 OK, I think we can start. So hello, everybody. My name is Roberto Lineros from the Instituto de Física Portugal in Valencia. And I will be the host of this webinar of the series of the Latin American Webinars of Physics. Today, we have a very interesting talk about the Smith Cloud and Dark Matter. So just to remind you that we are doing this transmission live thanks to Google Hangout. And don't forget to ask questions via the Q&A in Google Plus or via Twitter with the hashtag lowphysics. So if in case that you are not watching this transmission live in this moment, so you can follow us in, you can see, watch it again in our YouTube channel. And let me tell you that also we have a WordPress page in which we centralize all the information about the webinar. So you can go there and consult if you find other interesting webinars to watch. So the speaker today is Mark Polman. He's currently a postdoc in the Technician University of Munich in Germany. He received his PhD from the University of Hamburg. And much before, he followed the master and the bachelor of studies in the Universidad Simone Bolivar in Venezuela. So the title of his talk is Synchrotron Emission from Dark Matter in Galactic Subhalos, a look into the Smith Cloud. So Martin, if you can hear me and you can unmute yourself. I can start the call. How are you? Thank you very much for my invitation to this webinar. It's my first experience. Well, let's see how it goes. Yeah, I'm fine. How are you? I'm fine. So you are in Munich now, no? I'm in Munich in my office. Today was a sunny day, so it's a good day to be in a seminar. To be inside the office. Yeah, I'm inside the office. It's a bit of a anyway. So whenever you want, you can start your seminar and we're going to be listening to you. OK. Well, so now it's my turn to talk this series of Latin American seminars in physics. So my talk is going to be about the dark matter and searches for it using radio waves and a very interesting target, the Smith Cloud, which I'm going to explain to you about a few words of it. Before I start, I should also acknowledge my collaborators in Hamburg, Natasha Lacey, Robin Grumann, a student, Professor Günther Siegel, and Beggin, a collaborator, Michel T. Gatt. It's partly just a very small question. Can you go to full screen because we are watching today? Not full screen now. No, we can see all of your slides, let's say. OK, so probably then I should. Yeah, maybe if you can share the screen, share the full desktop, not only the window of the PDF. No, it's not a PDF. That's a point. Probably that's, yeah, maybe I export this. I make a PDF of it. And that will take a, OK, I did it on keynote. No, no, but when you share a screen, if you share the full desktop, not only the window, it should work the full screen. Yeah, yeah, yeah. OK. How do I do it? Yeah, let me try this one once more. Ah, OK, I see, I see. Now is it OK? Yeah, if you go to full screen, let's see. Now? No, we still watch the keynote window. You enter first screen. Still? Better, but we still see the stuff in the side. And now? No. No. Then, OK. So we have some technical problems. Yeah, but it's not a big problem. OK, yeah, because I probably, this is, ah, here, slight only, probably. Well, this is a bit better, right? Yeah, and now we can see much better. OK, well, and the cursor, I can use it. OK, so, yeah, I don't plan to speak two hours, so I summarized kind of my talking in these five parts, where I'm starting the introduction, but now I'm not co-ed somehow. I have to, OK, now it will work. OK, so the main motivation of this project and this paper is the motivation of many of my colleagues is we want to, at least, learn a lot about the nature of the dark matter. So there are several ways of doing this, and I'm going to focus on the indirect searches for dark matter, for the particular nature of dark matter. So as many of you know, so gamma rays are the golden channel that people have been interested in and indeed, so the Fermi-Lat collaboration, they put the strongest limits on the dark matter and the election cross-section. So I put some records with that, if you want to make a note. But, Martin, we can only see that your first slide, the introduction is in cloud and phenomenology. We are not watching if you, now you pass to the first. OK. I see. OK, now I know all the tricks to make this book. OK. OK, so the point is that there are other complementary channels where one can try to find a signal that you can interpret dark matter and, in particular, radio waves. And radio astronomy is a big field. And so my project or what I'm talking about today is going to be devoted to these signals. So this is, of course, the motivation of this talk. What I'm going to use a lot is that our galaxy or, in general, galaxies in the universe, have a rich substructure. And this manifests, this substructure manifests itself. When you look at the sky, you know that there are dwarf galaxies and some substructure. But these are usually the objects that people associate to the substructure, to the subhalos. But there are claims that there might be other kind of objects, the so-called high-velocity clouds, that can be also a manifestation of the substructure of the galactic halo. And as I will show in this talk, so there are simulations that support this idea called the references. In particular, I'm going to talk about one of these high-velocity clouds, or mainly the so-called Smith cloud. Now I go like this, and then I do this. And you see the slide number five, right? Yes. Very good. OK, so if you're not in the field, you might ask yourself, OK, what is the physical mechanism responsible for a radio signal associated to dark matter? Well, this mechanism has a name called synchrotron radiation, the same synchrotron radiation charge particles emitting in a collider, but a different frequency range. And well, in this talk, I'm going to talk to you about how we make predictions of this signal, in particular looking at the cloud, this Smith cloud. Yeah, and I flash you what we are going to show. We're going to explain, say, a few words about the spectrum of this signal, about the morphology, and taking these two predictions, we will set limits on the annihilation cross-section of the dark matter, and discuss a possible prospect for current and future radio experiments. So now I'm done with my introduction. I'm going to talk more about the cloud itself, what this cloud is. OK, now it goes like this, OK. So this is a comet-shaped high-velocity cloud, which you can see by the 21-centimeter signal, neutral hydrogen, and it's not going to go into details, but at the 21-centimeter, 1.4 gigahertz signals, you can actually, this is the way it was discovered, you can see this cloud. And nowadays, well, you know a bit about this cloud, you know that it's morphology and several constraints. You can already tell that in 30 millions of years, it will pass through the disk, through the galactic disk. It's actually not very far from it. In the next slide, you will see what I mean in a moment. So this is some artistic view of the orbit of the cloud. So as I said, in 30 millions of years, we'll pass through the galactic disk. But that means you can extrapolate backwards, and that means that 70 million years ago, it should have passed through it. And that raises several questions. And now, if that really happened, if the cloud survived a passage through the galactic disk, that means that this is very likely, because the cloud is much too big to have formed recently. That means that the cloud is supported in some dark matter halos, but there is a dark component in the cloud. And this is supported, as I will show in the slide you are seeing right now, that is supported by simulations, while the most recent publication on it was using MHT simulations displayed on the right-hand side of your screens. So there, you have a set of different conditions in your components of the cloud. In the first two rows, the cloud doesn't have any dark matter. Whereas in the following four, the cloud has a dark matter embedded in a dark matter halo. And as you see in the panel down, it is the clouds with dark matter. The ones that survive the passage. So if the cloud doesn't have any dark matter, they don't survive, instead the first two cases. So having this in mind, so that there are strong reasons to believe that this cloud is made of dark matter. So of course, you will want to look for annihilation signals of the particle dark matter using the Smith cloud as a target. Of course, it's not a new idea. People already, if Herman was in the collaboration, they looked at the Smith cloud using gamma rays. Because as I said at the beginning, gamma rays are the golden channel for indirect detection. And well, unfortunately, yeah, there were no positive signals of any discovery. And instead, they were able to put constraints on annihilation with cross sections. Of course, you might argue, the gamma rays are perfect channels for looking for dark matter. But they face limitations. So of course, the galactic plane is very active, and the physics is rather complicated. And there are four grants that are not understood, especially the galactic center. There's been long standing discussions of the four grants, millisecond pulsars, and so on. And you have the Ferby bubbles, and there are many problems that a gamma ray astronomer has to face in order to obtain or to discover dark matter when looking at a target that is so close to the galactic center. So our approach is complementary. So we also face limitations, which I will discuss right away. But we dare to obtain predictions of a signal of a dark matter annihilation. This is what I'm going to explain next. So the phenomenology involved in this project, I would like to say it's quite simple, but it's not. But I'm going to try to explain it in three major steps. So the first stage, so you have a dark matter that annihilates into several types of particles, among which you will have electrons or positrons, which are the ones that are going to be interesting for us. Because these electrons and positrons then will radiate, they will interact with the magnetic field, the cloud. And this interaction is responsible for the so-called synchrotron radiation. And if you plug the typical numbers for energy, electron or positron created by an annihilation and the typical magnetic fields in the galaxy, you get radio frequencies. Of course, phenomenology, as I said, is more complicated than just that. It's not like dark matter annihilated and then ballistically propagated to us. So the signal that emitted. But so the signal depends on the propagation of these particles and the diffusion models you take and several variables that are quite uncertain. So we just write the equations. The propagation is described by the diffusion loss process. And this is a question, we assume, spherical symmetry, that magnetic field is constant. And we will discuss the limitations of this approach. So think of this as a Bohr atom model for the cloud. And so we will get some resource that, in some cases, qualitatively describe how the signal will behave. But in some cases, you get actually quantitatively reliable results that you can compare to data. So once you take this assumption, you can solve this equation, actually, analytically. So you make a change of variables. It introduces so-called Synovsky variables that have a physical meaning. Probably not going to have the time to discuss them. But what they take home message is that you get an analytical approach to a very complicated problem, which is a prediction of a syndrome signal of a cloud. And as I advertised before, there are parts of our results. So several results that are actually quite reliable because they don't depend on some of our assumptions. So if you take a good look at our equation, and you realize that the electrons, they cannot escape the galaxy. So they are inside the diffusion zone of our galaxy. There is a maximum radius that they can reach. So that means that you integrate the diffusion equations over a sufficiently large volume. The first term will vanish because there are no escaping electrons or positrons. And what you're left with is something that doesn't depend on diffusion at all. So you get an equation that describes the number of electrons inside the volume or the number density of these electrons as a function of the source or the density of dark matter inside the cloud. And in the end, for the step that I'm not going to go into details in my slide, so you integrate that and you get a formula that resembles the flux formula that people use when they study the prompt emission of gamma rays from dark matter annihilation. Six, the formula, it looks exactly the same, which is not a surprise because we are dealing here with electromagnetic radiation as well. And the only difference is that instead of talking about the particle yield or the gamma ray yield of annihilation, we will talk about some radio yield or radio energy yield of this emission. In the paper, you can find a formula for this yield. And this is a formula you can apply to several cases, but you should always be careful when you apply it. So the central motivation is that the cloud is inside a larger volume where the electrons can propagate and at some point, the diffusion, the typical diffusion length, they reach that point where they cannot escape. I mean, the trapping volume is inside the observation volume. So that's what you should keep in mind when applying this formula. OK, I'm going to go through the ingredient that you cannot get rid of. So the energy losses, of course, when you compute the fluxes, you will have a result that depends on how the particles, how these electrons and positrons lose their energies while propagating. And so basically, when you use parameters, electron density, magnetic field, and also you need to use the CMB density, you realize that the interactions that yield to energy losses are you can, they are dominated by signature radiations or inverse components scattering depending on how strong the magnetic field is for high energies and for smaller energies, 1 GB as a reference value. So it's Coulomb interactions, the ones that will dominate. So we take that into account in our analysis. So now that we discussed the propagation, we are finally at the third stage in our analysis. I repeat, wind particles, dark matter particles, annihilate that create positrons and electrons, among many other things. This propagate in the turbulent magnetic field and the third stage, while they're propagating, they're emitting central turn radiation. The spectrum of a single electron or positron looks like the picture you have on the right-hand side, where this universal function that describes it and that is really flat at low frequencies, and then it has a cut-off. So once you have a spectrum for a single particle, you have the number density of electrons for every energy. You just integrate that, and then you get basically the flux. And well, from this slide, what you should keep in mind is the dependence of the frequency on both the energy and the magnetic field. That would be interesting in the next few slides, because now I can plot my results. So, well, this blue should have come as a surprise, but anyways. So these are our predictions. I emphasize these are very robust predictions, because they don't depend on the model you take on the propagation. So the spectra, if you integrate, so you take a signal to a big opening angle, you will get a spectrum that looks like the one we are showing here. And in our results, you see that there is a cut-off that we already predicted. And this cut-off is proportional to the square of the tag matter mass. And OK, I can go on. Now, we can use these predictions to get some constraints. But in order to do that, we need data, right? So what we did was, OK, we have an image of the Smith cloud. This image is in that frequency, 1.4 gigahertz. We have that image. And this image has a noise. And people who made that image, they report a noise level, a medium value of that. Well, we can compare our signal to that. So we assume that the tag matter is a conservative assumption. It's responsible for the noise. I mean, it was not observed, so it's below the noise. So it's kind of an argument here. However, you should be aware that it's not the optimal choice, because if you look at the blue lines, you see, for instance here, that the signal is several orders of magnitude suppressed respect to the maximum value you can get in a radio experiment. But still, so we use this data point to get some results. But before showing our limits, we got using that. Let me say a few words about the morphology. Because in this study, we could have stick to the formula, which is quite robust and everything. But we were also interested in knowing how the signal would look like if you find that tag matter by looking at the Smith cloud. So basically, the signal is featureless. And for the important aspect of our predictions is that it's universal or approximately universal. So it doesn't depend on the tag matter profile you use. Actually, the physical length scale will be the Sino-Basque variable, or the square root of this variable that I mentioned before. This will take over the meaning of a length scale. So you can actually even take a delta for the tag matter profile. And that won't change the results drastically. So this is, on the one hand, not very illuminating. Because it doesn't have any features. So you cannot constrain anything. You cannot constrain the mass function of the tag matter. But on the other hand, it's advantage. Because now you only need one length per meter instead of several. And of course, now this Sino-Basque variable, as I showed before, depends on the diffusion. So depending on your diffusion model, the shape of the signal will be different. So that's why we plot here the signal, the tag matter signal. So how it depends on the angle for several assumptions on the normalization of the diffusion coefficient. If you assume that the diffusion behaves like a power-loan energy, being the exponent of one-third. OK. And well, here they are. So if you take our results, and you trust on them, and you compare them to the noise level of the 21 centimeter image of the cloud, then you get this. So you get limits that are comparable to the ones of gamma rays and the electronic channels. So for instance, this green and dot dash and the blue dash lines, these are the limits that gamma ray people have manned. For instance, obtained a few years ago using the Fermilab data. And while these are our limits, so of course in the electronic channels, they don't compete with you guys. But in the electronic channels, so at some points in mass, we get improvements. And you wonder what is the physical meaning of this? Why there is a way for masses below 10 GB, our limits, stop being useful? And the reason is quite simple. Because of what I mentioned here, first, the cutoff frequency is proportional to the mass. So if you go to smaller masses, then the cutoff frequency is shifted to the left until the frequency we consider is goes beyond the cutoff. So what we are seeing in our limits here for masses that are smaller than 10 GB is that the frequency consider is above the cutoff frequency, and that makes the analysis basically useless. So alternatives, of course, you want to look at lower frequencies. And we have an ongoing experiment in the observatory called LOFA, the low frequency array. And while these guys, they might get the constraints I'm showing here. So these are the same channels as before. And the left hand side, so I'm showing LOFA, they have two types of antennas, the low band antennas and the high band antennas. So it took the sensitivities of LOFA for the frequencies I'm showing there, and basically took my predictions and compared them to the sensitivities of LOFA. And what you get is that for the frequency, 16 megahertz frequency, so the limits are better than the ones I showed before. But they are not, if you compare them to the ones of using the 21 centimeter line, they are not that better, but not that better. Here, I'm again showing the, I'm comparing actually to the limits I obtained before. However, the, sorry, I did something here. OK. In the intermediate frequency, one is still low, so compared to the, this is one order of magnitude smaller than the 21 centimeter frequency. So you get really limits that are way better in comparison to the ones. So this is the, this black line is the one that LOFA can achieve, in comparison to the one that you get with the 21 centimeter frequency. And this is for the BB channel, and so you see a great improvement. So the next step would be to try to convince people in LOFA to look at the Smith cloud in the high band antenna using, because, well, I've already had some feedback from you. Of course, from LOFA people, they say, like the lower you go to the frequency, it's more challenging. And so our prediction say that if you go too low, then the signal won't be that, the improvement won't be that great. So they would be happy to hear that they should go to their relatively high frequency range when looking at that target. So I think that's good news. And I'm basically done, because I cannot move to the next. OK. Basically done, because I'm going to go to the conclusions. And my conclusions, well, are reasons. We've seen that there are strong reasons to believe that this cloud is supported by dark matter. So the longer the time goes, the more arguments supporting this idea are available. That makes it a promising target for indirect panel detection. As I say, I would like to say that the dark matter phenomenology of Smith-Cloud using radio messengers is quite simple. Of course, that's a strong statement. But if you use our formalism and if you're convinced that it's reasonable to assume that the electrons and positrons won't escape the galaxy, then you might use it. And yeah, just by looking at the spectrum, how it looks like, you realize that low frequencies are the best range of low frequency. Then it's better suited for dark matter searches using radio waves. As an application, we use the 21 centimeter image of the cloud. And we were able to obtain decent constraints. But of course, low frequency experiments, observatories, rays, or whatever you want to call them, will be better suited for dark matter searches using radio waves. And of course, if you are able to collect, because low far, they can collect several frequencies. So the more frequencies, more images you can collect of Smith-Cloud at different frequencies, the more you can constrain it. And that's part of what I'm working on at the moment. And well, thank you very much for your attention. Thank you very much, Martin. So it was a very interesting talk. So I guess we can start with the questions. But before, I was just to remind the people that if you want to make a question, you can use the Q and A system. Well, I think Robert is gone. Yes, I think, well, Robert is gone. Now he's gone. He's really gone. Hi, Roberto Calle here. Well, in any case, well, we can go to the question, because Roberto made many questions here. I can read for you, Martin. OK. Well, the first question is if the Smith-Cloud is similar to the Bullet Cluster case. OK, sorry, guys. I have a problem with my computer. It's crashing. I already asked your first question, Roberto. OK, great. Great, great, great. OK, that there is similarities among the Smith-Cloud and the Bullet Cluster. I have this. The first question is because in the sense of, I mean, I did the question in the Q and A. So I wanted to ask you what is the similarity, because as you said, in order to the Smith-Cloud to survive requires that matter. And in some sense, the Bullet Cluster also the only way to explain it is to have dark matter in the case is following the. So in the sense, is there is kind of, for instance, for the Bullet Cluster, it's like a proof of dark. Maybe there is a kind of dark matter, but not modified gravity. In the case of the Smith-Cloud, is something similar? Right, that's a good question. No, so there is still offers of all this in this literature about the Smith-Cloud being supported by a dark matter halo. They still don't make strong statements. No, they say it's a claim that there are reasons to believe, but that is not strictly true that the cloud is. So it's an idea that has been evolving in the last 10 years or so. And well, there are every day more arguments, but I don't think you can compare it to the Bullet Cluster in the sense that you see the image, and then you see the lensing. And it's really obvious that there should be dark matter. And a modified gravity explanation of the cloud, you haven't seen. There are other explain explanations, of course, that the cloud is highly magnetized, and that explains why it came through or not actually an expert on the topic. But usually, alternative explanations are more based on other variables than the mass content of the cloud. OK, thank you. So I'm going to add another question. If my computer is not fully crashing again. I don't know if there are people in the audience, I mean like Hermano or Federico, if they have questions, you can do it now. Otherwise. I have a following question from the answer. It goes, well, there's a reason to think that the Smith cloud is a dark matter subhalo, is that it should be of an extragalactic origin. But I remember there was a paper measuring the metallicity of the Smith cloud. It was very high metallicity. And I think it plays against extragalactic origin of the cloud. Because if you have a high metallicity in the cloud, it should be galactic. But there were other problems. Like, well, it's very massive. It's difficult to explain. But I don't know if you take a look to those metallicity problems. Yeah, they mentioned it, especially in this, because the last reference, the MHT simulation I was talking about was already a few months ago. So it was really fresh. And they, of course, mentioned all these possible explanations. But to be honest, I don't remember the arguments against this cloud being of extragalactic origin. So it's usually believed that this cloud has been around for a while, and that it already went through the galactic disk and survived. That's the argument. So that it survived and now means that it should be of dark matter, which is this image, or drawing, let's say. Exactly that. Yeah, then I see your question, the Haslam map. Of course, there is this data. But you cannot resolve the cloud. So the cloud, if you want to see the cloud, you have to either go to the 21 centimeter, a bit shifted, because the cloud is moving at a certain speed, and the Lyman alpha. So from the 21 centimeter, you can learn something about the amount of neutral hydrogen and from the second Lyman alpha, you can quantify the amount of ionized hydrogen inside the cloud. And there were these studies of the nitrogen lines and the sulfur line. That's how people can learn about the metallicity of the cloud. And yeah, right. Does this answer your question? About the Haslam map, so second one. Yeah, yeah, yeah. OK. So I have more questions. I don't know if you're ready, because I have this probably with the computer now. But anyway, one that was about, have you are saying the prospect with low-far by why not with SQA? Yeah, SQA, for those who don't know, SQA is a project. So it's not existing yet. It's per kilometer array. It will be done probably in 10 years. I'm not sure, but a future experiment. And low-far is ongoing. So that's why we decided to show the limits that you get using low-far in order to motivate people in the collaboration to spend a few hours. And yeah, some main powers and these students starting this target. And the SQA, they will perform better, of course. But yeah, to be honest, I don't have any projections, any plots like the ones I show. Today, I don't have for SQA, maybe for next time, in 10 years. Yeah, when SQA is going to be almost ready. So because you were saying that one of the particularities of the Smith Cloud is gas, it's neutralizer and stuff. So in this case of the weather matter, it wouldn't be also interesting to watch, to observe the X-ray emission of the cloud if it is possible to observe. Maybe it's too small to resolve. Well, I think these nitrogen lines, and if they are already in the X-ray, I think some of them yeah, should well. I would expect that the weather matter would, if the electron had sufficiently high energy, it would radiate not synchrotron, but in brainstorming with the gas and also if there are protons or ions or whatever. Also, it would have some signature in the X-ray band. Yeah, that's an interesting possibility. But yeah, we haven't get the thought of that. Yeah, really, yes. So I guess, I mean, from my side, the last one is just curiosity. Do you know if there are other type of clouds, like the Smith Cloud, or is it just? Yeah, there are. This is a research field, so there are many other. But the Smith Cloud, what makes it special is that it's close, only 12 or so kilopasses away from the solar system. And that, well, you can actually make the drones like the ones that are in my image so that you can constrain the orbit. And well, there is strong arguments on the dark matter content of the cloud. So that's what makes this Smith Cloud more special than the others. OK, so I don't know. If the, I think, thank you for the answer. But I think I don't know if it's because I'm afraid to see the Q&A because I don't want to crash the system. But anyway, but I guess there are no more questions. No, no more. OK, so I guess we can close this webinar. So thank you, Martin, for your webinar. That was very interesting. And for all the people that are watching now the transmission, let's tell them that you can subscribe to this YouTube channel and continue following very hot topics in physics and astrophysical physics. And I hope to see you again in the next webinar that is in two weeks more. So see you to all. I hope you enjoy it. Bye.