Galaxies get fat and weird along with their black holes

By Phil Plait | June 2, 2008 4:01 pm

I mentioned earlier that all big galaxies have big black holes in their hearts. I also mentioned that the size of the black hole is related to various features of the galaxies, but that might be a little weird because, after all, galaxies don’t live in a vacuum*. There are other galaxies out there, and they sometimes collide, forming bigger galaxies. So why should there still be a relationship between the black hole and the galaxy it sits in?

Well, the idea is that when two galaxies collide and merge, so do their black holes. Complicated physical processes tend to favor the two black holes getting ever closer, until they eat each other and become one somewhat fatter black hole. At the same time, the galaxy formed by the merger around the two black holes also grows.

That’s the idea, at least. But there’s a way to see if that’s what really happens.

For example, when two galaxies collide and merge, their gas clouds collide. This can collapse them, forming stars. But since the event happens over a short period of time — compared to the age of the galaxies, at least — it’s called a star burst, or a burst of star formation. Massive, luminous stars form this way and stay bright for a few hundred million years.

Also, at the same time, it’s expected that a vast amount of gas will fall to the center of the new galaxy. As this junk falls into the black hole, it forms a flat disk which gets incredibly hot and bright. The inner part of the disk can outshine the entire rest of the galaxy combined. This kind of galaxy is called a quasar.

So if this idea is right, you might expect to see some galaxies that are quasars, and show signs of having had a star burst a few hundred million years in the past. Not only that, you’d expect them to look a little funny, distorted and goofy-shaped after having suffered such a cosmic collision.

Enter Mike Brotherton, astronomer, science fiction writer, blogger and, may I add, BABloggee. He and his team figured that they could use Hubble to look at a bunch of these galaxies and see what’s what. They went through a sky survey that had 15,000 quasars (!) and culled them down to 600 that looked promising. From those they picked the best 29, and when they got the Hubble observations what they found was pretty convincing…

Those are three of the 29. Look at them! Weird, twisted, distorted; just as expected. In fact, all 29 were pretty weird (check ‘em out yourself). They all bear the scars of recent mergers, and support very strongly the idea that "post-starburst quasars" are the results of violent collisions between galaxies, and, furthermore, black holes and their galaxies grow together.

Take a close look at those images. Right now, even as you read this (assuming you’ve read this far), the Andromeda Galaxy is barreling toward us at more than 100 kilometers per second. In a billion years, plus or minus, we’ll plunge together in an event of epic violence. If there is any gas left in our two galaxies — I haven’t been able to confirm whether it would all be used up by then or not, but at least one paper I’ve read (and quoted in my book, Death from the Skies!, about this) said it’ll all be gone — then we too will become a starburst galaxy, and if any gas gets dumped into our merging black holes a vast amount of energy will pour out. It’s potentially enough to cause problems, should that energy be aimed our way.

However, we do still have quite a bit of time to work out any potential problems. I’m not too concerned about it. But what a view it’ll be…



*Well, OK, they don’t live at all, and space is a vacuum. But I’m being all metaphorical here, so try to play nicely and follow along.

CATEGORIZED UNDER: Astronomy, NASA, Science

Comments (53)

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  1. Press Conference Experience | June 3, 2008
  1. Mark Roberts

    Why did I think of myself when I read this title?

  2. davidlpf

    You forget it there are no blackholes, the universe is electric and electric currents cause galaxies to collide not gravity. (just a joke)

    Cool find though.

  3. Ryan

    Perhaps the galaxy quasar pairs were once one very large galaxy that strayed into an area where it couldn’t stay together. Do black holes split apart in two ever?

    Also, is the idea that the galaxy-become-quasar has snatched the black hole from the center of the other galaxy and is running away with it? IE, does the other galaxy still have a black hole in the center after the quasar forms?

  4. davidlpf

    Ryan blackholes kind have a strong gravity that would prevent them from flying apart.

  5. Wildride

    Of course if Andromeda collides with the Milky Way, creating a quasar, what would the red spectrum tell us about it?

  6. davidlpf

    Wildride to mean if the spectrum is red shifted then it will tell us it is moving away from us.
    If you mean it is giving most of the light in the red part of the spectrum then the temperature is cooler then our sun, probably either a protostar or a low mass star.

  7. davidlpf

    wildride it could also mean there is a lot of dust between us and the object, dust scatters the shorter wavelengths and the longer wavelengths usually pass through better. Red is a long wavelength part of the visible spectrum radio usually passes through better because it is even a longer wavelength.

  8. Mark Tyler

    Is there an average age/distance/redshift for the objects in the picture? Were all these pictured collisions in the very distant past or are some of these current. Does the red color indicate a high redshift?

    Ok, just direct me to the paper.

  9. davidlpf

    from the linked webpage above in BAs post

    “Hubble snapped pictures of 29 post-starburst quasars in total, using a red-light filter that emphasized the starlight over the glare of the bluer quasar.”
    heres the link again
    http://www.mikebrotherton.com/

  10. Rimmer

    Don’t worry Wildride, some of us got your reference.

  11. davidlpf

    Now I feel like a real smeghead.

  12. Nathan Myers

    Galaxies do not, in fact, exist in a vacuum; they exist in a cloud of low-density plasma. You may find that inconvenient to think on, but the fact remains. Also perhaps inconveniently, the low-density plasma in its vicinity out-masses all the stars and black holes in galaxy itself. Your colleagues will all agree with you that all that plasma can have no possible effect on anything whatsoever, which might be a big relief to you, but the universe itself is not obliged to agree with them.

    More pointedly, if there really is a billion-solar-mass black hole there, it’s ludicrous to imagine it even slows down perceptibly when galaxies “collide and merge”. What force is supposed to operate on this billion-solar-mass object, passing thousands of light years away from the black hole presumed at the other galaxy’s center, to cause it to lose escape velocity? Even if you manage to stop or scatter its stars and its “gas” and dust, the black hole itself would have to keep going on about the same trajectory it entered with. (No, you don’t get to piss its kinetic energy away as gravity waves.) The result of all these galactic collisions would have to be naked black holes blundering about intergalactic space.

    This isn’t Star Trek. When you turn off the motor you’re not immediately sucked into the nearest gravity well, not even if it’s a black hole. Not even if *you’re* a black hole yourself.

  13. davidlpf

    The thing is the combined mass of the galaxies is a lot larger then either blackhole, therefore the gravity well for combined galaxy has more an effect on the blackhole. Also the linear kinetic energy is converted to angular kinetic energy. No you do not fall into a blackhole if you are outside the event radius, but once inside the event horizon then you fall into the blackhole.

  14. Nathan Myers

    davidlpf: So you’re saying each black hole’s event horizon magically extends out over thousands of light years to lasso the other? And linear kinetic energy is physically different from angular? Is this TOS, TNG, or STV? And what happens if we reverse the polarity?

  15. Zeugnitz

    Nathan, you seem to be confused.

    The central black hole doesn’t represent a majority of the total mass of a galaxy, it is only a tiny part of it.

    The event horizon has nothing to do with anything other than what we can observe. Gravitationally, it isn’t special; the gravity of a black hole extends far beyond it’s event horizon. If the Earth were suddenly compressed into a black hole, the Moon would continue to orbit the Earth and we’d continue to orbit the Sun, the only difference would be that the Earth would no longer reflect or radiate light.

    If your thinking is based on the fact that the central black hole would require a greater force to affect compared to most individual objects contained in the galaxies due to its mass, you should re-read your basic Newtonian physics. The reason the Earth’s gravity can be expressed simply as a constant acceleration is that the mass of the object being “acted upon” cancels out when you do the math. This is roughly correct on a galactic scale, as well, so the effect of the gravities of both galaxies would affect the central black holes in the same way as they affect every other thing inside those galaxies.

  16. Davidlpf

    Lets start off with some basics physics.First gravititional potential energy is the energy between two objects due to gravity. then there is Kinetic energy which is the energy from the object in motion.
    Now here is a litte experiment you can do yourself. Take a garbage can and spread a garbage bag ove the opening. Place something in the center like a grapefruit, then take a much smaller object like a marble and roll in straight line near the edge. The smaller object will start to go in circles. In space the garbage bag would be space-time the grapefruit the sun and the marble the earth or on a larger scale the grapefruit would center of a galaxy and the marble a star. The identation in spacetime is called the gravity well or potential well. When two objects are in orbit about each other the potential energy and kinetic energy are balanced.
    Now imagine galaxy A is not stationary and galaxy B is headed towards A. Each galaxy does have a large blackhole in their centers. When A and B collide gas clouds will generate new stars, older stars will combine into new stars creating the starburst. Now the gravity well for the two galaxies combined would be larger then one alone. The blackhole in B will find an orbit were the potential and kinetic energies are balanced. But since the two blackholes have close to the same mass they will attract each other. Blackhole A will start an orbit and the speed of the orbit will increase. Blackhole B will lose energy and start going closer to center of the system. Eventually the two blackholes will orbit each other faster and faster and closer and closer because the periods squared equals the length of semi major axis cubed (Kepler look it up). After a period of time the two will combine into one larger blackhole. Okay it happen faster if the blackholes start closer to each other they start out in after the collisions.

    Now what is next shot about Star Trek.

  17. Carl C Fields

    I wonder if the Andromeda galaxy IS indeed headed directly toward us. I realize there is a radial (directly toward us) component of Andromeda’s relative velocity. However, is there also an azimuthal component to the motion (which means we would “miss” each other, at least on this “pass”). That might provide postpone the collision by a few billion years.

    I sort of understand how the radial component of the relative velocity is measured. Is it POSSIBLE to determine an azimuthal component of the relative velocity for such a distant object (in the few-hundred-year time period since highly accurate sky coordinates for Andromeda have been available)?

  18. Mark Wilson

    @Carl.

    A quick back of the envelope calculation shows that a proper motion of 100km/s would show up as a position change of approximately 3 milliarcseconds per decade.

    A Wikipedia article led to http://www.journals.uchicago.edu/doi/abs/10.1086/491644 which places an upper bound on the proper motion at 100km/s based off measurements of M31 and M33.

    I’m not an astronomer, so I can’t comment on the reliability of the methodology or measurements included in the article.

  19. However, we do still have quite a bit of time to work out any potential problems.

    It sounds like a job for Bruce Willis. ;-)

  20. Thanks for the coverage, Phil. I crashed hard last night after a long tiring day and didn’t see this right away.

    Some interesting discussion, and I think mostly on the right track in the end. Black holes are indeed a small part of the mass of the total galaxy, and it’s the total gravitational potential that drags them along. The issue of merging black holes is complex, and there are some strange steps in which gravitational radiation makes them lose energy and come together.

    And I’ve just slept through the start of Tuesday morning activities here. Better get running!

  21. The Mutt

    I know college was a long time ago, but didn’t I learn that everything in the universe is moving away from us (and everything else)?

  22. “As this junk falls into the black hole, it forms a flat disk which gets incredibly hot and bright.”

    This is something that I’ve always had trouble visualizing. Why does stuff that’s gravitationally attracted tend to form flat disks? This seems to be the case at both the galaxy level and the solar-system level. My intuition says galaxies should be roughly spherical, not flat disks. My guess is that it has something to do with rotation — i.e. an initial sphere under rotation would bulge out a bit, and this bulge somehow cascades until you get a result that’s nearly flat. Then my next question is, why do galaxies rotate?

    (Obviously the short answer to all these questions is simply “gravity” :) )

    And also, if we step above the level of galaxies, and look at sets of gravitationally bound galaxies, are the galaxies in a set roughly positioned in a plane as well?

  23. Irishman

    The Mutt, while overall things are moving away from each other, local “objects” can be getting closer together. Take a bucket of sand and fling it in an arc. While all the sand particles are spreading away from each other, there are still a lot of collisions between sand particles that are next to each other.

    Colin M, I believe the answer is angular momentum. If the cumulative motion of all parts is not exactly balanced, the imbalances of the motion will start a rotation. The gravitational pull will prevent the parts from separating, and as they get closer together the angular momentum draws the rotation into a disk. Essentially the motions crossways to the overall system rotation get damped by the gravitational pull but the motions in line with the system rotation do not.

  24. Tom Marking

    “But since the two blackholes have close to the same mass they will attract each other. Blackhole A will start an orbit and the speed of the orbit will increase. Blackhole B will lose energy and start going closer to center of the system. Eventually the two blackholes will orbit each other faster and faster and closer and closer because the periods squared equals the length of semi major axis cubed (Kepler look it up). After a period of time the two will combine into one larger blackhole.”

    It’s important to note the relative sizes of the black holes in the Milky Way galaxy and Andromeda galaxy versus their separation distance. The Schwarzchild radius is approximately 3 km x the mass of the black hole in solar masses. Thus for the Milky Way black hole we have 12 million km (1.2E-6 light-years) and for the Andromeda black hole we have 600 million km (6.0E-5 light-years).

    Now, the odds of two objects with a diameter of less than 0.001 light-years approaching each other from a distance of 2 million light-years hitting each other head-on is almost infinitesimal. Much more likely is that they enter into some kind of elliptical orbit around one another. In order for the black holes to merge some kind of energy dissipation mechanism must take place. It may be the drag effect caused by travelling through the interstellar medium (although I’m not sure that can even affect a black hole) or something else, but without it the black holes have no probability of merging. I’d like this dissipation mechanism better explained. Also, have astronomers actually seen the merger of two galactic black holes before?

  25. Davidlpf

    Tom, what you quoted above is was meant after the two galaxies merge not when then millions of light years apart. Yes the intersteller medium between probably would help take away some the energy from the two.
    Sorry for the lack of clarity I wrote it at the end of a day when I was up for twenty hours and slightly peeved at whatever his name and his startrek insults.

  26. Tom Marking

    “Tom, what you quoted above is was meant after the two galaxies merge not when then millions of light years apart. Yes the intersteller medium between probably would help take away some the energy from the two.”

    Depending upon how the two galaxies are aimed at one another the two central black holes don’t have to come anywhere near each other after the galaxies merge. What happens if the central black hole of the Andromeda galaxy is headed straight at our sun? It will be 26,000 light-years from the Milky Way black hole after the galaxies merge. Without some mechanism to bring them together the two galactic black holes will never merge.

  27. Davidlpf

    When the blackholes get close together and finally merge then it is the realm of General relativity which I know very very little about but gravity is the donimant force.

  28. Tom Marking

    BTW, going from left-to-right and then top-to-bottom when I look at the 29 galaxies the only ones which show obvious signs of something weird happening structure-wise are: 4, 5, 7, 12, 14, and 20. The rest look like fairly typical galaxies.

  29. Wayne

    Mike Brotherton above mentioned the energy dissipation mechanism, “gravitational radiation”. Merging black holes are one of the few phenomena thought to produce sufficient gravitational radiation to be seen by current gravity wave detecting technology, although I haven’t heard that they’ve “seen” anything like that yet.

  30. Nathan Myers

    davidpjf, Zeugnitz: You’re still in the Star Trek universe. Two black holes passing thousands of light-years apart just might have a calculable effect on one another’s trajectories. The gravitation of the rest of the other galaxy is even more negligible: the closer the black hole comes to its plane and center, the more the gravitational effects of all its bits cancel out. You still haven’t identified any force to cause the two black holes to dissipate their kinetic+potential energy. ISM friction is negligible. Gravitational-wave radiation is negligible until after the holes have already (magically?) started orbiting closely. You’re in danger of leaving ST:TNG’s universe and entering that of the underwear gnomes: “1. Collide galaxies; 2: ???; 3: Profit!!!”.

    Mike: You started this. What is supposed to strip each black hole of enough energy to get it into a close-enough orbit with the other for gravitational radiation to amount to anything?

  31. davidlpf

    Nathan get thee to some first year physics books.

  32. Nathan Myers

    davidpjf: Get thee to an AP calculus text. Channeling Counselor Troi isn’t helping you.

  33. davidlpf

    Already have plus spent a summer working for an astronomy prof ok I did go any further in science but it is probably more the you.
    http://www.aavso.org/publications/ejaavso/v29n2/73.shtml
    The name David Fairweather is mine.

  34. davidlpf

    Nothing Nathan, no clever star trek insults, what magical force do you think is donimant come on I can not hear you at least I have tried to explain things ok my explaination is not perfect either explain what you think is going on and we can have polite conversation or stop trolling.

  35. Tom Marking

    “ISM friction is negligible”

    The more I think about this the more I realize that InterStellar Medium friction is zero for a black hole. If you think of an airplane wing moving forward through the air you can think of the air molecules hitting it head on and then rebounding backward. The molecules thus have a net momentum vector in the forward direction of wing motion. This must be compensated by an equal momentum vector imparted on the wing in the opposite direction (which we call drag).

    Now, replace the wing with a black hole. Do the air molecules rebound off it? Nope. There is no solid surface to rebound off of. Once they pass the event horizon they get sucked into the singularity. Maybe they cause some change in the linear momentum of the singularity but it would probably be offset by molecules falling into the event horizon on the back side.

    Of course, if the black hole theory is incorrect and the gravastar theory is correct now there would be rebound off the hard surface of the Einstein-Bose condensate (at least if the velocity is high enough, maybe?) and there should be observable drag. That may be one way to determine which theory is correct, by observing drag effects on black holes/gravastars travelling through a dense dust cloud.

  36. “Galaxies get fat and weird along with their black holes.” Big deal. Same thing happens to me when I eat too many Whoppers.

  37. Nathan Myers

    Sorry, Tom, ISM friction is negligible just because there’s not enough of it along the black hole’s path. Earth has orbited Sol for billions of years in a soup much thicker than ISM. Any friction felt by a gravitational body moving in the ISM (to the extent it feels it at all, and pretending it’s all magically neutral gas) comes from deflecting the particles it passes into different trajectories. The ones that hit it are negligibly rare vs. those that miss and end up going someway else than they would have.

    If they’re going to piss away a giga-solar-mass black hole’s kinetic energy, they’ll probably need to have it altering the trajectories of random stars it passes. A billion-solar-mass object passing a half light-year away would exert the same gravitational force on Earth as does the sun now. That is, planets and sun would all experience as much acceleration toward the black hole, during the time it’s nearby, as the earth does toward the sun now. I could imagine that extracting a fair bit of energy after a few billion passes, but why didn’t Mike say so?

    David: Let me get this straight. I’m supposed to propose my own “donimant” magical force to counter your unidentified “donimant” magical force? What exactly did this Prof. Turner teach you?

  38. davidlpf

    Lugosi that sounds like a whopper of a tale, I am pretty much the same boat.

  39. davidlpf

    Nathan I give up, I have better things to waste my time on then you.

  40. Nathan Myers

    David Fairweather: You have presented your credentials. Good day, sir.

  41. Tom Marking

    “If they’re going to piss away a giga-solar-mass black hole’s kinetic energy, they’ll probably need to have it altering the trajectories of random stars it passes.”

    Yes, so the whole concept of the superblackholes merging is very suspicious which is what I thought in the first place. What that means is that the superblackhole in the center of the Milky Way got as big as it is by eating local material and not by mergers with other blackholes. If the Milky Way has been eating other smaller galaxies in the local group then there should be other massive blackholes in other parts of the galaxy. I wonder how close the closest black hole is with a mass greater than 1,000 solar masses.

  42. Nathan Myers

    Tom: Actually I came to the opposite conclusion. It appears that an object of such a mass could, in fact, dissipate its kinetic energy by scattering stars it passes near (for an unusual definition of “near”). To a galaxy, what’s a billion stars, more or less?

  43. “Mike: You started this. What is supposed to strip each black hole of enough energy to get it into a close-enough orbit with the other for gravitational radiation to amount to anything?”

    I was off visiting local family this evening.

    Dynamical friction is the most likely effect to bring the black holes together. This is a subtle effect involving how a gravitating mass brings stars closer to it as it passes, and since they are closer when it leaves, they exert an extra retarding force. This dynamical friction can be very effective.

    Ah, there’s actually a wiki article:

    http://en.wikipedia.org/wiki/Dynamical_friction

  44. “Mike Brotherton above mentioned the energy dissipation mechanism, “gravitational radiation”. Merging black holes are one of the few phenomena thought to produce sufficient gravitational radiation to be seen by current gravity wave detecting technology, although I haven’t heard that they’ve “seen” anything like that yet.”

    Just before the press conference, Steve Maran announced to everyone that LIGO was going to be announcing a gravitational wave detection from the crab nebula. Haven’t seen that story break officially yet, but I haven’t looked today.

  45. “BTW, going from left-to-right and then top-to-bottom when I look at the 29 galaxies the only ones which show obvious signs of something weird happening structure-wise are: 4, 5, 7, 12, 14, and 20. The rest look like fairly typical galaxies.”

    It’s true that there are a few that are pretty normal looking, but many that don’t look too weird still show more subtle signs like asymmetries, faint tidal tails, shell structures, etc., characteristic of mergers or other interactions. We’ll be looking to quantify these in the near future.

  46. Irishman

    Tom Marking said:
    > Now, replace the wing with a black hole. Do the air molecules rebound off it? Nope. There is no solid surface to rebound off of. Once they pass the event horizon they get sucked into the singularity. Maybe they cause some change in the linear momentum of the singularity but it would probably be offset by molecules falling into the event horizon on the back side.

    Why would the front side and back side balance? The black hole clears its path of material, correct? So you have a homogenous (rough approximation) field on three sides and an empty path behind. Now there may be fall in from entrainment or whatever, but is the back going to see and equal amount of matter fall in? No. Ergo, the imbalance in matter added to the front vs the back. So how does this slow the black hole? How about momentum transfer. The totally inelastic collision (black hole absorbs entire momentum) creates a drag.

    Nathan Meyers said:
    > I could imagine that extracting a fair bit of energy after a few billion passes, but why didn’t Mike say so?

    Because astronomical events operate on huge timescales, so that’s the default assumption?

    davidlpf said:
    > Nathan get thee to some first year physics books.

    While I appreciate the simplicity of this response, it would perhaps be more productive to either attempt a layman’s explanation or find a link to something online related to the topic. If this material is truly as basic as you state, that shouldn’t be hard to come up with.

    > I have better things to waste my time on then you.

    Fair enough, but there are other people reading the thread who might benefit from at least a cursory explanation.

    Of course Mike Brotherton has replied.

  47. Torbjörn Larsson, OM

    D’oh! Sorry about the tag error, as well as the language mistake. Feels like nap time.

  48. Tom Marking

    “Dynamical friction is the most likely effect to bring the black holes together.”

    O.K. There’s an explanation that’s starting to make sense. According to the Wikipedia article the dynamical friction force is proportional to the square of the black hole mass which means it’s quite large for supermassive black holes.

    Mike, thanks for the explanation. Cheers.

  49. davidlpf

    Irishman sorry, but having Nathan coming in here commenting somthing is Startrek physics because it does not agree with he thinks is more then a little insulting. I may have something else say either tomorrow or next day. As I implied I am not an academic and work 9 to 5 job so it may it take to explain it better.

    Tom with that info from Mike Brotherton and wikipedia it makes sense to me the kinetic energy would be lost through friction with dust in the ISM.

    Nathan you really should learn to better deal with people. I think you were here on a earlier post saying that the BA did not how to read a graph and a paper about autism because he did not have degree in biology.

  50. davidlpf

    Take the Earth and moon, the moon actually does not orbit the center of the Earth but it orbits the center of mass which is just below the the surface of the Earth. If you scale it up to two stars that are about the same mass the center of mass is located more towards the center of the system. Two blackholes out side their event horizons are just massives stars so they would orbit each other their center of mass. Now lets start with a blackhole all alone in space and then another blackhole close so close that it starts orbiting the first blackhole. The second blackhole will start to “tug” at the first blackhole increasing its kinetic energy but since energy most be conserved the energy for the second Blackhole decreases.
    As time goes by they eventually start orbiting each other. That was my origional thought on it, does anyone know if the blackholes in the center combine or do they orbit each other in the core of the galaxy?
    Now increase the masses to a billion times as the sun also add in all the stars, and interstellar medium for dynamic drag effect and you get probably what really goes on with the blackholes as the galaxy combine.

  51. Torbjörn Larsson, OM

    does anyone know if the blackholes in the center combine or do they orbit each other in the core of the galaxy?

    I thought they were usually merging as galaxies combine to grow from the big bang on and most galaxies now are thought to have central massive holes, as this paper suggests. Also, I believe I have seen simulations (perhaps without dynamic friction) where at least one black hole can be ejected, this paper suggests.

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