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Black Holes are really only dangerous if you get too close
Who am I kidding ?
It turns out that they may be responsible for ending Star formation across the entire Universe
When we first realized that Black Holes could have masses of Millions
or even Billions of times that of the Sun
It came as a bit of a shock
they were discovered as the driving force behind Quasars
where matter is heated to extreme 
Incandescence
before it plunges into vast Black Holes
But if that weren't enough we soon realized that
every single decent sized Galaxy contains such a Supermassive Black Hole
While in the beginning of the Twenty-first Century
it became clear that Black Holes and the Galaxies that contain them
are very closely connected
The Bigger the Galaxy, The Bigger Supermassive Black Hole
that might not sound surprising
but what was weird was how closely they were connected
there's a tight co-relation between the mass of the Central Black Hole
and the mass of the star in the galactic bulge
that's the central bowl like part of a spiral galaxy
or the entirety of an Elliptical galaxy
and every bulge contains a Supermassive Black Hole
around 1/1000th of its mass
and there's an even tighter relationship between the Black Hole mass
and the speed that stars are moving in their random orbits within galactic bulge
the so called, Stellar Velocity Dispersion.
which itself depends on the total mass of the Galaxy including Dark Matter
so why shouldn't a Galaxy and its Black Hole be closely connected
A couple of things you need to know
Even though Supermassive Black Holes are Big
they're peanuts compared to the Galaxies they live in
their Gravitational influence should only extend to the stars
right at the very centre of the galaxy
they definitely aren't directly responsible for the speed of stellar orbits
outside their local region
Sure, Galaxies and their Black Holes probably grew together
but galaxy formation is an extremely messy process
so you'd expected a huge amount of variation in
Black Hole vs. Galaxy mass
Because of all of this,
the tightness of the Black Hole-Galaxy connection
has been a decades-long conundrum
that still is not solved
But one of the leading ideas
is that Central Black Holes
somehow extend their nefarious influence throughout the entire Galaxy
disrupting its ability to grow
To understand this
We first need to understand how galaxies formed
in short,
Small Galaxies collapse from the gas of Big Bang
then they smash together to make bigger galaxies
this is 'Bottom up Hierarchical galaxy formation'.
As opposed to 'Top-down'
which would have the large galaxies collapsing directly from the gas
Now, Based on our understanding of Physics of the Universe
especially the nature of Dark Matter and Dark Energy
we expect 'Bottom up Galaxy formation'
this is what we see in our simulations of the Universe
and also mostly agrees with what we see when we look out into the Universe
so galaxies first grew in clusters
in the densest parts of the Universe
Places where enormous worlds of Dark Matter
pulled in great rivers of material from all around
gas poured into the clusters from outside of the Universe
igniting bouts of extreme star formation called Star Busts
as galaxies grew so did their Black Holes
they would've started as a ready mass of seed Black Holes
formed by the very first generation of stars
they would fall to the centres of their local wispy protor galaxy
and when wisps merged their central Black Holes would also merge
all the while gorging on the rich gas supply in the early Universe
So, Black Holes grew as Galaxies grew.
then why is it so surprising that they appear so closely correlated
in the Modern Universe.
Two Things:
In order for the Black Hole to grow in lock step with the galaxy
you need to get a consistent proportion of new material
down into the centre of the Galaxy
That's challenging, given how messy and varied
the whole galaxy assembly process seems to be
Second
Observation seem to indicate that early Black Holes
actually grew faster than their surrounding galaxies
The 'Galaxy Black Hole Mass Relationships' seem to evolve
through the history of the Universe
many Supermassive Black Holes were in place early
leaving the surrounding galaxies to catch up
like a puppy growing into its giant feet
in fact we've seen quasars shining out from less than a Billion years after the Big bang
with masses of 10 Billion suns
Easily as large as the largest in the modern Universe
so if Black Holes and Galaxies are not growing in lock step with each other
from the same material
and they can't directly influence each other with gravity
how do they know what size to be?
how does a galaxy know to stop growing when its 1000 times larger
than the Central Black Hole?
perhaps the best contender is that the black holes kill galaxies
and by "kill" , I mean make them dead
which I guess is the usual sense of the word Kill.
But in Astronomy, a dead galaxy refers to its current star formation activity
in particular, the largest galaxies in the Universe
are the Giant Elliptical's
and we say they are 'Red and Dead'
not because they contain lots of old west themed planets
that's not yet known
but rather, its because they appeared to have almost no active star formation
and haven't had any for long time
The short-lived hot massive stars that give Spiral galaxies, like the Milky way,
their blue-white sheen
have long since died out in giant ellipticals
leaving them tinged with the orange-red colours of longer lived cooler stars
but from what we can tell those giant galaxies should've kept forming stars
Giant reservoirs of gas flowed into those clusters
from the outside Universe
In our simulations of the Universe
clustered ellipticals end up much bigger and bluer
Due to Billions of more years of ongoing star formation
A likely culprit is the Black Hole
it kills star formation in those giant galaxies
fixing the maximum size they could grow
the process is called 'Quenching'.
There are couple different ways it can happen via Black Hole
but all of them involve a Quasar switching on and blasting the crap
out of the galaxy's gas
rewinding a bit to the whole galaxy formation thing
when galaxies collide or even get stirred up a bit by eating a smaller galaxy
gas tends to get driven down to the center where its gobbled up by
The Black hole
well, some of it is
close to the Black Hole the gas forms a whirlpool-like accretion disk
heated by the energy that's long formed
it gets so hot that around 10% the mass of the infalling gas is converted to energy
in the form of light
and yet more of the gas is blasted outwards by
that same light
in colossal winds that rip through the surrounding galaxy
or its channeled into jets by powerful magnetic fields.
A Quasars's intense brightness and its powerful winds and jets
can dump an enormous amount of energy
into the gas through the surrounding galaxy
that can do two things
It can drive gas out of the galaxy and prevent new gas arriving
or it can heat the gas
either case is bad for star formation
in the later case is because hot gas can't form new stars
gas has to cool down before the force of gravity  can cause it to collapse into the stars
too hot and it stays puffed up
as well as killing star formation
quasar activity limits the Black Hole's own growth
it grows relating the new supply of gas
but then its energy output stops new gas falling into the Galactic centre
in the end you have this balancing act
a feedback process
the more massive the galaxy, the more stuff it can pull in to feed itself in the Black Hole
but the more mass of the Black hole, the better it is at shutting down the star formation
and its own feeding
the end result is that Black Hole mass and Galaxy size
are closely linked
and in the largest galaxies our red dead ellipticals
star formation is entirely shut down
whether or not black holes kill star formation or at least were the main culprit
is not established
but we do know that the star formation died
or is dying
not just in largest galaxies but across the universe
after the first stars formed around 150 million years after the big bang
the rate of star formation across the Universe slowly rose
it became concentrated in the most denser region of the Universe
where giant ellipticals assembled
the rising storm of star formation reached a crescendo around
8 to 10 Billions years ago and remain intense for few Billion years
following that the work of star formation shifted towards the
outer regions further from the giant clusters
for the past several Billion years the cosmic star formation rate
has been dropping
and continues to do so
this is mainly due to star formation shutting down in the densest parts of the Universe
which as I mentioned is strange
given i should still have plenty of gas
there are other explanations besides quasars and Black Holes for the shut down
star formation has to be a somewhat self-limiting process because
intense starbursts leads to intense supernova activity
which could also heat or expell gas and limit further formation
for the largest galaxies the shock experience
by gas falling in from the outside universe can be enough to stop stars forming
all of these probably play some role in killing star formation
they are accomplices to the crime
but many astronomers think Supermassive Black holes are the main suspect
here's one final clue
compare the cosmic history of black hole growth
to that of star formation
they grow together and peak at the same time
around 10 Billion years ago
right before star formation begins to die
now some very massive black holes existed earlier than this
somewhat perplexingly
but this peak in black hole growth represents the peak of the quasar epoch
when the most outrageously luminous quasars shone from the most
ridiculously massive galaxies.
Surely enough to shut down those galaxies' further growth.
In the modern universe Giant Dead galaxies harbor
fossil quasars
supermassive Black Holes whose close connection to their surrounding galaxy is a clue
incriminating evidence suggest a long and violent struggle
a feedback cycle of growth and quenching
that neither quasar nor star forming galaxy would survive
and perhaps for the best - for the comfort of the life bearing world
to leave raging starbursts and fiery quasars to an earlier epoch
of Space-Time.
Learning astrophysics is tough
thankfully there are online tools that can help
like Brilliant a problem solving website that teaches you to
tackle difficult topics and think like a scientist
By breaking up complexities into understandable pieces
and instead of just passively listening to lectures
you get to master concepts by solving interesting and challenging problems
so whether you wanna learn about special relativity, quantum physics
or brush up on your complex algebra and differential equations
you can learn more at Brilliant.org/spacetime
Last time we talked about all the cool elements that get made when neutron stars collide.
Your comments
were gold.
A number of people wondered whether the Fermi Paradox
might be explained by the fact that neutron star collisions are rare
so that only lucky parts of the galaxies have
a high abundance of the products of those collisions.
actually...
although these collisions are rare
they are common enough
that most of the galaxy gets a decent amount
of all the stable products of neutron star collisions.
On the other hand,
the presence of highly radioactive stuff
with very short half-lives
is a telltale sign of recent merger.
But, that stuff probably isn't essential to life.
Speaking of half-lives, Anthony Schroeder picked up a silly mistake
I said that the half-life is the average decay time of a radioactive element
It's not.
It's the amount of time for half of the given sample to decay.
And that's not the same as the average.
In fact, it's longer by around 44%
or one over the natural log of 2
Thanks Anthony, I guess I need to retake nuclear physics.
Tricky asks us why we don't see more strange matter from neutron star collisions.
Now, this is a cool point.
It may be that some neutron stars are actually "Strange Stars"
whose nuclear material is composed of strange matter
which means up, down, and strange quarks.
If such a star was disrupted,
then we would expect to see blobs of strange matter in the debris.
But the thing is, strange matter is probably only stable under extreme pressure
Release it from the embrace of the neutron star
and all the strange quarks probably decay into boring old up and down quarks
which would join together to become protons and neutrons
which means you are back to where you started from.
Even if strange matter is stable as so called "Strangelets"
These are electrically neutral, so they don't have an electromagnetic signature.
By the way, you wouldn't actually "see" strange matter.
But you might see the result of it's decay
a particular spectrum of radiation and cosmic rays.
or in the detailed shape of the gravitational wave signal before collision.
People are looking.
Nadira Rahman notices that their cat loves the sound of my voice
and moves closer the the screen every time
Actually that's just because Mittens can sense my allergy and like all cats
is drawn to my suffering.
