The Supermassive Podcast - 28: How to Kill a Galaxy

Episode Date: April 30, 2022

How does a galaxy evolve and die? Izzie and Dr Becky find out with help from Tim Davis at Cardiff University. Plus, Dr Robert Massey from the Royal Astronomical Society answers your questions and explains how to spot galaxies in the night sky. Thank you to Brilliant for sponsoring this episode.  Head to brilliant.org/supermassive to start free courses in maths, science, and computer science. The first 200 subscribers will get 20% off. The link below has more information on the RAS Public Talk with Astronomer Royal Martin Rees, Space exploration and the search for extraterrestrial life - with humans and with robots. https://ras.ac.uk/events-and-meetings/ras-public-lectures-and-events/space-exploration-and-search-extraterrestrial A Year In Space by The Supermassive Team will be out on Oct 13th 2022. Pre-order here: https://www.amazon.co.uk/Year-Space-Supermassive-Podcast-Astronomical/dp/1472299507/ref=sr_1_3?crid=1O3PS3QUCPP2A&keywords=the+year+in+space&qid=1650904438&s=books&sprefix=the+year+in+space%2Cstripbooks%2C94&sr=1-3 The Supermassive Podcast is a Boffin Media Production by Izzie Clarke and Richard Hollingham. 

We are thrilled to say that Brilliant is sponsoring this episode of the Supermassive Podcast. Now, there's no denying that podcasts are a great tool for learning, but so are videos. You know, I love a video anyway. Brilliant is an interactive STEM learning platform with a huge range of courses across science, maths and computer science that get you to learn by doing. They guide you through these interactive, engaging, hands-on courses so that you really get to grips with the concept yeah and you really have to see them the visuals are incredible i know from my way of learning that's just what i need to make sure i really get a grasp on any topic and they're a great way to just get an overview and learn the basics of lots of different topics

and ultimately it's fun yeah like i found it quite fun to work through some of the machine learning ones. You know, we use machine learning all the time in astrophysics and I'm sort of forget the basics. Sometimes you talk about something so much and you just learn it and you're like, oh, actually, how does this specific bit work? And sometimes it's nice to go back through and remind yourself of that as well.

Yeah, and I really need to get stuck in onto that new everyday math course that's gone up this month as well. Yeah, I think we all could do with pushing up on our math skills, right? And also, if you get stuck, Izzy Brilliant provides these in-depth explanations to break down the material for you even more. Yeah, probably going to need that. So head to brilliant.org slash supermassive to get started for free. And the first 200 subscribers will get 20% off.

In the end, it all comes down to one thing, and that's, do they have gas left in these galaxies? Maybe this question from Twitter is just going to cause drama. Ooh, astrophysics drama. Sometimes you even see gas and stars moving differently in galaxies. Hello, and welcome to the Supermassive podcast from the Royal Astronomical Society with me, science analyst Izzy Clark and astrophysicist Dr Becky Smith-Ess. This month we're continuing our exploration of galaxies. Last month we covered how they form and this time it's all about their evolution and their eventual death. So for anyone that didn't hear part one last month, I mean,

where have you been? We're not offended, it's fine um so i'll give you my brief guide to building galaxies i'm excited for this okay number one it's all about getting gas into one place number two galaxies come in different shapes and sizes and three according to becky the roundish blobish galaxies are boring and i think that covers everything yeah definitely just about I think those main two types spiral shapes and the the blobish ones they're actually called elliptical galaxies I feel like I should say that right and they formed through mergers of galaxies as well which is kind of one of my favorite things especially when you see those amazing Hubble

images of them merging together that's really cool oh we love it and if you don't know Becky is our galaxy expert so Becky was going to be taking us through galaxy evolution later and I also spoke with Dr Tim Davies from Cardiff University about the death of galaxies but as usual Dr Robert Massey the deputy director of the Royal Astronomical Society is here too so Robert if we look at our galaxy, the Milky Way, that is on track to merge with Andromeda, our closest neighbouring large galaxy. So does that count as death? And if so, Secret P on Twitter wants to know, how horrible could that be? Absolutely awful, truly horrible. No, Secret P, I am joking, I i promise the two galaxies are going to

merge in about four and a half billion years time so although the sun will sort of still be around and somewhat warmer the earth will actually probably be uninhabitable by then so and even if we've moved on to another star at that point the individual stars in each galaxy you have to imagine these vast collections of stars that make up galaxies. They're ridiculously far apart, so they're very, very unlikely to collide. So you might see things like a little burst of forming stars if the hydrogen gas in each galaxy combines together, and some stars might get thrown out in the collision. But even if the sun was one of those, and as I mentioned, we're probably not going to be living around the sun by that point anyway, they'd still hold on to their planet. So i think there are bigger things to worry about i'm i'm happy to say it's not something that troubles me too much

i also have to do a little plug at this point as well which are the the editorial team looking at me but um i should say in a quick switch back to a couple of episodes ago um we've got a fantastic lecture that relates to book club where martin reese the co-author, is giving a talk at the RAS online on the 17th of May where he's talking about the end of astronauts. And also, if we send robots out in the solar system, will they find life? So that is a quick plug. Have a look at our website to catch up with that. And I should also mention, to go with the theme of the presenter on this, another podcast,ffins has an interview with martin reese and his co-author about the end of astronauts so either listen to space boffins or listen to martin reese talking about it in his lecture or better still both i love the segue there robert like you

didn't even try to take away that's two things together how can you connect the collision of the end of a galaxy what about the end of astronauts? That's very true. That's a nicer segue than I should have imagined something. But it did seem a little discreet. Lean in. I say lean in. Lean in a little bit.

Yeah, lean in all the way. It's Robert. Anyway, we'll catch up with you later in the show to take on some list of questions. Okay, Becky, are you ready for this? I'm so ready. Let's get into galaxy evolution. As we said it all sort of begins with all this gas and this dark matter is pulled together to form a young galaxy so what happens after that initial formation? Yeah so I mean the dark matter essentially helps the gas clump together so that it becomes dense enough that you can form stars

and they seem to form essentially in these big clumps so i guess we call them star clusters today but really we call them clumpy galaxies because you can see sort of glowing gas and then embedded in it is literally just these huge big blobs of star formation the universe was much denser back then so then you had a lot more merger of these clumpy galaxies too. But as the universe expanded, galaxies got further away from each other. They're essentially, everything was able to settle down from that initial sort of like, it's just forming in clumps wherever they could, you know, be dense enough. And eventually because of these mergers as well, that sort of set a lot of things spinning, the gas then sort of

spreads out and settles down into a disc like a flat frisbee shape right and then you start forming stars from there and this is we think how you start to get these beautiful spiral galaxies as well from that point then the spiral galaxies merge together and give you the blobish things as well so there's lots of different processes that can give you all of the things we see today. But essentially, that's how you get from a clump of dark matter and gas in the early universe to something we might recognize and call a galaxy. Okay. And so what are the processes that affect galaxies over their lifetime? Yeah, I mean, it depends on whether a galaxy is left completely alone and isolated

or whether they're in a big group or cluster of galaxies and that's not that rare because as the universe was much denser in the past there were areas that were denser more galaxies formed there and then as the universes expand if there was any gaps they've sort of expanded into voids and left some galaxies isolated as well and so you can imagine i mean tim's going to talk more about this later, I think. But if you imagine, if you have a galaxy in a group, you're going to have much more things like mergers, interactions, quite horrific. It's essentially sort of like removing gas and forming stars and what forms stars and what stops stars from forming and what changes galaxy shapes, all of those kind of things. Essentially, all these processes can massively disrupt a galaxy on whatever path it's on. can massively disrupt a galaxy on whatever path it's on.

But if you have an isolated galaxy, it's really in charge of its own destiny, if you want to put it like that, right? It's completely isolated. What happens to that galaxy depends on what that galaxy does internally. So where does it send its gas? Does it funnel it towards the middle through spiral arms or through what we call a bar structure as well? You have this sort of like long linear structure of stars that moves around the galaxy that and when we picture spiral arms and bars

moving we often picture them as the stars moving but it's not the case it's more like a density wave like what you see in traffic on a road or sort of like a mexican wave moving through the galaxy as stuff clumps to give you these structures um but essentially all the processes that could affect it throughout that lifetime whether it's something that's happening internal or something that's happening external, it's are you giving it more gas to form stars or are you taking that gas away? And essentially the amount of gas a galaxy has is what you can use to predict, well, what's going to happen to a galaxy in the future. Okay, okay. And so that's quite interesting, though, that going back to say those bars or those spirals, they're quite dynamic then what you're saying it's not just like it's not a static thing no and i think we when we picture them we think of them as quite static structures and they're just sort of very slowly spinning as these big static structures but actually there's so much movement within those

structures themselves and sometimes you even see gas and stars moving differently in galaxies you know we think it was stars form from gas so they should be the same but if you have something like themselves and sometimes you even see gas and stars moving differently in galaxies you know we think it was stars form from gas so they should be the same but if you have something like a big merger you could end up with what's called counter-rotating disks so the stars are going like clockwise and the gas is going anti-clockwise it's the weirdest weirdest thing i was gonna say that i find that really hard to compute yeah wait how how just how does that happen well how about this one as well sometimes you can have a counter-rotating core so that the I was going to say that I find that really hard to compute. Yeah. Wait, how? How? How does that happen? Well, how about this one as well?

Sometimes you can have a counter-rotating core so that the core in the middle, like the yolk of the galaxy, is rotating one way and then the gas in the disc on the outside, like the egg white of the fried egg, rotates the other way. This is so complicated. But now I'm going to ask, why does that happen though?

Or is this going to be one of those, we don't know, this is what we're trying to find out we think it's mergers we think it's the only process that could do something like that maybe a minor merger for something that changes the middle section where you have something that's like 10 of a galaxy's mass comes in and drops in and you know changes stuff up and a major merger to make those completely counter-rotating stars going one way gas disk going the other way. Yeah, okay. And so when it comes to size,

how big can a galaxy grow, essentially? That is a great, great question. I mean, if there's gas there and it's cold enough, so you need coal gas to form stars, right? Because you have hot gas, the molecules in the gas have so much energy that they can essentially resist gravity trying to clump them together and they'll never get dense enough for nuclear fusion to to kick in essentially yeah so you need so you need gas and you need coal gas to keep forming stars obviously galaxies also merge together and that makes them grow bigger as well

so you'd think essentially that time was the only thing that would limit how big a galaxy can grow as long as there's gas there and if it keeps merging together it could just go bigger and bigger and bigger and while that does play some part in it in terms of what's the maximum mass we can see today in a galaxy it's not the whole story so the way we actually sort of probe this and test this is actually we sort of go out and do a big survey of the universe and the galaxies we can see and we say essentially like count how many small galaxies do you see how many big galaxies do you see and everything in between and you end up with this distribution really of of the galaxies that you see we call it the luminosity function because actually we're not measuring mass we're measuring

brightness but if a galaxy is brighter it has more stars therefore it's more massive and so the two are correlated so and you can also do the exact same thing in a simulation as well you can go out in a simulation and count how many small galaxies to make how many big galaxies do you make and if you've got all those physics in there you know that they should match and you've got everything right and in sort of like the 1990s people started realizing actually they don't match there's something we were missing in simulations and slowly but surely people realized that they were essentially making too many massive galaxies. They were growing too big in simulations. And there was obviously something

that was missing in the simulations. And essentially people played around with it for a while, adding different things that they thought they'd missed. And it turns out that the Sims match the observations when you include supermassive black hole burps. Okay. For listeners, I was just like cheering for the mention of a super massive black hole and and this is something that we did allude to last month we find them at the center of every galaxy so how do they influence galaxy evolution

yeah i mean so it might think weird there was a burp from a super massive black hole right and how does it have an effect on how big a galaxy can grow? So actually growing a supermassive black hole, sending gas towards the middle of the galaxy to where the black hole is, not all material actually makes it into the black hole. They're not just endless Hoovers. What essentially happens is that the material ends up on what we call an accretion disk. So because the black hole is spinning,

it pulls this big cloud clump of material in, hydrogen gas, and it pulls it into a flat disc. You know, say you set a bowl of pizza dough above your head spinning and it flattens out into a nice pizza base, right? Same thing that happens to this gas around a black hole. And it gets heated up, you know,

because it's traveling so fast, has more energy, heats up and starts to glow with radiation, x-ray radiation, ultraviolet, everything, all the bad things, right? And so that radiation actually then can exert a pressure outwards. So as the photons of light hit into other things, they exert a pressure. And so they exert a pressure on the material that's still trying to come in to feed the black hole. And so as that more and more material enters the accretion disk, that pressure builds and builds and builds until you can reach a certain point where the material is actually turned around and ejected back into the galaxy again.

And so that can happen, so we call it outflow. It can also happen as a jet as well. You might have seen images of these huge jets from black holes too. And so what you end up there is because you've got essentially plasma in your accretion disk you've got charged particles the electrons are no longer in the atoms anymore and so you've got moving charged particles which means you've got a magnetic field and so those charged particles can be funneled by a magnetic field up into one of these jets that extend you know like huge distances and so these are the things we think can affect the galaxy they essentially drop heat into the galaxy or they can just pick up material and move it so as i said you either need gas there and you need cold gas heated up the gas no stars forming there's no gas there no stars forming and so this is why we think that they can affect galaxy evolution but also what's

the biggest galaxy you can get because essentially like the the growth of the supermassive black hole is like putting the brakes on and And so we think that the maximum, like top end of galaxy's masses is around about 10 trillion times the mass of the sun. Wow. How? Okay. Yeah, but it would be bigger if it wasn't for black holes.

Okay, okay. And so then I'm thinking, then what role does dark matter play in all of this as well? Yeah, I mean, well, it does kickstart the formation. So you've got to be thankful, I guess, for the dark matter for that, because if it wasn't there, literally there's not enough matter to clump together to get it dense enough to form stars and galaxies and stuff like that.

Ultimately, it helps to keep the galaxy bound and essentially increases what we call the gravitational potential of that area in space. And if you have galaxies clustered together as well it keeps those bound because it adds extra mass into the area and helps them clump together in what we call this halo of dark matter you could also ask well does dark matter end up growing the black hole which is a fun question to ask i actually had this sort of like what i what I cast as the only eureka moment I had during my PhD,

which is when I was writing up and was probably half crazed at that point. And I was like, what if? I had to control the black hole and I ended up like racing down the stairs to speak to my simulation friend. And they were like, eh, yeah, we've had this thought, probably not.

Essentially, like if you have it on a direct trajectory, it will end up in there. But otherwise that accretion disk process, it doesn't actually happen with dark matter um essentially like if you have it on a direct trajectory it will end up in there but otherwise that accretion disk process it doesn't actually happen with dark matter because we think it's what's called collisionless that you don't get these collisions between particles that help them lose energy so that they actually spiral into the black hole and don't just orbit the black hole but we roughly think there's probably about 10 percent of the mass in supermassive black holes could come from dark matter like that's the maximum kind of amount we're thinking but that's all like speculation completely it's like simulations of a thing that

we don't really know that much about um so okay okay well speculate away it sounds interesting so maybe this question from twitter is just going to cause drama which is more influential dark matter or a black hole astrophysics drama my favorite drama um that is a really good question actually and i i think it's one that's quite hard to answer because at the beginning i'd say it's dark matter yeah massively so you know it helps them form especially during the formation the the black hole in the center it might not even exist yet we don't know whether the galaxies form around the black hole or if the black hole forms from one of the first stars going supernova. So in the early stages, I'd definitely say dark matter.

But then I think in the late stages, in the past six billion years or so, something like that, I'd definitely say supermassive black holes, especially if we're coming at it from a simulation perspective. But this is the interesting thing. So in simulations, people have been like, it has to be this process of black hole burps these outflows jets stopping galaxies from growing too big that's what's regulating like the evolution of galaxies but we don't have like concrete evidence of it happening

observationally like across the whole population and that could be because these burps don't last very long or the time scales for the stars to then die after the burp happens is too long. So you can't really find that smoking gun. And this is essentially what I'm working on is trying to find that smoking gun of, yes, it's the black hole that's responsible so that we can actually agree with the simulations people

who are like utterly convinced that this has to be what is happening. And the rest of us are like, oh, hang on a minute. Wait a sec. Oh, amazing. Well, thank you, Becky. That has answered all of my questions.

All of them? Some of them. Okay, so we've covered the birth and evolution of galaxies, but there is no denying that death is a key part of that evolution. But what does it actually mean for a galaxy to die? It's a question that I put to Tim Davies from Cardiff University. When we talk about alive and dead galaxies, it's sort of a misnomer in a way, because there's no way a galaxy itself can actually die. But what we typically mean by that is, is it still forming stars? Is it still able to increase itself to keep going

as a system? And so the main way we know that is we look at, for instance, the colors of galaxies. So very massive stars are typically very blue. They produce a lot of light in the ultraviolet, for instance. Whereas older stellar populations, all those big stars have died and you're left just with the smaller stars that live longer and those emit more light in the red. So you can have a look at the colours of galaxies, for instance, and that will tell you about how many stars are forming in those galaxies and allows us to pinpoint those red galaxies that are dying. Yeah, okay.

So what is that process? How is it that a galaxy can actually die and stop forming stars and end that process? So that's a big question and a big thing that we look at in galaxy evolution. So there's a whole variety of ways. So in the end, it all comes down to one thing and that's, do they have gas left in these galaxies? Because gas is the fuel from which new stars form. And so you have to look at processes that are going to affect that gas. And so one of the places that that gas can be really easily affected is if these galaxies are in a dense environment. They're in a galaxy cluster, for instance, with lots of other galaxies. Okay. And so does that mean, you know, if you've got

more galaxies around you, it's a bigger cluster that they're all having an impact on each other and therefore it can, what, speed up that process or just impact it in different ways? Yeah. So it provides different pathways to what we call it quenching, you know, putting out the fire in galaxies essentially. So the first one of those, we call it ram pressure stripping. And that's a, it's quite a familiar process, actually, in a way, because it's very similar to what happens if you're on Earth and you are driving in your car, you, as a passenger, ideally, you roll down the window, stick your head out, and the wind blows all your hair back.

And that's exactly what's happening in some of these galaxies because these galaxy clusters have a big atmosphere of hot gas, sort of at a million degrees. And as the galaxy moves through that plasma, it's exactly the same thing. They feel this headwind blowing against them and pushing the gas out of the galaxy. So you can actually just completely strip away all that material from the galaxy and stop it forming stars directly. That's one of the first ways we can do it.

And just quickly, is that what's the picture that's behind you? Yes, it is actually. Yeah. So behind you, Tim, there's this picture of a galaxy. When I first looked at it, it looks like a jellyfish. You have that sort of mass at the top and then these long blue strains which look a bit like um the sort of is it tentacles of jellyfish i mean i'm a physicist don't quit don't quote me on what those are but you you can see how all of the matter within it is just being sort of almost blasted away from that core yeah that's

exactly right we even call these things jellyfish galaxies. Amazing, of course you do. They're having that gas stripped away by this interaction with the plasma in their cluster. Okay, so that's one way. You said there's quite a lot of different ways to do this. So what's the next one? So another way you can do this

is if you have tidal effect. So you get a galaxy and because you're in a cluster, you've got lots of other galaxies around that if one of them happens to come close, it's very unlikely they'll merge in a cluster because they're all moving too fast relative to one another. But what they will do is have a tidal effect, just like the tides on Earth. They're caused mostly by the moon and a little bit by the sun and their gravity on ours. And exactly the same thing can happen in galaxies. You can get a tidal effect that rips the gas out of that galaxy again,

stopping it forming stars in the system that's affected. And so that sounds quite brutal. I mean, how quickly, over how much time does that take? Yeah, so in galaxy evolution terms, it's fairly quick, but we're still talking about, you know, hundreds of millions of years. Yeah, okay. A billion years. Okay, okay. Just from perspective, good to know. And then I've heard that there's like a third one.

This is kind of related to the first one, actually. So galaxies, if they're sitting in isolation, they have a halo of hot gas around them. We call it the circumgalactic medium or the hot halo. And cooling from that can help fuel a galaxy and keep it forming stars. So if you can remove that hot halo from the galaxy, you remove its reservoir that it can get future fuel from. And so what you can do is you can ram pressure strip that gas, leaving the rest of the gas in the galaxy alone, if you want, and you'll then prevent it from swarming stars on a longer timescale. So that takes longer, you know, several billion years then

to start to affect the star formation. But it will eventually, and that will eventually lead to the death of the galaxy. Yeah, okay. How is it that we know all of these different methods for galaxies to die? How are you actually studying these? So there are a variety of ways to do it. You can look at galaxy populations, so red versus blue, for instance. That's how this was done to start with. We just looked at the colours of galaxies and we saw, for instance, that in dense environments, there are many more red galaxies than blue galaxies.

And that told us, OK, something about environment is important. Then we can go further and look at what that might be. What we do a lot nowadays is look at the gas in galaxies. As I said, it's really the crucial component in all of this because it's that fuel for future star formation. And so we can look at that through optical emission lines, for instance. So certain elements shine at different specific frequencies, and you can look at those and see how that material is being pulled out of galaxies. You can look at the emission from the colder materials as well, so the molecular or the atomic gas,

this cooler material that's most directly related to the forming of stars. And that's typically done with radio and millimeter telescopes like the VLA or the ALMA telescope in Chile that some of your listeners may be familiar with. Yeah. And so for you, what's next in the field? What are some of the big questions that you're hoping to find out or just want to better understand? I think at the moment we have a pretty good idea

of these different pathways we've chatted about, the ways that galaxies can die. But what we are not sure about is how important they all are relative to each other. So what dominates in a given situation? And so what we need to do to work that out is to use all of the new observational facilities that we've got, all these new techniques, and really get large samples of these galaxies. Because we have to do this statistically. Unfortunately, we can't follow individual galaxies in real time. I'd love to be able to do that, but you can't. You'd be there a long time. So that's going to be really exciting. And we can combine that with theoretical methods as well. So in a simulation, you can follow a galaxy, you know,

in real time, or as the computer chunks away at it. And as those get better and better and get more and more realistic, we can also study it there and compare that with what we're seeing in the real universe. So that's how we're going to make progress here is by combining that theory with the observations. Thank you to Tim Davies from Cardiff university so clearly there are a few ways that clustered galaxies can die becky but can you expound on lone galaxies what is that process and how important is a black hole in all of that yeah for lone galaxies it's a bit different as i was saying before essentially they're left through their own devices this is something that's quite understudied actually and it's how does lone galaxies growth of black holes and the processes that affect them all tie together.

Because we think that if you have a spiral arm or a bar, it can funnel gas to the center and dumps it in the middle. Where do you think, okay, well, maybe stars will form in the middle, but in that process, it gets heated up. And so again, it's sort of useless for stars forming. So that in some ways could kill a galaxy or at least kill the outskirts of a galaxy. But then of course, if you dump it in the middle, it could end up in the black hole. The black hole could then have an outflow,

which then also kills the galaxy as well from the inside out. We call it secular evolution. Almost like describing very internal, like tortoise slow and steady wins the race kind of evolution is what it means like no mergers basically and it's sort of very much an interplay of what the galaxy does and then what happens to its black hole in the center i kind of like to say the galaxy shoots

itself in the foot because it fed its own black hole okay then does the shape of a galaxy have any influence on its death? Yeah, 100%. I mean, we actually see, so when we look at galaxies and we look at what colours they are and we see that red galaxies, you know, tell us that they're dead and blue galaxies are still soffling, we know that 70% of the red galaxies are blobish shaped and 30% are beautiful, pristine spirals.

And so we understand that there is some link there between shape and and how things die and so yeah the the spiral galaxies that have bars or don't have bars for example these long thin structures in the middle they actually do have different proportions of how many of those are dead as well the barred galaxies have a higher proportion but again actually proving that it's the bar doing the killing, you know, doing the murder is very, very difficult. I actually have this really cool diagram that I show at the beginning of talks where you have all of the different processes that could kill a galaxy on like a graph where it's split into four quadrants, like internal, external, slow, fast process. And I put them all up and I'm like,

yeah, we all agree probably that they're all in these four quadrants. But then what we've not agreed on is how all of these interlink. And then I add all these arrows that essentially show that like one leads to the other and one leads to the other. And it's such a mess. It's so difficult to piece out. But one thing we do know is that different processes affect spiral galaxies and blobs. Okay. Okay. So it's obviously hugely complex lots of things

come into play so is there a relationship between the shape of a galaxy and then its black hole does that have an influence as well on its death yeah so actually we see more massive black holes in the blob galaxy in the electrical galaxies i really should call them by their official name rather than blob so supermassive black holes in blob galaxies are much bigger than the ones in spiral galaxies right we think that they've grown more massive probably through the fact that you've had a galaxy merger and you've also merged the black holes and probably there's been more gas tumbled towards the center because they're more massive they then can have more of an impact in terms of the amount of energy that they can then inject into the galaxy around them too. We then see these blob galaxies more often in big groups and clusters as well.

And so what you end up with is sort of this, a bit of a dichotomy. We have these two different types of what we call black hole feedback. This black hole burp scenario where you can, you know, kill off a galaxy by growing a black hole. this this black hole burp scenario where you can you know kill off a galaxy by growing a black hole and you end up in clusters probably seeing these jets more often than not which usually perpendicular to the galaxy right they come out top and bottom they don't really go through the galaxy but what they do hit is um the gas that permeates between galaxies in the cluster it's very diffuse but if you heat it up and then you've got a galaxy moving through that gas, essentially you can then heat the gas in the galaxy from the cluster gas that's sort of surrounding it. And so it's sort of like a second hand sort of way of killing them in a way. Whereas

you then have maybe in a, in a, you know, an isolated spiral galaxy, you instead might have a much smaller black hole that you might not get one of these huge jets from but you might get what i what i call like an outflow or a wind before where you just sort of like turn around the material and then you you heat up the gas surrounding it and it's much more direct but it's possibly not necessarily as like like instantaneously effective um as well so there's lots of people trying to piece out what's going on where how effective each type of different feedback is as well again lots of work of people trying to piece out what's going on where how effective each type of different feedback is as well again lots of work in simulations but also in observations to try and uh pull these out as well so i look at the isolated galaxies trying to just figure out

without all of the the mess that mergers bring what actually can happen um on its own so we can actually unpiece that which might be going on in all of these other galaxies as well that have had a merger like i said tortoise slow and steady kind of way underlying but we only ever see the effects of the merger so there's lots going on but you can kind of think about it as yeah two different types of feedback for the different galaxies this is the supermassive podcast from the royal Astronomical Society with me, astrophysicist Dr Becky Smeathers and with science journalist Izzy Clark. Last month we mentioned some very exciting news. The multi-award winning team behind the podcast are bringing out a space annual called

The Year in Space and we have a date for it. It's going to be out on the 13th of October, 2022. And so you can now officially pre-order it. Yay, so exciting. I love pre-ordering books. There is a link in the episode notes if you would like to as well. We're going to be talking about everything from launching the James Webb Space Telescope

to exploring Mars. And the year in space will take you through the most exciting space news from the past 12 months and also is going to be looking to the year ahead there is a lot to squeeze in there and I can confirm I've really enjoyed writing about Webb and we've even found a home for all of your questions that we haven't had time to include on this very podcast so thank goodness for that. Yeah, I'm so, so excited. And also, that's not all we've got to celebrate, is it, Izzy? Are you now an award-winning science journalist?

That is true. Yes, I am. Yeah, no, it's really exciting. I won something called a Webby Award, which is basically an award for the best content on the internet, is how they so cool yeah so i i don't mean to betray everyone but i make other podcasts so my one uh called ocean matters won that one i was up for another one and we lost to a very very dear cause close

to our heart the uh nasa's podcast about the James Webb launch. So if we're going to lose to anyone, I understand. I understand. It's okay. Yeah. Don't worry. I didn't have a deciding vote. Oh, okay.

Yeah, yeah. There's you voting like, James Webb, vote for me. Who do I vote for? No, it's fine. I'm so proud of you. It's amazing. Congratulations. Like a Webby is so cool.

Yeah, it was very unexpected, but very fun. Right. So I think we should get back to galaxies and onto our listener questions. Robert, can I bring you back into this chat? We have this question from Helen McMillan, which I hope you can help with. She asks, what is the lifespan of a galaxy? And does everything just cool off when a galaxy dies? Yeah, so most galaxies, Helen, are really quite old and including our own they form not long in other words within the first

couple of billion years after the big bang so they're really old you know more than 10 billion years old and defining death for one is a sort of movable feature it depends how you describe it but i suppose most of us would think about when they're kind of going dark. So the stars are in the galaxy exhausting their fuel. There isn't enough hydrogen left to make new stars. And so then you're talking about the existing stars slowly consuming the fuel they've got and then reaching the end of their lives. So the longest lived stars are the small stars, red dwarfs. They've got a lot less mass than the sun.

They live a lot longer than the sun. They're cooler than the sun. and they consume their fuel very slowly and some of those could take as long as a trillion years to reach the end of their lives so that's the kind of time scale you're looking at many many times the the lifespan of the sun and some of the some of these events you know the stars will cool down some of these events will eject hot gas out of galaxies but eventually like it or not if you wait long enough, hundreds of trillions of years, longer and longer and longer, then eventually things will cool down as the universe gets bigger

and galaxies cool down one way or another. Okay, thank you, Robert. And Becky, here's one from Delta Tesseract on Twitter. They ask, why do the outer arms of the Milky Way spin around the core faster than expected? Okay, so when we look at the Milky Way, or any galaxy really, we find that it is brighter in the centre and fainter on the outskirts, right? So it means, because, you know, if it's brighter there, there's more stars, it means there's more mass on the inside of a galaxy too, right? And if we think about how the planets orbit in the solar system,

Mercury, much further into the sun, is moving much faster than Neptune on the edge of the solar system. And that's essentially gravity and Kepler's laws of motions and everything we would expect when it comes to gravity. That's how we'd expect it to behave

if all the stuff is concentrated in the center. So when we look at the Milky Way and we don't see the same thing as the solar system, instead of a drop-off, we actually see it flattening out towards the edges and actually it being the fastest part of the galaxy that's rotating at the edges. It sort of raises a little bit of a red flag to us.

And it means either something's wrong with gravity or where we think the mass is is not where the mass actually is in the galaxy have you combined that with the fact that we've also got other evidence for more mass being somewhere we don't think it should be in in galaxy clusters and also simulating how galaxies form you need more mass there to make them come together which is this idea of dark matter right in order to get the universe's structure to emerge you know we also then come to this conclusion well then there must be more matter on the outskirts of the milky way than on the inside despite the fact the inside is brightest so this is how we come to this idea of of it being unexpected of there being dark matter that's on the outskirts

of the galaxy as well which makes it rotate faster on those scales so the idea of dark matter is the leading theory people are instead also looking at modified gravity theories too you know that idea hasn't been put to bed it's just that most of the places we tested general relativity which is our best theory of gravity that we currently have it's come up trumps everywhere and we have all this other evidence as well for dark matter and so that is the leading theory that we have at the moment okay i think we should get that on t-shirts just printed cause dark matter and robert i really like this question from timo hines and he asks are there any known events that could stop star formation not only in a single galaxy but in an entire galaxy cluster? Well Timo all at once that seems

pretty hard you know you can have these interactions across clusters and so on that Becky was describing but to do that sort of thing to the whole cluster at once is really tough and when you look at the kind of clusters that are there you tend to see a mixture of elliptical galaxies or it isn't much dull blobs sorry becky as uh the blob galaxies where star formation isn't really happening anymore and galaxies like ours where it is so you tend to see that sort of blend of them so it's hard to imagine that all shutting down at once uh and yeah i'm just thinking some of the things actually you can see in the sky show those characteristics too. I mean, obviously, yes, you can get processes that shut down star formation individual galaxies relatively quickly. The collisions can strip the gas out, black holes, as we've heard before, and so on.

But to do it across a whole cluster would be really quite a big deal. Some of them are quite big as well. So it's hard to imagine all that happening at once. Amazing. Thanks, everyone. quite a big deal there could be some of them are quite big as well so it's it's hard to imagine all that happening at once amazing thanks everyone and if you're listening and you've got a question for us then do send it in you can email podcast at ras.ac.uk or pile in on our monthly thread on twitter it's at royal astrosoc so i think that is almost everything for this month but robert they can be harder to find, depending on where people are in the country or even in the world. But what would be your top tips for looking for galaxies?

Well, I was thinking about this, and I think actually the best top tip for looking at a galaxy is to look at the inside view of our own, because the Milky Way, have you ever seen that in the summer or the autumn skies? In a dark sky, without the moon in the sky, away from light pollution, all of those things. It's absolutely stunning. And it does give you this wonderful impression of the inside view of the galaxy we live in with that wonderful arc of light going across the sky. But if you want to see other galaxies, you're basically going to need a telescope.

There are one or two that you can glimpse with your eye. In the southern hemisphere, actually, you can see the satellite galaxies that go around our own, the Magellanic clouds. But if you're up in the north, it's a little harder. But right now, in the spring sky for the northern hemisphere, the autumn in the southern hemisphere is not too bad. If you have a small telescope, you can look at a constellation Virgo. And if you're in the northern hemisphere, you follow the plows handle or the tail of the great bear down to Virgo. And you need star maps for this. And that's got a cluster of about perhaps 1,500 galaxies.

And you can see quite a lot of those with a small telescope or even a big pair of binoculars. Now, I should stress that all those fantastic images you see of galaxies with telescopes, even with amateur telescopes, are a lot than usually than the view you get with the eye what you tend to see is something that looks like a faint fuzzball but it's worth reflecting on the fact those faint fuzzballs have probably each got hundreds of billions of stars in them and in Virgo they're 50 million light years away and another site as well is actually there's a few galaxies in Ursa Major in the Great Bear the ridge around the plough itself and that this time of year in the northern hemisphere, that's almost overhead later in the evening. And there are three or four galaxies there, M81, M82 and M101.

And they're a bit brighter than the ones in Virgo, but they'll still mostly look like fuzzies. OK, so keep an eye out for fuzzies. And what else is there to look out for this month? Well, it's not bad. There's a fair amount going on at the moment, although we are getting to the point where the days are getting longer, so the opportunities for looking out at night are getting a bit shorter. But the very start of the month, at the beginning of May, is a good time to try and spot Mercury, if you've never seen that, in the evening sky, quite low in the west after sunset.

And again, it helps to have a map or one of those apps on your phones that you can check where it is to confirm it but if you try looking about 30 minutes after the sun goes down so if you're in the south of england around 9 p.m or so you should be able to spot it i actually took a picture with my smartphone a couple of days ago so it's not it's not too hard to find get a pair of binoculars maybe but obviously don't use them till after the sun sets because you don't want to look at the sun with a pair of binoculars maybe, but obviously don't use them until after the sun sets because you don't want to look at the sun with a pair of binoculars. And if you've got a small telescope, you should be able to see a phase as well because it moves between the sun and the Earth and it shows the same kind of phase as the moon does. And there are a couple of other things that are really nice to look forward to as well.

If you're up early on the 16th of May, as the small hours of the morning starting at about 2 30 in the morning so i'm not not sure about my podcaster co people here but you know that's not a time i'm usually up and about at my time don't go to bed exactly nothing good ever happens at 2 a.m so if you do this there's a total lunar eclipse visible when the full moon moves right into the shadow of the earth and it can turn a beautiful red color and it's best for you from the americas this time but in the uk the rest of western europe we can catch the start of it before sunrise so 2 30 onwards you should see the moon going into the lighter bit of the earth shadow over in the west and then it will get into the darker bit around 4 30 and then the eclipse moon will basically disappear as the sky brightens then

it'll set as the sun rises but you know it could be quite a nice event if you're patient enough and up and about that time of day and the sky is clear and all the usual usual caveats we have to add and then uh finally there's some a meteor shower as well the eto'o aquarid shower on the fourth to fifth of may or around them it's better in the southern hemisphere actually but if you but you do see some meteors here and if you get a dark sky maybe you'll see a few an hour just might be quite a nice thing to look out for and the final thing i want to say of course because we're reaching the end of ramadan at the beginning of may and i'd like to wish any muslims who are listening a happy eid al-fitr and if you

want to have a look for the new crescent moon which uh marked the beginning of each islamic month then the you'll be able to see that actually really well in the uk after the sunset on the 2nd of may and you know quite apart from whether you're celebrating or not it's a really beautiful sight you see this beautiful thin crescent with the the illuminated earth shine the bit that glows from light reflected off the earth as well so hopefully there'll be some really nice photos of that and do share them with us on twitter thanks so much robert and so i think that is it for this month and thank you very much to brilliant for sponsoring this episode we'll be back in may with an episode all about gas giants and becky i'm warning you as the producer of this show we have

to talk about all of them equally and you cannot just focus on sat. Well, I want to. No, fine. Fine, fine. I'm putting my foot down. Consider me a scolded child. Don't forget, everybody, to tweet us if you have any questions. It's at RoyalAstrosoc on Twitter or you can email podcast.ras.ac.uk and we'll try and cover them in a future episode.

Until then, though, happy stargazing.