Thanks to Brilliant.org for supporting PBS Digital Studios 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 as 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’re definitely and 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 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 is 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 massive 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 that 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 short lived hot massive stars in Spiral galaxies like the Milky way their blue-white sheen have long since died out in giant ellipticals leaving them tinged to 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 its centre where its gobbled up by The Black hole 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 field 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 the 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 to the Black Hole but the more mass of the Black hole, the better it is for shutting down the star formation and its own feeding the end result is that the Black Hole mass and the 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 galaxxies 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 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 plays some role in killing star formation they are accomplices of the crime but many astronomers thinks 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 peek 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 peek at black hole growth represents the peek of the quasar epoch when the most outrageously luminous quasar shone from the most ridiculously massive galaxy surely enough to shutdown those galaxy’s 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 live 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 and interesting 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” who’s 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.