Galaxy ripped to its death

By Phil Plait | March 2, 2007 7:17 am

What immense force is tearing this galaxy apart?

The spiral galaxy in the upper right of this newly released Hubble image is getting shredded– you can see the bits of it getting torn off to the lower left, streaming out behind it. It’s disintegrating before our eyes… well, it’ll take a few million years, but still. Things are looking grim for it.

There are actually two causes for this galaxy’s imminent demise. Both are because of its environment: it has the unfortunate circumstance of living in the massive galaxy cluster Abell 2667. Here’s the long view (our doomed friend is in the upper left corner):

Abell 2667 is a rich cluster, fraught with galaxies. It’s located about 2.7 billion light years from the Earth. I can’t find a good estimate of the entire cluster mass, but one paper quoted the mass as approximately 70 trillion times the mass of the Sun inside a radius of 360,000 light years. For comparison, or own Milky Way Galaxy is about 100,000 light years across, and has a mass of roughly 200 billion solar masses. So this cluster has at least 350 times our Galaxy’s mass. In other words, big.

That mass translates into a lot of gravity, and most of it is concentrated in the core. That means tides will be an issue. The gravity an object feels from the cluster core depends on its distance. But galaxies are big! So one side of a galaxy might feel significantly more gravity from the cluster core than the far side. This stretches out the galaxy, and if the difference in force is big enough, it can shred it.


This is in fact part of what’s going on with that tattered galaxy. The tides from the cluster are literally pulling it apart. There’s more, though.

Inside a cluster is a lot of gas. A lot. As galaxies orbit around the cluster center, they stir that gas up, heating it. In Abell 2667, there is so much gas that a galaxy passing through it feels a pressure, called, appropriately enough, the ram pressure (think of holding a lit candle while riding a bike to get the idea). If the galaxy is moving quickly enough, and the gas is dense enough, the galaxy can have all its gas stripped away, where it will merge with (and heat even more) the cluster gas.

Again, our poor galaxy there in the upper left is getting slammed this way as well. As the tides shred it, its gas is being forcibly blown out. The gas also getting compressed, and you know what that means: star formation. And you can see it! That stream down and to the left is bluish and clumpy, exactly what you expect for gas clouds getting compressed and forming new stars. The color is from newly born very massive stars, which are hot and blue. Those stars live a short, violent life, dying in supernova explosions. Keep an eye on that galaxy; it’ll have some fireworks going off in the next few millennia.

Those supernovae heat the intercluster gas up too. How hot is it? It’s between 10-100 million Kelvins. Hmmm, for comparison’s sake, water boils at 373 Kelvins– that’s 373 with no millions or anything after it; that gas is fracking hot, hot enough to glow in X-rays. One technical paper quotes the total energy emitted in X-rays to be a numbing 1045 ergs/second. So, in X-rays alone, that’s a trillion times the Sun’s total energy — in all wavelengths — every second. And that’s just from the cluster gas!

It would suck to live there.

Oh, one other thing. I’ve talked about gravitational lensing before, where the mass of a galaxy or cluster can warp the images of galaxies behind it. Well, that’s happening here too. Did you notice the really weird elongated galaxy near the core in the picture above? Here’s a closeup:

That stretched out goofy galaxy is actually probably a relatively normal spiral, distorted and twisty due to having its light passing through the warped and bent gravitational field of Abell 2667. The spiral is about 8 billion light years away, give or take, three times farther away than Abell 2667.

Actually, there’s still one other thing. In that last image, the core of the cluster is visible to the right. That’s the gravitational center, the bottom of the pit, so to speak. The cluster gas is streaming into that center, where it piles up. Dense gas cools faster than more tenuous gas, so towards the center the gas emits energy in the form of light (from optical up to X-rays), which cools it off. This is called a "cooling flow". Evidently, those blue streamers you can see there are from the gas flowing in; as it compresses it — what? Bueller? Bueller? It forms stars. Some are massive, hot, and blue, which is again exactly what you’re seeing.

So even as a galaxy dies in the outskirts of the cluster, its stripped gas forms blue stars… and when the remaining gas finally hits rock bottom, its destiny is the same. Eventually those stars will merge with the immense galaxy at the heart of the cluster, which will continue to grow as it eats its own.

It’s a galaxy-eat-galaxy Universe out there. I’m glad we have a front row seat.


Comments (27)

  1. seaducer

    So, I guess all that hot gas makes life like on our planet impossible? But without that super heated gas, let’s say for argument’s sakes there tidal forces pulled the galaxy apart but didn’t heat it, would a planet like ours still be able to survive? If we were in the middle of that would we even know what was happening?

    I know the distance between Earth and the Sun have a lot to do with our being here, but is all that contingent on the Sun’s position relative to the other stars in our neighborhood?

  2. Tom K

    I hate to threadjack, but I just saw some Bad Astronomy on, of all places, Astronomy Photo Of the Day. Here’s today’s entry:

    That’s not so bad, but click on the picture to go to the uncropped version. If the sun is behind the Earth, where is all that light coming from and making shadows that point away from the camera? Studio lights, perhaps?

  3. Tom K

    Dang, that was yesterday’s entry. Click here for today’s:

  4. Gary Ansorge

    Great shot of the stretched galaxy. 8 billion LYs? I wonder what it would look like today(I mean at that particular point in space). Dang, it’s really hard to talk about things occuring so far away in real time. This language needs some adjusting, from such terms as apparent time(what we see right now) vs real time(what’s actually there, now).

    Anyway, great coverage, Phil. Tanks,

    GAry 7

  5. owlbear1

    All those stars without a Galactic home, how sad.

  6. kingnor

    question: if there is so much gas in this cluster that it’s ripping apart this one galaxie, why arn’t the other galaxies getting ripped apart?

    Same with the streaching, shouldn’t we see much more streaching warped weirdness than just the one galaxie?

  7. Awesome post Phil. I love it when you take an image like that and break it down for us. :)

  8. Front-row seat, sure… But, personally, I’m more happy not to be on stage.

  9. Amanda

    Tom: If you look under the picture on today’s Astronomy Photo of the Day, it says “Illustration Credit & Copyright”. Just to clarify.

    As for this really awesome picture of the galaxy cluster, I’m amazed. I was really hoping you’d talk about the elongated galaxy. :) Thanks, BA!

  10. Navneeth
  11. DrFlimmer

    Indeed, very amazing! And another proof why I like astronomy so much! And a very nice explanation phil, thanks for that!

  12. Mark Martin

    I don’t think it would suck to live there- nothing would live there! :)

    It’s impressive to think of a super-hot gas occupying the gaps between whole galaxies. It’s like that region of the Universe is an oven, and those galaxies are roasts.

  13. ioresult

    Our galaxy is also filled with hot gas. Ok, the gas isn’t as hot as that gas in Abell 2667, but it’s there nonetheless. And we’re protected by it. By the Sun’s wind. The solar wind extends up to about 100 AU. So I imagine there is a margin there. Galactic wind would have to be a lot more violent to affect us.

    What I’m wondering, is how much more violent must the galactic wind be to affect us? Would we be protected from Abell 2667’s harsh environment if we were in such a cluster?

  14. Arthur Maruyama

    IANAA (I am not an astronomer), but let me venture to answer kingnor’s question.

    One thing to remember: this galactic cluster is about 2.7 billion light years away so the view that we see has a lot of other galaxies in it which are NOT part of the cluster. One example is the nicely shaped nearly face-on barred spiral galaxy in the top half of the last picture–since neither it nor its light are being distorted by the center of Abell 2667, then that particular spiral has to be much closer to us.

    I also guess that a survey of the members of Abell 2667 would find a high percentage of elliptical galaxies (if it isn’t exclusively so aside from the spiral being torn apart) which can more easily retain their shape due to their relative compactness. I imagine that there are a number of apparent ellipticals which are really the stripped centers of spirals–perhaps a research paper could be done on this subject.

  15. When you say it would suck to live there, don’t you really mean it would suck for humans to live there? (Yes, I know you’re being facetious.) For all we know, one of those galaxies has developed life that has evolved to fit those conditions well and they look at our frigid, silent (yes, I know) galaxy and marvel at how inhospitable it would be.

  16. David S-D

    Wow, what a great post! It’s fun to hear about tidal forces, ram pressure, gravitational lensing, and the like when you can see it all in pretty pictures. Thanks, Phil.

  17. Phil – great posting. And I just wanted to say – I just heard the Joe Rogan part III audio, and I have to say – you held your own. The only thing that disrupted the discussion was Joe’s insistence in talking over you and cutting you off.

    It was extremely entertaining non the less, and I would love to hear a part IV happen.


  18. wright

    A point, Darmok. Though the form and chemistry of life that could arise, let alone survive and thrive in a shower of x-rays like that… the mind boggles.

    No doubt you’re right: any sentient creatures that evolved in that environment would have all kinds of theories about how life could never develop in such dark, cold places like our own galaxy.

  19. Hello Phil,
    just to say that i really appreciated and enjoyed this post: vivid and clear as usual!

  20. Nigel Depledge

    Another very nice article, Phil. Fantastic.

    I have one nit-pick: you call 2.7 billion light-years away (that’s nearly a gigaparsec!) being in the front row??

  21. Truly awesome. Let the theists have their little gods. I’ll take stuff like this any day.

  22. jackd

    I’m trying to understand the size comparison here: [an estimate of part of Abel 2667’s mass is] ”
    inside a radius of 360,000 light years. For comparison, or own Milky Way Galaxy is about 100,000 light years across

    The Milky Way is that big? Meaning that from a similar distance, it would obscure a really large portion of that second image?

  23. icemith

    ioresult, correct me if I am incorrect here, (never wrong), but isn’t it a fact that the density of particles, namely hydrogen atoms, or more correctly, molecules, that are heated and this is why they are observed? The space in which they exist, however they came to occupy it, has no “temperature” itself, only the matter in it.

    Now if the observed temperature is different, and in this case, seems to be wildly different, this could only be because of the greater density of that hydrogen. If the dynamic range of density approached a million, would that still be mostly empty space, compared with our concept of solid matter here on good old Earth? With many more atoms being heated, then the average temperature would rise, but the space between the atoms is a void and at -273degrees Kelvin, in other words – no temperature.

    As I see it, we *could* exist in that space with our present technology for space exploration, but that is obviously a pipe-dream, its, aah, how many light-years away? And I said exist, at least for as long as our lunches lasted, and if we discount any massive cosmic/x-ray or other mildly annoying radiation.

    Bytheway, (love that word, but not the usual initials, and I once new a kindly old couple with that name), is it only hydrogen atoms that exist in space to form those clouds? Do other progressively heavier atoms get to travel interstellar? And finally, the heavier atoms that appear on Earth, that we are made from, were these produced locally, ie, in our galaxy, or could they have been imported from a far distant galaxy, or a series of galactic supernovas, each building up the heavier elements?

    I need to know this. *Before* the next supernova would be fine.


  24. Irishman

    Pic on APOD today:

    Tom K, nice catch. I didn’t look at the full pic till you commented.

  25. JB of Brisbane

    I tried to post this before… I’ll have another go.

    In “The Boy In The Bubble”, Paul Simon sings a line I have always liked, and I feel is pretty appropriate for this post:

    “The way we look to a distant constellation that’s dying in a corner of the sky…”

    Okay, Bad Analogy: this is a galaxy, not a constellation, that’s dying, but I thnk we can afford a bit of artistic licence here.


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