Here Be Monsters: Tales of the Hot Universe

today the talk will be about astrophysics and in particular the

monsters of the universe the speaker will be Julie lava check Lauren dough

normally we're very proud that we gather scientists from slack to slack from all

over the world but in her case things had to be gathered from all over the world just so that she could be born she

is the child of immigrants from two very different places who came to Montreal she was educated in the french-canadian

system went to Cambridge did a brilliant thesis there on the structure of some

active galaxies came here to Chi pack the astrophysics Institute of slack and

Stanford on an Einstein fellowship and she's been working with people here on

x-ray observations of the biggest and most ferocious things in the universe

and that's what she'll talk to you about tonight so let's welcome joy

okay can everybody hear me yes right okay so hi everyone I'm really happy to

be here and so today I'm going to be talking about the most powerful and

mysterious objects in the universe okay and it's because of their power that I call them monsters and so

basically what I'm gonna do is I'm gonna tell you I'm not gonna tell you exactly

what these monsters are yet okay instead let's pretend for now that they're

basically dragons okay and instead what I'm gonna do is I'm going to tell you a very interesting story about a very

special object one of these monsters and what we'll do is we'll go through the

steps that astronomers did that kids took decades ago when they discovered this object and we'll piece together all

the clues and finally we'll come to the conclusion to see what these kinds of monsters are actually okay and so to

start off basically we're gonna go on a small journey okay what we're gonna do is we're gonna take our telescope so any

kind of telescope that we want and we're gonna point into a very particular region in the sky okay so not just any

one but one in particular and this region is called a Cygnus constellation

okay so this is where our story takes us so this constellation is quite a

well-known constellation in the northern sky so we can actually see it from here and it's based so it's a lot of these

stars that come together so the constellation is traced out by these green lines here what's interesting is

perhaps there's this very bright star here which happens to be the nineteenth brightest star in the sky so quite

bright but this is not where we're gonna focus our attention to what we're gonna do is we're gonna focus our attention

here on this particular star called Etta signe okay and what's actually

interesting is that we're gonna zoom in and this part of the sky so let's do this right now okay if you have a very

nice telescope this is the kind of image that you'll see of this region of the sky and as you see it's a very beautiful

image it's made up essentially of a lot of stars you have a lot of activity

going on a lot of newly formed stars that are happening but there's actually something very

different that's going on in this region of the sky and what's different is based on a discovery that was made in the

1960s what happened in the 1960s was that astronomers launched rockets that

for the first time observed the universe in x-rays okay and they didn't know what

they were really gonna see but they found that in this region there had to happen to be a very very bright x-ray

source and this source was named Cygnus x1 because it was in the Cygnus

constellation and it was the first x-ray source discovered there now what I'm going to do is I'm going to show you

where the sources so the x-ray emission isn't actually coming from all over this region here it's only coming from one

very very tiny region from here okay so where this red little square is it's

coming from this very tiny region and just to give you an idea of what we're looking at this is what we would see in

the x-rays okay this as I said already this is one of the brightest x-ray

sources in the sky now the problem with this is that took for an object to emit

x-rays it takes a very particular kind of situation so not all objects actually do this only a very small percentage of

them do and to be able to emit x-rays you basically need the object to be extremely hot okay how hot I'm gonna

explain this maybe in a bit easier way yes so this is a very typical

explanation for what's happening so basically what you do is you take your oven or stove top and when you turn it

on so initially it's black but as soon as you turn it on it gets heated up and then it starts to glow okay it starts to

glow red and what's happening is that objects basically are gonna emit a certain kind of light depending on their

temperature all right and just to give you a better understanding this is basically what happens so depending on

the temperature of the object you get different kind of light that it's that it's emitted by the object so what

happens here in the case of our stovetop as it goes up to about 400 degrees Fahrenheit it's mostly going to emit

emission in what we call the infrared so we're going to get a lot of light in the infrared but some of

is actually going to go all the way up to the visible into the red portion here and so our eyes are actually not very

sensitive and they can only catch a glimpse of what's happening so our eyes are only can only see what's happening

in the visible however we can use telescopes to try and probe these different regions here so radio infrared

up to x-rays and so I'm gonna focus today on this part here so on the x-rays

basically what this is telling you is that for an object to actually emit x-rays you need it to be extremely hot

so we're talking about several millions of degrees Fahrenheit so very very hot

so whenever you look at something in the sky that's emitting x-rays it's basically telling you that something

very hot something very energetic is happening in that particular region and so coming back to our object before this basically

what we do is we use these telescopes and so I'm going to talk about this telescope here and so coming back to our

object so basically it emits a lot of x-rays so what's happening what's making

it so hot that it's actually doing this and recalling that this is one of the brightest sources in the sky so it has

to be extremely energetic in this region and all of it is coming from this very tiny square here and so what astronomers

did was when they figured out that most of this x-ray emission was coming from this little square they said okay let's

try and see what's actually there is there an object is there for example a supernova remnant and so the reason why

they thought about supernova remnants was that at that time they knew that supernova remnants actually emitted

x-rays and so supernova remnants are basically what's left over once a very massive star has exploded and so you see

these beautiful supernovas and so they said maybe that's what's happening here maybe that's what is emitting all of the

other x-rays so they looked for one and they didn't find one okay what they

found instead was a very massive star okay so this is a part of a diagram that

was taken from the original paper so these were one of the discovery papers back in the 1970s that found this

massive star to be associated with this x-ray emission and so they derive the masses of the

stars so we're basically talking about a very massive so essentially between 15

to 30 times the mass of our Sun so a very massive star however the problem is

that astronomers knew at this time that when you looked at other stars that had

the same kind of mass they didn't admit that much x-rays okay they only they emitted about 1000 times less than what

we were seeing so something else something different was happening here but they also notice is that this star

was moving it wasn't actually stationary and it was moving in a very particular

way it was moving just like this as it's illustrated in this diagram what this diagram shows is

the speed of the star that's coming towards you or receding from you so

basically what you do is you go on this plot here as it goes up this curve as the velocity becomes positive it

basically means that the star is going towards you and as it's going down it's going away from you and it's repeating

this motion towards you away from you so when astronomers see this kind of diagram they become really excited why

because this basically means that this star is part of a binary system okay

what do I mean by binary system I basically mean that this star is in

orbit with another object for now we don't know what this object is and the two are orbiting around each other okay

and this is causing the star to move here in this orbit and so for now let's just focus on this movement of the star

and I'm going to explain this diagram a little bit more in detail so what's happening is that as the star orbits in

this direction it's actually going towards you okay so the star is up here in this diagram but as it moves in the

diagram at this point it's not going towards you're away from you it's just going from left to right okay so it's

here it has a velocity of zero but as it continues in its orbit in this phase here it's going away from you and you

continue and continue and this is what happens and this is why you get this kind of diagram so this is why it means

that this system here is part of a binary system but the question is with what what's the

other object okay so we know that these massive stars have as I mentioned do not

really emit a lot of x-rays okay so that's this basically means that perhaps

the x-rays are actually coming from the other objects here okay so what is it

could it be a star could it be some other kind of exotic object we don't

know for now but what astronomers did know is that when they looked at this diagram here okay they can actually

calculate just how massive the other companion has to be because the the

intensity of the velocity so how fast the star is orbiting around the other object and the the period here of this

diagram basically tells you a lot of information about the system and it tells you the information about the

other object so just how massive this one is and what they found was something quite interesting they found that this

object was very massive okay we're talking about at least three times the mass of her Sun at the very least okay

so this meant that technically it should be maybe some kind of other star and yet they weren't seeing it they could see no

kind of signature of any star mission so what was going on so this was part of

the second clue okay now the third one is that when they studied the x-ray emission that was coming from this

object they actually noticed that it was varying so yes the source was emitting a

lot of x-rays but this kind of emission was varying quite significantly and when

they looked at in even more detail the kind of time skills were talking about microseconds so on microseconds the

source the amount of x-rays it was emitting was varying by several factors okay so a very huge amount and this is a

problem in astronomy why because it tells you that essentially the object the size of the object has to be quite

small to be able to vary that quickly because the information time it takes for the information to code from one end

of the object to the other it depends essentially on just how fast you can transmit that information and the

fastest thing that we know of is like okay nothing can go faster than light travels at about 200,000 miles per

second it's extremely quick and so that limits just how fast the information can

travel and so how small your object is because we know it's varying on microseconds all right so this basically

told astronomers that the object to be able to vary this quickly had to be very small so that the information could move

from one end to the other and so the question is how small so they calculated this and what they found was that the

object had to be very very tiny we're talking about hundreds of kilometers wide okay and there are only three

things that we know of in astronomy that can be extremely small yet extremely

massive and these three things all come from the deaths of stars okay so the

first kind of object is a white dwarf so what happens is that you have very so if

you have a very normal star for example a star that's about the mass of our Sun this star is basically burning fuel in

order to fight gravity but the problem is that stars can't burn fuel forever they run out at some point and so at

some point gravity wins and everything collapses and so what you get for a star

that's like our Sun you end up with something that we call a white dwarf so basically you have this kind of object

now the problem with white doors is that we know that they're actually much bigger than hundreds of kilometers

they're typically thousands of kilometers wide and so basically white dwarfs are too big to explain this

object so astronomers said no it's not a white dwarf the second object they examined was neutron stars

okay so neutron stars are what happens is that if you have an even more massive

star to start off this one collapses into a white dwarf when it dies but at

some point since the mass is so big even the white dwarf can't support the pressure of this mass and then the white

dwarf end up collapsing and forming a neutron star so an even smaller object and we know that neutron stars are

typically a dozen kilometers wide so yes they're quite small however

they we astronomers know that neutron star can only go up to a certain mass they can only be up to about three solar

as times the mass of our Sun okay if it's above that mass what happens is

that gravity wins over again there's too much gravity the star collapses even further and so what do

you get you get a black hole okay and so basically white dwarf was too big

neutron star it was impossible because they knew that the mass of this object was more than three times the mass of

our Sun so the only possibility they were left with was a black hole and so

what is a black hole well by definition this is what a black hole looks like

okay essentially black holes are objects

so compact that nothing can escape escape gravity is so strong that even

lights cannot escape that's why they're called black holes but in terms of this

size there is a definition of just how big black holes can be so this is what we call the event horizon which measures

the radius within which nothing can escape okay so within this radius not even light can escape that defines the

size of a black hole it's approximately given by what we call the Schwartz your radius which is given

by this equation here but I won't go into detail but this equation but just to give you kind of an order of magnitude if you take something that's

as massive as three times the mass of our Sun so something similar to what we were seeing in Cygnus x1 then the size

of the black hole is tiny we're talking about nine kilometers so black holes are extremely compact massive structures

very powerful so this is what astronomers thought was going on here

and so but the problem is that I just told you that black holes don't emit light because nothing can escape from it

not even light but we know that this object here was emitting a ton of x-rays so what's going on well what happens is

that these two here these two objects are so close to each other there are

reading around in Turners so quickly they're so close that the black hole is literally tearing apart the star and

just to show you this kind of looks like so this is an illustration of what we think is going on so you have a gigantic massive star

and you have the black hole here and basically the two are so close to each other that the black hole is literally

tearing the star apart and as it does this it drags material material starts

circulating forming what we call an accretion disk which will eventually be

accreted onto the black hole and as material settles into this accretion disk what happens is that the material

here is traveling very quickly okay almost at the speed of light it's being attracted by this immense gravity that's

created by the black hole and as material does this there's a lot of friction that's created so the material

essentially heats up due to this friction very it's very similar to what you do when you rub your hands together

try and get them warm this is the same thing there is friction it heats up but

the friction is so intense that it heats up - we're talking about 20 million degrees Fahrenheit and even more so

extremely hot and this is why this source here is beaming in x-rays this is

why it's so bright in the x-rays and this kind of object this is what I call a monster something so powerful that it

can literally tear up a star although as we saw the size is tiny so very very

tiny we're talking nine kilometers well so this is this is what's going on now astronomers in the last couple of years

have studied this object quite intensively and now we know that the

star here we have a very precise estimate of its mass about twenty times the mass of our Sun and this black hole

is actually fifteen times the mass of our Sun so very massive and so this is

the kind of object I'll be talking about for the rest of this talk and so just to give you kind of a glimpse of what I'm

going to talk about so I showed you one example of one kind of black hole that we see these are what we call

stellar-mass black holes so the remnants basically of the death of a very massive

star so we're talking about an initial star that had about a mass of 20 times

the mass of our Sun and this collapsed due to gravity when it no longer could fuel its burn its fuel and so

these kinds of stellar masses a black ghost typically have this kind of mass however what I'm going to talk about for

the rest of the talk is what we call supermassive black holes okay so these are black holes that are

gigantic they're at least a million times the mass of our Sun so huge we

essentially think that almost all galaxies have one of these monsters at its center

okay so I'll talk about this and then finally I'm going to talk about the kind of research that I do so I work on these

supermassive black holes but I actually work on the biggest ones that exists okay and so I'm not gonna tell you just

yet how big they can actually get I'll tell you that at the end but basically this is a kind of object I study so

really really big supermassive black holes and so let's go and move on to

this kind of optic and so we're gonna take another journey this time we're gonna go somewhere very different in the

sky and just to give you an idea of where we're gonna go so basically this is a very nice illustration of what we

think our own galaxy looks like ok so we are part of a galaxy called the Milky Way it's very difficult to know exactly

what it looks like because we're in it and so it's difficult to see what it looks like from outside but this is the

kind of picture we we think it looks like and so you have these beautiful spiral arms the Milky Way is basically a

spiral galaxy this is where we are located here okay so we're approximately

26,000 light-years away from the center and what I'm gonna do is I'm gonna focus here right at the center here this is

where we're gonna look at and as we'll see there's a very interesting monster right at the center here and to kind of

tell you show you what this monster looks like we're gonna use the Chandra

x-ray Observatory so this is a very very nice telescope this is one of the telescopes I use I use for my research

it was launched in 1999 what's really amazing about this telescope is that it

significantly increase the resolution of x-ray images it went from a factor of

about 10 when it was launched so it was ten times better resolution and I'm

gonna show you just an idea of what kind of improvement it made thanks to this telescope here so

if we take a picture of the center of our galaxy so we've focused our camera here right at the center this is what we

would see before Chandra okay so you see before Tantra there's some structure but

you don't really know what's going on now I'm going to show you what you see with Chandra this is what you see okay

so this is the C amazing thing about Chandra it's kind of like the Hubble but

for x-ray astronomy we get amazing images out of it so this is a very very deep image of the center of our galaxy

called the galactic center okay so basically what this means is that imagine you took your camera okay

and you took a picture of the night sky but you open the shutter and so that it

collects lights for over twenty six days this is how long this exposure is so it's a very very deep image you can see

a lot more objects the deeper you go and so you see there's a lot of structure here and I'm gonna talk about two or

three of these and then focus on one particular object so there's interesting structures here so for example this

structure here which is called Sagittarius b2 happens to be a very cold

molecular gas okay and we know that cold objects don't really emit x-rays and yet

it's showing up in this image so what astronomers think is going on is that this amount of cold gas is actually

being illuminated by all of the x-ray photons that are coming here so all of the x-ray lights and as this light goes

to the molecular cloud it gets reflected and that's how we see it light up okay

this is an interesting structure another interesting structure is here which is called 1e 1743 this is one of the

another candidate so we think that what's actually going on is that there's another binary system here consisting

again of a black hole and a very massive star and so this is another candidate

and we know about 30 systems of these in the Milky Way okay so Cygnus x1 isn't the only one

there are about 30 other candidates where we think there's a black hole that's tearing up a very massive star

here you have another interesting structure which is basically a bunch of massive stars that are all packed

together due to gravity so that's interesting but I'm gonna focus here okay so this is a very well known object

called Sagittarius a and as you can see it emits a lot of x-rays so let's zoom

in right on this portion here okay so very tiny but let's zoom in there and let's see what it looks like in the

x-rays so this is what you get okay so the same square here is the same region

as we were seeing this tiny square here so we're zooming in quite significantly this is an even deeper Chandra image of

the very very center so the very galactic center it's over 34 days of

exposure so ultra ultra deep and we can see a lot of structure but we can see in particular one object here which stands

out and this is what we call Sagittarius a star so let's zoom in even more so you

can do this with Chandra and what you see is there's this fuzzy cloud here now it's really interesting is that

astronomers found that is what was actually happening is that a lot of this gas was being expelled okay so something

was expelling the gas and so the idea that you can get is there's actually one

supermassive black hole at the center here that's accreting material but that it's also expelling a lot of material and the

reason why we think that it's a supermassive black hole it's based on a very important discovery that's been

made over the last 15 years and it's a discovery concerning the galactic center

okay and I'm going to show you this discovery here and so basically what they did is that astronomers observed

the stars at the center of our galaxy so on Sagittarius a star and this is what

you see here so all of these fuzzy blobs are individual stars okay and what we'll

see is a video that was made over the last 16 years okay so it's a lot of data

these are the real images so it's not a simulation this is really what you see

okay and so just to show you I want you to focus in particular on this star here

okay as you'll see so the stars are gonna move

and this is what you see okay so the video is repeating over four cycles and

zooming in each time and so what you see is that the stars are moving okay but

what's really incredible is that this one here is moving very quickly and then

it goes back up and then it goes back up okay what's happening so basically you

have a star that's going in a essentially an orbit it's orbiting

around something it's orbiting around something that we cannot see that's black okay and so very similar to the

way astronomers worked on Cygnus x1 based on just how fast the star is

moving about this invisible point you can calculate the kind of mass that has to be there of this this object here

that's essentially black and when you do this the kind of mass that you get is four million times the mass of our Sun

so gigantic and so we're talking about

an extremely massive dark object now the other interesting thing is that if you look at the kind of scales you're

dealing with so when this star actually approaches there's kind of like a

beeping that's it okay okay I will

ignore the beeping and so when this star actually approaches here okay the kind of distance it reaches is

about six light hours it's very very

tiny just to give you an indication six light hours corresponds to approximately the size of our solar system okay so it

takes light about eight minutes to reach but to go from the Sun to earth okay and

for six like hours it's approximately the size of O that is not me okay

so this is a black hole

and okay so I'll continue a little bit so this is one of the really the strongest pieces of evidence that we

have for these existence of an invisible object that we cannot see extremely massive a supermassive black hole okay

so four million times the mass of our Sun all of this has to fit inside the solar system of a solar system so very

very compact and so people have been taking these data for over 16 years now

and they've actually measured the orbits of about a dozen of these stars so they get a very very good constraint on the

mass of this object and I'm going to show you other interesting pictures very

soon it started beeping okay and so I'll

show you a picture as soon as this comes on line of something that's happening now very that's very interesting about

Sagittarius a star so the supermassive black hole at the center of our galaxy there's actually a cloud that's

approaching this black hole okay this cloud is named g2 and we've known about

this cloud for a couple of years now and so we've seen it approaching this supermassive black hole and actually now

this year this is when this cloud is supposed to interact with the black hole and so we'll be able to see exactly how

black hole accretes mass and this is going to be an amazing opportunity and

I'm going to show you a picture very soon of this cloud that's actually approaching the supermassive black hole and we can just about now we can start

seeing it being torn apart okay but stay

tuned for the results because this year is going to be extremely exciting concerning that and at some point I will

show you this okay so there are two leading theories

so one is either gas okay but there's actually a group that published a paper about a year ago that suggests that this

cloud is actually a star yes so we don't have the technology to resolve the

clouds we don't know just how small it is so now we just see a fuzzy image but we'll see once it interacts with the

black hole because that will be very different if it's a star or or a cloud

of gas so we will see it's starting to we can see it kind of starting to spread

out we can't resolve it but we can see that the velocity has been changing and so this is consistent with it being torn

apart although we can't resolve a cloud but yeah I can take yeah let me take

questions okay thank you yeah yeah so

you see very different things oh yes okay so I showed you a picture of

what the galactic center looks like in the x-rays and so the question is if I understood correctly what it looks like

at other wavelengths so there's actually a very interesting discovery that was

announced about a week ago about an image taken in the radio okay and what

you see okay cool and I want to show you

that image and then I'm gonna come back here okay

okay so this is the image so basically what you see in the purplish colors this times is the x-rays okay so you see the

same structure as before but in the radio here you see a lot of this kind of

weird material here and there's actually something very faint in the x-rays here

that you may not have seen initially but there's a jetty structure okay and what

they're suggesting is that there's actually a jet coming out of the Senate the supermassive black hole it's

interacting with material here creating a shock front and this lights up in the in the radio okay this is a very new new

discovery so we'll see if this actually works but coming back okay and I'll take

more questions at the end yeah and so let me show you just once one

more time this video because it really is amazing and so you have all of these stars orbiting and then in particular

this one and so these again are the real images so there are a lot of simulations out there but these this is a real thing

this is really what's happening and concerning the cloud g2 this is basically what we see okay so this is

the zoomed in version of the previous figure these are the individual stars

okay so we can't actually resolve the Stars so they're just fuzzy blobs and this is the cloud so we can track its

movement from 2006 2010 2013 this the black hole is located approximately here

okay so it's starting to interact and then it's gonna do something very amazing we still not sure how it's gonna

react it might just flip over we don't know we'll see okay but time will tell

okay so it's very interesting but this is this is what's happening now

okay but although Sagittarius a stars or the black hole at the center of our

galaxy is very interesting it's actually what we call very quiescent so there is some activity but not that much okay and

there are other black holes what we call active black holes that are very different these are the exciting ones

okay these are black holes that are recruiting a lot of material and what's happening is that in some black holes

they're able to emit what we call Jets okay and these are Jets of made of

relativistic particles so particles that are very energetic what's very strange

about these Jets is that they seem to be extremely thin okay and one of the ways we explain this is that essentially

what's holding them together what's making them so thin our magnetic fields now when you take if anybody has done a

physics class okay if you take magnetic fields and you take relativistic particles and these relativistic

particles are accelerating in a magnetic field what do you get you get what we call synchrotron emission okay so this

is a very typical kind of emission that we see in in the universe and basically synchrotron emission Peaks in the radio

so when astronomers study these kinds of structures here they use for example

Chandra to study the hot gas that's on the accretion disk that's heated to millions of degrees Fahrenheit but they

use what we call radio telescopes to study these jetted structures here okay so although radio lights isn't very

energetic it indirectly traces very energetic particles that are accelerating in magnetic fields and so

we use a combination of these two kinds of telescope to understand these objects and this is basically what I do for my

research and so I'm going to show you some examples of these active black holes okay now to do this we have to go

on we have to look at this in a very different region of the sky so not at the center of our galaxy not towards the

Cygnus constellation but what we're gonna do is we're gonna go away from the galaxies okay so our neighboring galaxy

is called Andromeda it's located up approximately at a distance of two million light-years from us but what

we're gonna do is we're gonna go ten times further than that okay and what we'll hit we'll hit a very interesting galaxy

called Centaurus A so Centaurus a is the galaxy that contains the nearest active

supermassive black hole okay and we'll just see how powerful this kind of object can be this kind of monster can

be and so this is a beautiful optical image okay of the galaxy what you see is

very typical this is a very large galaxy this kind of stream here is essentially

dust and so dust absorbs light and prevents us from seeing what's happening at the center so this is typically what

you see however if you look at the same exact image this time in the radio we're

calling that radio allows us to trace these Jets of relativistic particles this is what you see same image but in

the radio only you see something completely different okay this is why it's so important to combine

different kinds of lights to try and understand what's going on this is why astronomers use all kinds of telescope

to try and understand the physics behind what's going on and so just showing you

this image again this is a radio now what I'm going to show you is the same exact image but this time in the x-rays

this is what you see so this is a beautiful image taken again with the Chandra x-ray telescope you see

something totally different you see yes the jets light up in x-rays you see these beautiful

interactions here that are lighting up in the x-rays but you also see a bunch of point sources okay we think that a

lot of these points are actually these some of these binary systems that I talked to you about where you have a

stellar mass black hole and a massive star and the two are orbiting close to each other and the black hole is

shredding the star this is where we think at least some of them are doing but it's you see something completely different and so combining all of these

together this is a picture that we can build we have a very massive galaxy okay

so a galaxy typically is maybe fifty thousand light-years across okay so it

takes light fifty thousand years to cross it and what you see is that you see these beautiful Jets that are on the

scale of the galaxy so gigantic Jetts and these are being created by one single object by one single supermassive

black hole at the center of this galaxy okay so this is the power of black holes although they're tiny in size they're

literally about a billion times smaller than the Sun the size of the galaxy although they're tiny they can create

enormous structures extremely powerful they can have a very important impact on their surrounding medium and by

extension their their galaxies okay and this is the kind of thing that I study

however there are black holes that are even more active than this okay so this

is just one example and we'll see the the things that are even more powerful and to do that we have to go somewhere

else in the universe and what we'll do is first of all I'm gonna show you what the universe looks like okay so I'm

gonna show you one of my favorite images and so this is a very nice image taken

from the Sloan Digital Sky Survey basically what it did is that they looked at the sky and they said I'm

gonna count all the galaxies in the sky and I'm gonna place them on a diagram so

every single point you see here represents one galaxy we're calling that

each galaxy contains billions of stars okay so this is how big the universe is this is huge and just to point out this

kind of scale is only a tiny tiny tiny fraction of the actual universe universe goes beyond these kinds of light-years

okay those is very very tiny and yet you see that there are a lot of galaxies and

what's interesting is that you see that these galaxies aren't randomly distributed there's actually structure

there are these very beautiful filaments here okay and these filaments tend to

meet up and what we call clusters of galaxies okay now before I get there I'm

gonna there's I want to make one analogy that the reason why this has been always so interesting to me is that it reminded

me a lot of what neurons actually look like in the brain so you see just

neurons here and then connection by the foot like filaments and so it's very similar what you see in outer space so

to me this has always been something very impressive but I'm gonna focus

these clumps here which are called clusters of galaxies and I'm going to show you one example one very famous

example of a cluster so basically clusters of galaxies are consist of hundreds to thousands of galaxies that

are bound together by gravity so this is one example here this is one of the most massive clusters that we

know of think thousands of galaxies and so you see all of these yellow points

here representing galaxies so this is a beautiful image taken with the Hubble Space Telescope now what I'm gonna do is

I'm going to show you what we see in the x-rays okay so we're calling again that x-rays trace is very hot energetic phenomena so

the exact same image this time in the x-rays okay I don't know if you were

expecting that but basically basically what this means are that clusters are actually x-ray sources okay so it's not

only the accretion disks around black holes but clusters are also x-ray sources so why is this

and so just combining these two images here and so to try understand this I'm

gonna recall that basically when we look at x-rays we're looking at things that are very very hot so we're talking about

at least 20 million degrees Fahrenheit and the reason why clusters happen to be

very ex luminous sources is because they're very massive okay so we're talking about the most massive

gravitationally bound objects in the universe these clusters and because of that whenever gas falls into the cluster

it gains a lot of energy because of the potential well and as it does this it

gets heated up to extreme temperatures it gains a lot of velocity this translates to a very high temperature

gas and so much there so massive that the gas gets heated up to about 20

million degrees Fahrenheit and even higher okay and it's simply because they're extremely massive and anything

that falls in there is gonna gain a lot of speed it's gonna get heated up and it's gonna meant a lot of x-rays and so

this is some of the objects I've been studying and the x-ray emission actually tells us a lot about what's going on and

I'm going to show you two typical examples of what we see of the x-ray emission and clusters of galaxies

and so the first one is a very famous cluster named the bullet cluster okay so some of you may have seen these images

and so the bullet cluster was discovered as several years ago and it's very

interesting because what you see is this is an optical image so it traces the galaxies you see there's a bunch of

galaxies that seem to be kind of linked here and then there are another group here so what's going on so basically

what I'm going to show you next is the same image but in the x-rays this is

what you see ok so what this tells us is that yes okay clusters emit x-rays but

there's a lot of structure going on here there's this very weird shape structure

here and what this actually looks like is what we call a shock front very

similar to what happens when a bullet goes through the air or water creates a

shock front here ok so what's happening okay basically in this cluster here is

that initially this massive structure consisted of two smaller clusters of galaxies so one cluster that contained

these galaxies and another one contained these galaxies now what happened is that when these clusters collided okay so

they they literally collided against each other what happens is that the galaxies are what we call collision

lists so it's very rare that two galaxies are you going to directly hit each other and so they're just going to pass through okay so galaxies just

passed through and so what you see is one bunch of galaxies here one bunch here so they just pass through however

the hot what we call intra cluster gas the one that you see in the x-rays is

not collisionless this gas will heavily interact okay and so what happens is

that at some point one of the clusters went faster than the speed of sound broke the sound barrier and created one

of these shock fronts here and this is what you're seeing so very similar to what happens with bullets but we're

seeing this on astronomical scales okay so it's really gigantic gigantic skills

so we're talking about much much bigger than the size of an individual galaxy now the other kind of structure that

see that I'm very very interested in okay and so again this is based on the

x-ray mission that we've seen clusters of galaxies so to show you this kind of emission I'm gonna focus on one of my

favorite objects which is the Perseus cluster so the Perseus cluster is again a very massive cluster of galaxies

contains hundreds to thousands of galaxies all bound by gravity the center of the class the cluster is actually

located here okay so this is a beautiful image very big cluster but the center is

here and as you can see there's something very strange going on with this galaxy it doesn't look like the

other galaxies so what we're gonna do is we're gonna zoom in right here on this galaxy so this is what we see so this is

the central galaxy in the Perseus cluster okay so again Perseus cluster contains

hundreds of galaxies and basically what you see is you have a lot of structure okay and you see these beautiful

filaments here and a lot of people including here at Stanford study these

filaments because very difficult to explain how they got there but I'm gonna focus on what we see in the x-rays okay

because we're tracing the hot universe so the same image but I'm gonna show you the x-rays so we definitely should see

some kind of x-ray emission because we know clusters emit x-rays and so this is what we see okay

so again I don't know if you were expecting this but this is for me one of my favorite images okay and I'm gonna

explain what's going on so this is the x-ray emission in blue okay so you see a lot of x-ray emission and this is coming

from the hot intra cluster gas but you see a lot of structure here here here

now what we're gonna do is we're going to look at the same image but I'm gonna overlay what we see in the radio because

maybe this is gonna tell us a bit more information but what's going on and so this is what we see in the radio so the

pink blobs are where all the radio emission is coming now we're calling that the radio emission is a very good

indicator of synchrotron emission so the emission coming from the Jets created by a black hole so what's actually going on

is you have the central galaxies this galaxies here contains a very active

supermassive black hole this supermassive black hole is creating Jets of relativistic plasma that we see in

the radio but as these Jets propagates through the hot intra cluster gas they

literally push the hot gas away okay so they're literally just pushing it away

doing work against them the medium here and this is what we're seeing so you

have the black hole Jets and they're pushing all of this gas away and these are what we call bubbles in astronomy or

x-ray cavities and this is the kind of object that I study now the reason why

this is so remarkable is that before if we did not have the x-ray information if

we only had the location of the galaxies here and only the radio we did we wouldn't know how the radio actually

affects the surrounding medium okay because we still don't understand exactly what Jets are made of or how

they're made however we see through the x-rays the actual work that the Jets are

doing on the medium so we literally see that they're pushing it away and so we can quantify just how energy you need to

literally push away this amount of gas okay and just on the scale indicator here this is a typical size of our

galaxy okay so these bubbles are the size or even bigger than the size of our

galaxy and these bubbles are create being created by one single black hole

so this is just how powerful they are now what's really interesting is that as

you can see the structure here these this structure way up there actually represents an older cavity okay which

has risen buoyantly and this is exactly what we see with bubbles in water okay so bubbles and water what they do

is they rise towards the surface they just rise there and this is exactly what we're seeing here these bubbles just

rise through the inter-cluster medium towards the medium that's less dense and so

they're just gonna rise up and this is what we're seeing this is very interesting except we're seeing again on

astronomical scales so much different skills now one interesting discovery

that was made a couple of years ago is looking at this Perseus cluster of galaxies and looking at the

x-ray emission that you're getting away from this bubble here so this these are the bubbles that we were seeing before

but if you consider this region here and you study the kind of emission that

you're seeing as a function of what we call radius so as a function of how far you're going away from the bubble well

you see something very interesting okay you see this kind of emission so this is

the amount of x-rays you're getting okay as a function of what we call the radius so the distance from the bubble as you

go further away why you see our waves okay these are gigantic waves okay these are

much larger than our own galaxy so very very big waves and actually what's

interesting is that x-ray emission this basically is telling us that the density of this gas is varying according to this

and it's also telling us that the pressure is essentially changing now

what do you get when you see waves of changing pressure they're basically

sound waves but in outer space and for

those that yes exactly for those that have any talent in music

fortunately I don't at all but this is the kind of sound that you would hear from these sound waves so our ear cannot

hear them but just to give you a kind of indication of what you would hear so

this is really interesting things now you can actually see these sound waves if you play around with the image so you

have your image here of the Perseus cluster this is another more zoomed out

image of the x-ray machine you're getting so the bubbles this is the outer bubble I showed you now there's actually

a lot of structure around these bubbles here okay and if you play around with

the image to try and bring out these structures a bit more this is what you get okay so this technique here is

called unsharp masking it's a very common technique that we use in astronomy to try and bring out all the faint features it's actually a technique

that's used also in biophysics to try and bring out faint features in for example MRI scans and so this is just

some that we use and you see these beautiful waves just coming out of the cluster and

these are sound waves and these are sound waves being generated by the creation of these bubbles as they're

being created and they're created by the central supermassive black hole now the question is just how powerful are these

actual sound waves and these cavities so we see that they're gigantic so they should be extremely energetic and I'm

going to show you just how powerful they are and so to kind of give you an example in comparison of just how

powerful so if we take a typical energy of an atomic bomb so this is one of the

most energetic phenomenon that we know of on earth and so we know that this is a kind of energy that one atomic bomb

liberate okay so we're talking about 10 million tons of TNT now I'm gonna show

you another example just to show you how energetic these things are but for this

example we're gonna take the Sun okay so the Sun is very bright okay and the sky

is emitting a lot of energy now let's assume that it's emitting this kind of energy for its entire life okay so we

can get an estimate of just how much energy the Sun will have liberated in its entire life and the kind of number

that we get is gigantic okay so there should be 28 zeroes in there okay and I

actually looked up and there is this number exists it's called an octillion I had no idea before but it exists and so

this is the kind of energy you get from the Sun now I'm going to show you the kind of energy you get from one single

bubble created by the supermassive black hole this is the kind of energy you get

so there's 37 zeroes in there okay this is known as an onion gigantic number

okay it's hard to conceive just how energetic this is and all of this energy is coming from one single object okay so

extremely massive so this is the kind of thing that I study now what's really

interesting is that I'm going to take this Perseus cluster here so this is a

very nearby cluster this is why we study it so much because we have such great data on it and I'm going to show you on

the same scale okay another cluster with another set of cavities so this is call

MSO 735 it was discovered quite some time ago but it's very interesting why

because the bubbles in here so what you're seeing is the x-ray emission okay

and the blue you see the radio here in the pink colors these cavities here are

the bubbles that we were seeing very similar to in Perseus however this is the appropriate scale okay so these

cavities here are 10 times larger than the ones in Perseus and again these

cavities are being created by one single black hole located at the centre of this cluster so just imagine the kind of

energetics and this is an indication this little diagram here is the size of

our galaxy okay so this is what we're dealing with these are the monsters these are the biggest monsters among the

monsters this is what I kind of what I study now I've studied a lot of these

objects here and what's interesting is that you can ask the question what kind of black holes you need to be able to

create such big cavities okay and so what we find is that you actually need a

very particular kind of black hole and so I'm going to come back here to this diagram that I showed you earlier of

what black holes look like so this is the event horizon which defines the radius within which nothing can escape

not even light the size of this horizon depends on the mass of the black hole

the bigger the black hole the larger the event horizon okay

what's actually interesting and that I didn't mention is that black holes are actually quite simple mathematically

okay why because they are completely characterized by three parameters the

mass okay well and then the other parameter is what we call the spin the spin measure is just how fast the black

hole is spinning around itself these quantity is it tends to be what we call

a normalized quantity so zero means it's not spinning ones mean means it's spinning at its maximal value and this

is related to the fact that a black hole can't spin faster than the speed of light and nothing can go faster than the speed of light so that determines an

upper bound now the charge is technically mathematically a third parameter however we think that all Astrophysical

black holes have a charge equal to zero the reason for this is simply because when we look at the universe we see that

it's essentially largely neutral okay there's nothing that's tends to be charged according to a certain charge so

black holes if one initially was created and had a charge a positive charge for example it would quickly neutralize

because on average the universe is neutral so we think that essentially Astrophysical black holes don't have a

charge so we're left with two parameters and these two parameters completely determine the properties of the black

hole it determines just how powerful they can be just how powerful it can create Jets determines also just how

much mass it can accrete at least the maximum value and so when we look at the be the big cavities here we can ask

ourselves what kind of black holes do we need to create them and so what we come up with is actually quite interesting is

that it seems that an easy explanation would be either that it's highly spinning or as we'll see it's related to

the mass and so what's interesting is that astronomers can actually measure the spin of black holes

okay now this now it may sound weird because how do you measure the spin of an object you cannot see okay but we can

do this and the way we do this is we use general relativity so it's very

interesting because general relativity predicts that the kind of phenomenon

that's gonna happen related to the accretion disk varies depending if the black hole is spinning or not so if you

take a non spinning black hole what happens is that the material as it gets secreted

can only a creep to a certain radius this radius is known as the inner most

stable orbit what happens is that as soon as the material crosses this orbit it just plummets directly to the black

hole however what happens when you deal with spinning black holes this inner

most stable orbit changes what happens with spinning black hole is that there's a very interesting effect okay called

frame dragging so spinning black holes as they spin they tend to drag with them

space-time okay and as a like that with them what happens is that the material gets dragged in closer to

the black hole okay so the inner most stable orbit here gets closer so when we

study black holes and we're looking at the x-ray emission coming from these accretion discs what happens is that when we look at

this kind of black hole the material only goes to a certain point near the black hole and so it gets heated up to

up to a certain temperature however when we look at spinning black holes what happens that the material goes closer so

it feels the gravity even stronger okay and it gets heated to even higher temperatures and we can actually measure

the difference in temperature between these two systems and this is how we can constrain just how fast the black hole

is spinning and so this has been done for about 30 systems maybe 40 systems so

far where we've actually measured the spin of a black hole it's been done for stellar-mass black holes so black holes

in these binary systems which are tearing up a star but it's also been done for supermassive black holes and we

tend to see a variety kinds of spin so they're not all highly spinning they're not all non spinning it's a variety and

so astronomers are trying to understand is there a link do we that does this tell us about maybe the accretion

history of the black hole so they're trying to do this and so the question is for our big cavities an easy way to

create very powerful Jets is if you have a spinning black hole and the reason for

this is very interesting is that actually the higher the spin the more powerful the Jets that can create so you

can create very powerful Jets simply by having a spinning black hole now the other possibility that astronomers have

envisage is that perhaps what we're dealing with are is simply the most massive black holes in the universe okay

so these are extremely powerful they were located in the most massive galaxies in the universe which are found

at the Centers of these clusters so they should have very massive black holes and so the question is just how massive and

there have been a couple measurements of these black holes at the Centers of clusters so far and I'm going to show

you what kind of numbers you get and but before I do that just to give you a kind of scale of what how just how big they

are so basically I'm gonna start with the stellar-mass black holes so we know that they have typical masses between three

and four times the mass of our Sun now recalling that the size of the black holes with the event horizon so the

radius within us nothing can escape not even light it's given by this equation here well you get four stellar-mass

black holes is that their size is essentially the size of the bay area approximately okay so big but not that

big if you take supermassive black holes okay and we saw one example of a very

famous one located at the center of our galaxy called Sagittarius a star we saw

that the mass of this black hole was about four million times the mass of our Sun so significantly larger so if you

plug this into this equation what kind of size of the black holes you get you

basically get about twelve million kilometers which is approximately twice the size of the Sun so you have two suns

that fit within this black hole here okay and now for the biggest black holes

okay so the biggest black holes that we've measured so far so the masses are about ten billion times the mass of our

Sun okay so gigantic black holes they're about ten thousand times bigger than the

black hole at the center of our galaxy this is the kind of measurements that we get and two of these gigantic black

holes are located at the Centers of clusters of galaxies and so just to give you an idea of what this kind of scale

means okay so this is the size of the black holes but I'm plotting here and this little arrow here is Neptune's

orbit so basically this black hole is several times the size of our solar

system it's humongous all the hue huge black hole several times the size of our

solar system it's very hard to envisage just how big it is but it's huge okay

and so these are the monsters among the monsters these are the really gigantic black holes and we know of at least four

of these that exists in outer space and two of them are at the Centers of clusters of galaxies

now I'm gonna just to finish on that bang I'm gonna end here I will take

questions but I just want to let you know that there are a couple people here that will also gladly answer questions

and so they're all wearing helmets okay yes so there are three of them here

today unfortunately one had to cancel due to emergency but essentially there is arena here I will try to pronounce

her last name Judah's leva she's actually working on clusters of galaxies

and she's finding very interesting results concerning the outskirts of clusters and concerning the sound waves

that I showed you and so do ask her questions after the talk Andre here is a

graduate student at Stanford he is also working on clusters of galaxies and is

very interesting he's actually published a nature paper so this is a very prestigious journal he published a nature paper earlier this year

concerning a very interesting result about clusters so do ask him questions too about this paper and there's a

finally Sam Skillman so he's the theory guy he's the guy who simulates what we

think is going on with the physics of clusters collides clusters and simulations so he's a theory guy so

please feel free to ask him questions too and I will stop here

SLAC National Accelerator Laboratory