What kills galaxies? | My Astrophysics PhD Thesis Part 1

this is my astrophysics PhD thesis you will see it is rather hefty because well there are three years worth of research just written up on these pages now I finished this up in 2017 so it's been about three years since I finished writing this and to be honest before now I just couldn't bear to look at it any more because I spent so long on it but so many of you were asking and requesting in the comments to know what my PhD research was about so this one is for you because you know three years probably long enough it's about time we crack this thing open all right so my thesis title was the influence of morphology Aegean and environment on the quenching history of galaxies which is a bit of a mouthful so let's break that down okay starting with galaxies which was the real focus of my PhD I was trying to understand how galaxies evolved over time in the universe so a galaxy is basically an island of stars about a hundred billion stars also on average in a single galaxy so for example our Sun is one star of rendre billion or so in the Milky Way galaxy our own galaxy then we have morphology right morphology is a basically a fancy word for shape but essentially looking at what shape a galaxy is is it a spiral shape or is it more of a sort of a blob around shape then you've got a GN a GN stands for active galactic nuclei I something is going on in the center of these galaxies we think these are essentially supermassive black holes that are growing so essentially if you have a galaxy and it's got a supermassive black hole in the center how does that affect its evolution then you've also got environment when we talk about environment of galaxies we mean whether they are basically in a very under dense environment so that pretty much left alone to their own devices or whether they're in a very dense environment so a group of galaxies or a cluster of galaxies that are all very near each other in the universe now we've got this term quenching history quenching is essentially the word we use to mean stop forming stars so quenching history is essentially looking at how many stars a galaxy has been forming throughout its entire lifetime and specifically if it has ever stopped forming stars and it's no longer forming any new ones so essentially what my thesis was about was how the shape the supermassive black hole in the center and the environment of a galaxy could somehow stop it from forming stars or because that didn't all fit in a youtube title how do black holes kill galaxies so I just said at the beginning the research that went into this thesis took me about three years to do now a PhD in the UK is typically three to four years it was bang on average in terms of how long it took me but the actual write up itself of the research I had to do it in two months and that was because I found out that the job that I've been offered post my PhD I had to start in January 2017 and this was right at the end of September 2017 so I had to hand it in in December right at the beginning and I had two months to do that I already had a holiday books in October as well it's the Caribbean which I thought about not going on but I went anyway and it was great and I still managed to get this right now but it was a lot easier because two traps is worth of the work actually about three chapters actually were already published in two journal articles so I had already really done a lot of the writing up it was just sort of the final research chapter and then like the introduction discussion and conclusion that really needed to be written up in those two months so essentially those three things that I talked about at the beginning the shape the supermassive black hole and the environment each make up sort of three core chapters in this thesis now if you do want to read it which I don't understand why any of you would want to but I have put a link in the description for those of the do I warn you though it's not exactly like bedtime reading okay so let's skip to the introduction see where we start shall we get past all the list of figures and everything first introduction the first sentence of the entire thesis is understanding the physical processes which have shaped our universe is the fundamental goal of astrophysics I couldn't have put it better myself but Becky essentially we want to understand how we went from soup of particles right after the Big Bang to these huge structures that these galaxies are made up of billions of stars now there's a lot of people that focus on those initial stages going from the soup to the very first stars and the first galaxies I focus on more recent evolution so what's been happening and say the last billion years or so to stop galaxies from swarming stars to change their shapes you make it so that they're supermassive black hole is growing what is going on in that kind of era and the way that we study these galaxies is basically just record their properties when we observe them with telescopes so the very obvious thing you can look at is the shape which is why one of the first diagrams in the entire thesis is the very famous diagram from Hubble of the Hubble tuning fork essentially let's put all of our round galaxies on this side and we'll get steadily less round until we get to the spirals and we'll put the spirals on two tracks those with bars and those without bars how we classify galaxies hasn't really changed since this original diagram of Hubble's from the 1920s now another property we also look at is the color of a galaxy is it more reddish in color or is it more blue in color now that's obviously very easy to say if we're just looking it with our eyes we can sort of just tell which one is redder and which one is bluer but we really want to be able to prou amateur eyes that ie we want to be able to put a number on how red or how blue a galaxy is so the way we do that is to observe galaxies through filters so filters the only let through specific colors or wavelengths of light so for example we will observe it through a filter the only let's do red colors of light or green colors of light or blue colors of light and then what we do is we record how much light in total we've received through that filter and we put a number on that that's what we call the magnitude sort of the magnitude of light that's received from that galaxy in that filter so if a galaxy has an equal magnitude in a red and a green and a blue filter then to our eyes that will look white because it's a combination of all three of those colors but if a galaxy had a higher magnitude in a red filter than it did in a blue filter then that would mean the galaxy would look red so what we do to calculate a color is just take one magnitude off the other so if we take the blue magnitude off the red and we get a positive number then we know that that galaxy is red but if we get a negative number when we do that we know that the galaxy is blue now one thing we know is about galaxies when we look at a huge population of them say a million or so galaxies is that you end up with a lot of red galaxies and a lot of blue ones but not so many green ones now I know that they shouldn't technically be dubbed green it should be like white ones but you know green is in between red and blue to astronomers so we've dubbed in green ones and we call this area of parameter space that the colors of galaxies the Green Valley and it's a well known plot in galaxy evolution studies which is why it is now figured to in my thesis interestingly actually the Milky Way is thought to be in this Green Valley region as well which i think is really cool now you might be wondering what makes a galaxy red green or blue well it's all about the mix of stars in a galaxy so the hottest stars in a galaxy burn very blue think about like the flame of a Bunsen burner or your hob or your cooker that is a blue flame and it's much hotter than say a candle flame which burns much redder just like the cooler stars in a galaxy how much redder now the hottest stars are so hot because they're burning their fuel at such an incredible rate that fuel is the hydrogen gas inside the star that they're using for nuclear fusion and the reason they have to burn their fuel so fast is because they're so so big that gravity is pulling in at such a rate that they have to burn or feel so fast to counteract that so that they don't collapse it to keep the whole system's stable it means that despite the fact that they're you know hundreds of times bigger than the Sun they only live for something like hundreds of thousands to millions of years unlike the Sun which doesn't have to burn its fuel at such a high rate and despite the fact that it has a lot less of it lives for maybe ten billion years so when he Galaxy forms a bunch of new stars it tends to follow them all across the sort of range of sizes that it can it's a little bit of a random draw about which sizes you're gonna get but it'll be a generally representative sample of all the different types of stars that you can't have so the reason that we think we have so many red galaxies that sit at the top of this diagram is because you know galaxies normally if they had enough fuel and continued burning stars they'd stay in this blue region but as soon as that process stops they move through the green valley and up the red sequins as they get redder and redder and only the red stars are left now a lot of people have said this process of moving from blue to red through the green must happen very very quickly because everything piles up here and we don't see as many galaxies in that Green Valley region remember that's why it's called a valley but of course we need to test that so the first bit of research I did with into how fast this process is happening and crucially if it depended on shape because typically we see that about 90% of these elliptical or blob shaped galaxies are reddish in color and about 70% of spirals are blue in color so obviously there's something to do with shape here going on as well as the color changing but what is it so the first thing I had to do when I arrived as a little fresh-faced PhD student back in 2013 was build a model of what I thought was actually going on here so let's assume that most galaxies will happily just keep forming stars at a constant rate if they have enough fuel hydrogen gas to do so but then at some point sometime quenching starts this process where these star formation rate starts to drop off and the galaxies stops forming stars and after that time the star formation rate the rate of which the galaxy forms new stars will decline at some rate and that rate could either be incredibly fast or it could be incredibly slow so I decided to go with modeling this is an exponential curve and that allows you a bit of freedom to either have an incredibly rapid shut off that practically just looks like a step function from constant to zero or you could have a much slower decline that's more actually like very linear sort of very like straight line down and then everything in between so this model gives you a lot of flexibility so there's rate that the quenching is happening at is the number that I wanted to get out but all I had at a time was something like 260,000 galaxies with their colors of how red and how blue they were so I had to have some way of comparing so I took my model of how many stars forming every single year in a given model and I said okay if we're forming that many stars then what would the color of that many stars look like from a best model of what color different sized stars are so thankfully other people have spent their entire careers developing those models to say if you have a generic population of stars with the you know nice spread of masses then if it's this age what color is it another hundred thousand years later what color is it another million years later what color is it and I could take those and essentially add them together in such a way that it would give me a population of stars ie a galaxy that had a constant star formation rate up until some time at which point it started to drop off and so what I did was write a code that basically searched for which model best fit a galaxy is using something called MCMC Markov chain Monte Carlo using Bayesian statistics as well for all of those who are interested there is an entire chapter basically in my thesis devoted to explaining this bit of code this model that I had written I named it stop I because it reminded me of Staffie from Mario so it was sort of named after Mario's little star friend and it makes me happy every time I think about that and basically this chapter is there to describe it but also prove that it works right if you write a code like that you have to prove that it can recreate like known colors or they could recreate a population on average as well and so all the tests of that were also in this chapter too so by chapter 3 I actually wanted to use this model in code that I had written to study some of the galaxies that I had about 260,000 them from a survey of the sky called the Sloan Digital Sky Survey which is essentially a good chunk of the northern sky with about a million galaxies observed in total in it now a good fraction of those had also had their shapes classified by a project called Galaxy Zoo so this is an online citizen science project using a website that you will go to you will get shown an image of a galaxy and get asked to classify its shake so that people like me can do research that information so that was launched back in 2007 it's still going though to this day because well astronomers don't stop taking pictures of the sky so there's always new galaxies that need classifying so that we can do more science in different signs with it but it's not just me that uses those classifications for this one project there's hundreds of people all around the world both professional and amatuer using them to do their own science because they're all available on the web now - which is great I'm all for open science so then once I had this code that I'd written to say okay if I observe this color of a galaxy what rate is the star formation dropping off at and they had 260 or thousand those galaxies each with classifications of spiral or blob what I could do is run them all through to get the best fit model the distribution of all the different rates you'd see in those galaxies and then by comparing say red spirals to green spirals or red spirals to red blob galaxies we could work out what was going on in order to give us a large amount of red galaxies and especially red elliptical galaxies and then not as many green galaxies at all to give us this Green Valley region and so what my results ended up showing was the four elliptical galaxies these blob galaxies that transition from blue to red did happen very very quickly which is why you get such a large amount of them with red colors but for spiral galaxies that transition is happening much more slowly and so there's an intersection in the green region of that diagram that I showed earlier where you have slow moving spiral galaxies and very fast-moving elliptical galaxies and it's the overlap of those two populations that gives you the Green Valley and I speculated that essentially it's all to do with the different mechanisms that drive galaxies from blue to red so for example mergers of two galaxies are thought to destroy the spiral nature of the galaxies so if that happened to two galaxies in the blue area of the diagram then they're very quickly going to turn red and they're going to change their shape whereas red spiral galaxies of somehow become read but retained the spiral shape so that must have happened much slowly with the less violent process than a merger that was the big conclusion of basically this chapter but also the very first scientific paper that I ever got published as well moving our now to chapter four which is my favorite chapter of my thesis because it's a supermassive black hole Oh hold your horses there filming Becky this is editing Becky step it in and print a foot down because I am not even halfway through the footage that I filmed for this video on my thesis and we're already gone past 15 minutes so we're gonna split this into two parts just so my research doesn't suffer because I'm editing so much so the next part will be on how growing supermassive black holes and the environment affects a galaxy and how we think it can shut down the star formation in a galaxy and what I found but then also the big picture as well and those are some of my favorite chapters of my thesis so hopefully I'll see you there for that one I'm literally like mid to film session at the minute like just ignore all of these cables but look what just came in the post at first I was annoyed the doorbell weren't cuz it interrupted but oh my days look what it is all the way from America June 2nd II in that Green Valley region remember that's why it's called a valley I said that very well should entire Valley the Green Valley oh we love the whelk should I do I just still love that I got it printed in Oxford blue as well it looks so good it's blue diamond thesis is blue diver died advocate aid up you

Channel: Dr. Becky

Views: 56,727

Rating: 4.9637718 out of 5

Keywords: astronomy, space, physics, astrophysics, university, college, collegelife, universitylife, academiclife, phdlife, scientist, astrophysicist, cosmology, drbecky, Becky Smethurst, Rebecca Smethurst, galaxies, supermassive black holes, telescope, spirals, stars, colours, ellipticals, mergers, collisions, thesis

Id: l9TOXO80u98

Length: 18min 14sec (1094 seconds)

Published: Wed May 27 2020