT. What's really nice about astronomy is that there are loads of observatories that you can just apply for, and it's completely free to apply. You can get as much data as you need, as long as the merit of your application is really good.

And the GMRT is one of these telescopes where literally anyone can just put in an application. It's completely free. It's like really heartwarming that it is such a meritocracy. That are not having to pay like a billion rand or whatever to get time. So, yeah, so we put a application into the GMRT. It's quite a sensitive telescope, so yeah.

Jacinta14:06

Well, congratulations on getting time with GMRT. I'm sure a group of international experts reviewed your application and decided to award that time. How much time did you receive and what did you do with the data?

Tariq14:17

That proposal was made by my supervisor before I started the PhD, but I have since put in a couple of proposals and one of them has gotten time. So with the data, although it is free to apply, the data is presented to the user in quite a raw format. So there is quite a lot of work that goes in from the raw data to science quality. Yeah. And I sort of had to learn the different sort of software packages that one needs.

Jacinta14:48

How much time did you receive on the GMRT?

Tariq14:50

Oh, this is about eight nights for observations. Or actually I think there are also days included, because with the radio, you can also observe during the day. So eight... about eight days. Hmm.

Jacinta15:02

So now I guess you've processed all of the data. You've learned how to reduce it. And you've been doing some science or that at the end. So what have you found?

Tariq15:11

Well, we... there were three galaxies which we observed. For two of the galaxies, there's clearly like no detection. For one galaxy there's sort of a hint of a possible detection, but you can't really say for sure because the signal's not strong enough. In hindsight, the proposal was maybe a little bit too ambitious because HI is so faint and a few nights per source is really not much.

But it was quite exciting that we got some hint, and we'll do follow-up observations on that.

Jacinta15:43

Yeah, you certainly have to start somewhere. And it's really exciting that you might have found a hint of lensed hydrogen, which would definitely be some pretty big news. But you told us in your conference presentation that you are trying to find out some extra information about these systems. You made some non-detections here, which is what we call things that we didn't detect, but can we actually still get any information about these galaxies from the non-detections?

Tariq16:13

This was quite an interesting point that was raised by my co-supervisor, Deane. He's quite a smart guy. So originally when I presented him with the non-detections, I just presented them as upper limits to him. And I said, you know, we can be sure that what we're looking for is not anything brighter than, say, this factor times the noise. And he was like, "No, no, no, no. But if I'd be leaving out information." I was like, "I'm leaving out information?". And then yeah, he actually... if you look closely, even if we didn't make it a detection, there's actually a lot of other information that we can sort of leverage to get more information about the neutral hydrogen.

So essentially we know from the optical Hubble and HDSS data, we know exactly where the galaxy should be. So we know exactly where to look. And we also know the statistics of the noise. Say you buried something in the ground, not too deep, quite shallow, and there's sort of little mound maybe left over from what you buried and maybe after some time, this is where the noise is coming in. So the ground has shifted a bit and, you know, slightly buried your signal. This is what, the analogy that I'm trying to get of like this hydrogen signal that's being buried in the noise, but we know where to look. You can tell, like if whatever was buried in the ground, if it was really huge, then you would expect there to be, even if it's buried... with the probability that there would be a positive noise or that there would be a slight positive bump in the ground... the probability of that is higher. If the object that you buried is higher, or if HI fluxes is better. And the same, if there's no hydrogen in the galaxy, then there's sort of equal probability that is sort of a slight indent in the ground or mound over the ground. So, we basically came up with a way of getting the statistics of the thing that we did not see based on knowing what noise looks like, and exactly where to find it.

Jacinta18:22

It's complicated to explain, but I think you did a really good job. So we can find out some information about this object, even though we can't detect it, but we do know where it is, or at least we know where to look and that itself gives us some extra clues. Right?

So are you going to publish this work?

Tariq18:40

Yeah. So the plan is to publish it for the end of the week, but these sort of research deadlines are always seem to be pushed back.

Jacinta18:47

Well, good luck. You used the GMRT telescope in India for this work. Do you think South Africa's new radio telescope, MeerKAT, will be used for this in the future?

Tariq18:58

Yeah. So I've actually got some MeerKAT data just waiting to be used. Just had my hands full with this paper, but planning on getting to it as soon as possible.

Jacinta19:09

Now that is exciting. I can't wait to see the results on that.

Tariq19:12

Yeah, especially we've got some... so the previous data, it was... the lenses were galaxies, which are sort of well-behaved, but are sort of smaller scale. And this cluster stuff is really like thinking big.

Jacinta19:27

Yeah. So it'll be really exciting to use entire clusters as huge magnifying glasses to detect very distant hydrogen.

Tariq19:35

I think the lensing phenomena is going to be quite different. In the galaxies, what was nice is that it was quite well-behaved so you could characterize it well. The thing is in clusters, the models of the cluster is more uncertain. You have these huge collections of galaxies that have been merging and you maybe have multiple of these clusters emerging together. And so there's sort of more uncertainties involved. So we are just going to have to divert up techniques that are more related to blind... sort of blind surveys where you don't really know exactly where to expect the detection, but you sort of search over a larger area.

Jacinta20:20

Right. So instead of choosing one tiny patch of the sky to look at, because you suspect something's there, you just kind of stare at an entire big patch of sky and maybe find something?

Tariq20:30

Exactly. Yeah.

Jacinta20:31

How do you feel about the environment of astronomy in South Africa?

Tariq20:35

Astronomy is booming. So it was pretty exciting to be in South Africa. It's unbelievable how many astronomers there are in the country now.

Dan20:55

Thanks. You recorded that a while ago and very interesting work. Do you know what Tariq's up to these days?

Jacinta21:01

Yeah, so Tariq, he did submit his paper and he's actually now finished his PhD. This chat with Tariq actually happened quite a while ago. Well, before COVID hit. That's why we were together in person at a conference. And yeah, he's now finished his PhD and is working as a data scientist.

Dan21:19

Great. And Shilpa you interviewed very recently, I know. There was quite a lot of fuss around Shilpa's recent paper. I know I got many phone calls about it and gladly passed them onto her. So I assume you spoke to Shilpa about that too.

Jacinta21:37

Yeah. So Shilpa was looking at hydrogen gas in clusters, and trying to detect it in those systems. So clusters are, you know, groups of groups of galaxies. And the more galaxies you have in your environment, the less likely you as a galaxy are to have hydrogen gas. And so Shilpa is trying to understand how much hydrogen gas is in galaxies that are quite far away from us, and therefore kind of existed back in the history of the universe.

And she also does a little bit of work on gravitational lensing. So she explains to us her Master's research, which she did at the University of Pretoria.

Dan22:16

Great, looking forward to it.

Jacinta22:26

With us today we have Shilpa Ranchod, who is a PhD student at the Max Planck Institute for Radio Astronomy in Bonn. Welcome to The Cosmic Savannah, Shilpa.

Shilpa22:36

Hi, thank you. I'm very happy to be here.

Dan22:38

Can you tell our listeners just a little bit about yourself to begin with?

Shilpa22:41

Okay. So my name is Shilpa Ranchod. I'm originally from Cape Town in South Africa, and I also did my BSc in physics and astronomy. And my BSc Honours with NASSP at the University of Cape Town. I then went on to do my Master's at the University of Pretoria. And now I've just started my PhD at the Max Planck Institute for Radio Astronomy in Bonn, in Germany.

Jacinta23:10

And you've just moved there?

Shilpa23:11

Yes. It's been about three weeks now.

Jacinta23:14

Three weeks. Oh, in the northern hemisphere. How are you finding it so far?

Shilpa23:18

Starting to get a little chilly.

Jacinta23:21

Already?

Shilpa23:22

A bit of an adjustment. Yeah. But it's been been great so far. Yeah.

Jacinta23:28

All right. Great. So you've just moved there. And before that you were doing your Master's at the University of Pretoria. What was your Master's about?

Shilpa23:37

So my Master's just broadly was searching for HI or neutral hydrogen gas in galaxy groups and cluster.

Jacinta23:46

Alright. Great. So now I know a lot about HI, neutral hydrogen gas. I did a lot of research in that for my PhD. But for our listeners at home, we have done episodes on this in the past, but maybe we have some new listeners. So could you tell people a little bit about what HI is?

Shilpa24:05

So HI, as I said is neutral hydrogen. It's the most abundant gas in the universe. And it's the essential ingredient for star formation. Even though it is so abundant, it is extremely faint and difficult to observe. It can only be observed with radio telescopes.

Jacinta24:26

All right. And you have been using one of South Africa's very powerful radio telescopes to study this.

Shilpa24:32

Yes. I've been using MeerKAT and it really returns some very awesome results.

Jacinta24:38

All right. So tell us about your data. What was it of, and what did you find?

Shilpa24:42

So the first part of my Master's was part of MIGHTEE, which stands for the MeerKAT International GHz Tiered Extragalactic Exploration. So that's quite a...

Jacinta24:55

mouthful.

Shilpa24:56

Yeah, but it's... this is one of the MeerKAT large survey projects. So this is part of the HI component of MIGHTEE. And MIGHTEE just observes a very large portion of the sky and the MIGHTEE team worked very hard to reduce and calibrate the data so that it was ready for me to work with. And with this, we just looked through it by eye to find any blips of HI. And with that, we just discovered a galaxy group.

Jacinta25:23

Okay. So what is a galaxy group?

Shilpa25:25

So a galaxy group is a collection of galaxies that are gravitationally bound to each other. Which means that they evolve together and influence each other's evolution.

Jacinta25:37

Okay. And so had this group been found before? Did we know about it? What was special about it?

Shilpa25:43

So these galaxies in this group, they've been observed many times before with optical and infrared telescopes, but they were assumed to be individual and unrelated because they were against the background of hundreds and thousands of other galaxies. But by detecting mutual hydrogen emission from these galaxies, we were more accurately able to tell the distance of those galaxies from Earth. And that led us to believe that, instead of just being close together in projected space, they were actually close together in a three dimensional space. And this led us to believe that they are gravitationally bound and a galaxy group.

Jacinta26:23

Okay. Yeah. Because when we look up at the night sky, you know, we are seeing stars and little dots of light, which could be galaxies or planets or anything. And we're essentially seeing it in 2D, because we can't tell how far away each of these things are. And so, you're saying that with these radio astronomy observations with of the hydrogen gas, you could tell how far away each of these galaxies were. And therefore you could tell that these particular ones were all close to one another.

Shilpa26:49

Yes, exactly. What was even more exciting about this group is that it was extremely neutral hydrogen rich, and this led us to believe that it was in the early stages of formation.

Jacinta27:00

Okay. So the galaxies each had a lot of hydrogen gas. So why does that mean we think it's in the early stage of galaxy evolution?

Shilpa27:07

Because dense environments like galaxy groups and clusters, they're quite violent. Well galaxy clusters are anyway. And in the process of joining and evolving in a dense environment, galaxies can lose a lot of the neutral hydrogen gas. And because these galaxies still had so much, it shows that they hadn't been a part of the group environment for a very long time.

Jacinta27:31

Okay. So you found kind of a young group of galaxies, right? Okay. What does this tell us about galaxy evolution?

Shilpa27:40

Well, this in itself doesn't tell us too much, but it is a unique snapshot of the galaxy group in its evolutionary timeline. And this along with other observations of massive groups like this and simulations, can put together the whole picture of how galaxy groups evolve from the beginning to the end of their evolution.

Jacinta28:02

Okay. So you mentioned that you used data from the MIGHTEE survey. And listeners may remember from episode 31, where I was my own guest on this podcast, that I used data from the MIGHTEE survey with MeerKAT to study giant radio galaxies.

MIGHTEE has several different components and you're looking at a slightly different component or a different way of looking at the sky than the data that I was using, but we're part of the same big collaboration. Why hadn't this group of galaxies been found by other telescopes before? Had other telescopes just not looked at this part of the sky? Or is there something special about MeerKAT and MIGHTEE that helped you find them?

Shilpa28:40

Yeah. I mean the latter. MeerKAT has such great sensitivity. It's able to pick up emission that has never been detected before. And as I mentioned earlier, HI is extremely faint. So it is thanks to MIGHTEE's great sensitivity that we were able to detect this group.

Jacinta28:58

So you actually published a paper on this, right?

Shilpa29:00

Yes. In July.

Jacinta29:02

Congratulations on getting a paper out during your Master's. That's really a big achievement. And I saw that you actually put out a press release for this discovery. How was that experience?

Shilpa29:13

Oh, that was a whirlwind. There was a lot happening. A lot of phone calls, interviews. It just went on and on and on, but it was extremely exciting to have the opportunity to communicate my work to the general public and also get some recognition that way.

Jacinta29:31

I heard you on the radio. Did you do any TV as well?

Shilpa29:34

I did. I did one on ETV news, so that was quite exciting.

Jacinta29:38

How cool! Was that sort of your first experience doing that sort of thing?

Shilpa29:42

Yes. Yes, I definitely would do it again.

Jacinta29:44

Well, you were fantastic. I heard you and it was so good. Yeah. What, so did you find that there was a lot of interest from the public and from the media and this sort of thing?

Shilpa29:54

Yes. I think people were particularly excited about MeerKAT and just the fact that a South African telescope has been able to produce world-class results. I think everybody is quite excited about that.

Jacinta30:08

Definitely. It's something that, you know, the people of South Africa and of Africa can be very, very proud of. So it's, yeah, it's really awesome to see that our students are, you know, using this and making some big discoveries already.

So tell me Shilpa what are you... what have you been working on since then?

Shilpa30:25

Well, that was only one half of my Master's. For the rest of it, we were looking at, or searching for neutral hydrogen in galaxy clusters, which are similar to galaxy groups, but much, much bigger. And because these environments are so turbulent and violent, there's a lot less HI gas in galaxies that reside in clusters.

So, that was a challenge, but with MeerKAT, and it's really amazing sensitivity, as I mentioned before, we were hoping to detect HI in these clusters that also much, much further than the group that we discovered was. Yeah. So these are the red shift of 0.3 to 0.4, which is very far away. I dunno what that is in... parsec.

Jacinta31:12

Yeah. Just suffices to say that that's a lot further away than these other galaxies that you were looking.

Shilpa31:17

Very, very far away.

Jacinta31:18

Yeah.

Shilpa31:18

Billions of light years away.

Jacinta31:20

Yes.

Shilpa31:20

I guess. Yes. So, yeah, so we used a statistical method in order to actually detect the signals from the galaxies in the clusters.

Jacinta31:32

Now I'm gonna hazard a guess that you used stacking to do this.

Shilpa31:37

Yes. It is. Yeah. So stacking is a statistical method that co-adds the signals from the HI signals from many galaxies in order to reduce the noise and get a higher signal-to-noise ratio. And then hopefully make a detection.

Jacinta31:56

In one of our mini episodes previously, we spoke with student Andy Firth who was also kind of playing with HI data cubes and trying to do stacking. He used a slightly different sort of technique than what you are... you are doing... but I think that what you're working on is similar to what I did for my PhD. Right? Maybe you might know about that?

Shilpa32:18

Yes. No, I know very well about that. I think I've referenced your paper quite a few times in my Master's thesis.

Jacinta32:25

Ah, well, thank you.

Shilpa32:29

But, yeah, so we looked at three different galaxy clusters, and in two of them we didn't find any HI emission even after stacking.

Jacinta32:38

Oh, right.

Shilpa32:39

But in one of them, we did make a three-sigma detection. So that was quite exciting.

Jacinta32:45

Okay. So, for our listeners, what does three-sigma mean? Is that a strong detection or...?

Shilpa32:51

It is not the most strong detection. I think we can confidently say that there is definitely some HI there.

Jacinta33:00

So had this not been found before in this particular cluster?

Shilpa33:03

No. In fact, these clusters, two of the three clusters had never been observed in HI emission before. So this was really something new that we were trying out with MeerKAT.

Jacinta33:14

Okay. And so you made sort of like a weak, but not too weak stacked detection. So that means if I could just repeat what you said before... So instead of just looking at one galaxy for a long time to try and detect the HI signal from it, you look at many galaxies. I don't know how many you looked at.

Shilpa33:34

Well we observed the whole cluster and because the cluster is so far away, it fits very well within MeerKAT's primary beam. Yeah. So we were able to observe the whole entire cluster, but we were only really able to stack a few clusters, a stack of few galaxies, because we only had a few spectroscopic red shifts.

Jacinta33:54

Right. So you only knew the exact kind of distances to a few of these galaxies.

Shilpa33:59

Yes, exactly.

Jacinta34:01

Right. And that's essential information that you need in order to stack. And so you combine the signals of these galaxies together and you made a statistical detection, so you didn't directly detect hydrogen in any of them individually, but combining them all together, you can detect a slight detection, right?

Shilpa34:18

Well, yes, an average amount of HI between those galaxies.

Jacinta34:23

Okay. And what did you find out about the average properties of hydrogen gas in this cluster?

Shilpa34:28

So what we did look at was the HI deficiency parameter of these classes, including the ones where we didn't detect any HI, but we found that it was consistent with what we predicted. So there isn't much HI in this cluster or in any of these clusters because we don't expect to see too much HI in clusters.

Jacinta34:52

So how much hydrogen gas did you find in these galaxies when you did the stacking?

Shilpa34:57

Okay, so we found of order 10 to the 10 solar masses of HI gas between these galaxies.

Jacinta35:04

Okay, so that sounds like a lot. Is that a lot?

Shilpa35:06

It is a lot and it's much more than we expected. So we do think that this amount was falsely boosted due to the fact that a lot of the galaxies that we stacked were really, really close together. And sometimes when that happens, it's called source confusion. The signal can be artificially boosted as some of emission from one galaxy is counted as part of another.

Jacinta35:31

Okay. So you like double counting some of the hydrogen signal from several different galaxies together. And so you're kind of adding up too much. Is that right?

Shilpa35:40

Yes, exactly. And with our limited redshift information and with the limited resolution of MeerKAT, we weren't able to model this any further.

Jacinta35:51

Okay. Yes. Well, I know all about the confusion of confusion. When I was using... when I wrote my papers on stacking, this was back in 2000 and, what year was my paper? 2013.

Shilpa36:04

13.

Jacinta36:05

Okay. I'm glad one of us knows. Okay. Back when I wrote my paper in 2013, MeerKAT didn't exist as captain exist. These other telescopes didn't yet exist.

So I was using the Parkes radio telescope in New South Wales. And that has a much, much, much worse resolution than MeerKAT. And so almost all of my galaxies were confused together. And so I was double counting hydrogen all over the place. And one of my papers, my second paper didn't actually get to be published because there was so much confusion. We couldn't even figure out what was going on.

So I don't know why I'm talking about my own research so much right now. Apologies for that. But just to say that like this stuff is actually really hard. So just round of applause for doing this in your Master's. It's really amazing stuff.

Shilpa36:53

Thank you.

Jacinta36:55

And you also mentioned to me earlier that you work on something called gravitational lensing. We spoke earlier to Tariq Blecher, who I think is one of your collaborators, one of your colleagues. And do you work on something to do with that as well?

Shilpa37:10

Yes. So I'm sure Tariq explained very nicely, but galaxy clusters are particularly great for observing the gravitational lensing phenomenon. And we can observe very distant galaxies that are behind these clusters because their light is magnified by the clusters because they're so massive.

And through this, we were hoping to detect even more distant HI. Unfortunately, we didn't make any detections, but yeah, hopefully that will be done with future MeerKAT observations.

Jacinta37:48

Okay.

Shilpa37:48

Yeah. I just would like to advertise maybe quickly that there's another paper coming on the HI stacking and the gravitational lensing search that's been submitted to (MNRAS). So yeah, you'll hopefully see it on the arXiv.

Jacinta38:02

All right. Watch this space. Good luck with that. And so now you have moved onto your PhD in Germany. Are you working on something similar?

Shilpa38:13

No, I'm doing something completely different. I'm still going to be using MeerKAT, but I'm not looking at HI, neutral hydrogen, at all. I'm going to be working with magnetism and polarization.

Jacinta38:27

Magnetism! Oh...

Shilpa38:28

Yeah.

Jacinta38:29

Cool. Tell us more about that.

Shilpa38:32

Mm, maybe in a year, once I've actually gotten stuck into my work.

Jacinta38:37

Okay. Okay. Fair enough.

Shilpa38:38

Yeah, at the moment, everything is very new. I only really started about a week ago.

Jacinta38:44

Okay. But, so you'll be working on the fact that galaxies have magnetic fields?

Shilpa38:49

Yes. So, the data that I'm working with is observed as part of the Max Planck Galactic Plane survey. Which is the MeerKAT survey of the galactic plane, and a little bit above, and a little bit below.

And what I'll be looking at is the extragalactic sources that are in the background of the galactic plane and characterizing the polarization and magnetic field properties.

Jacinta39:16

Well, thank you so much for joining us today, Shilpa, and explaining all of your exciting work. Do you have any final messages for listeners?

Shilpa39:23

Yes. I mean, for any people out there that are interested in science and interested in astronomy, I mean, this has been such a great experience. There's been so many opportunities to work with as part of like large international collaborations to go overseas. To live overseas and yeah, I highly recommend it. With MeerKAT, there have been such exciting results. And I think we can only look forward to what the SKA will bring. So yeah, for everyone out there, that's interested in astronomy, particularly women of colour, don't be shy. Yeah. Jump in. It's a great career choice.

Jacinta40:02

Absolutely. And where can listeners find you online if they want to follow you?

Shilpa40:07

I'm not too big on social media, but you can follow me on Instagram. That's helloshilpa, H E L L O S H I L P A. Or if you have any questions about my work, you can contact me on email: shiplaranchod@gmail.com.

Jacinta40:25

Great. And we'll put those details on our show notes on the website. Thank you so much Shilpa once again for joining us and a huge good luck for your PhD. I'm sure you'll be fantastic and do keep in touch and let us know how you're going.

Shilpa40:38

Thank you very much.

Jacinta40:47

Yeah. So, Tariq and Shilpa both talking about how amazing MeerKAT is which, you know, we sing its praise as every single episode. And this is, of course, the radio telescope in the Karoo region in South Africa, and just how powerful and sensitive it is, and the fact that... again, you could just turn it on and then just see this group of galaxies there that no one had ever noticed before. Yeah. It's just testament to how powerful this thing is, and really exciting that Shilpa could make this kind of a discovery in her Master's.

Dan41:16

Yeah, for sure. I mean for students to have access to this MeerKAT data is incredible. And then for Shilpa to get the recognition