The galaxy that ate Detroit

By Phil Plait | February 5, 2008 8:46 am

Actually, it’s The Galaxy That Ate A Whole Lot Of Other Galaxies… And Got Gas.

Check it, babies:

This new Hubble picture release (American version) of the elliptical galaxy NGC 1132 has some way cool stuff going on. Usually, when I see a new Hubble image, I like to look at it for a few minutes and see what I can see before reading the actual text. I do this to see what I can figure out on my own, but also so that I don’t get prejudiced by the press release. The PRs are generally very good, but if I read it first I may bias myself against seeing something that I would otherwise miss.

In this case, I was pretty close. My first thought was, big deal, an elliptical galaxy. They are very pretty and very interesting, but generally not worth a press release. But then I spotted something odd: the core is very bright. Hmmm, is it an active galaxy? Maybe the supermassive black hole in the core of NGC 1132 is furiously gobbling down matter, and spewing out vast amounts of energy in the process.

Then I looked at the Chandra X-ray image (superposed on the visible light image taken by Hubble):

Hmmm, the core isn’t that bright. Active galaxies, as a rule, have tons of X-rays pouring out of the center, and the core would be the brightest thing in the X-ray image. So NGC 1132 isn’t active, or is at best mildly active. Then I noticed the weird distribution of X-rays. X-rays usually come from very hot gas in galaxies, and ellipticals tend not to have much gas in them. Also, most ellipticals, if they emit X-rays at all, tend to have a nice symmetric distribution of them. It would look like a smooth cloud of light surrounding the center.

That’s not the case here! The gas is lumpy, and there’s a lot more "above" the core than "below". Hmmm… Aha! NGC 1132, I says to myself, says I, is a giant elliptical. It has been busily eating other galaxies, building up its mass. Now it’s huge, and the asymmetric distribution of X-rays indicates how the hot gas inside has been roiled up by collision after collision of smaller galaxies.


From the press release:

In visible light NGC 1132 appears as a single, isolated, giant elliptical galaxy, but this is only the tip of the iceberg. Scientists have found that NGC 1132 resides in an enormous halo of dark matter, comparable to the amount of dark matter usually found in an entire group of tens or hundreds of galaxies. It also has a strong X-ray glow from an abundant amount of hot gas ‚Äď an amount normally only found in galaxy groups. Its X-ray glow extends over a region of space ten times larger than the 120,000 light-year radius it has in visible light. An X-ray glow that is equal in size to that of an entire group of galaxies.

Kewllllll. Galaxies tend not to live alone: they usually travel in groups. The Milky Way, for example, lives in a smallish cluster called the Local Group, which has us, the Andromeda galaxy, the Triangulum galaxy, and a few dozen smaller galaxies (what my Mom would call pitzulahs, "little things"). When you travel in packs, collisions are likely, and over time a medium size galaxy can swallow lots of little ones to become a Monster Galaxy.

NGC 1132 is such a monster. Like I said before, ellipticals tend not to have much hot gas, but NGC 1132 is lousy with it. It got all that matter by eating smaller gas-rich galaxies. As they spiral in to the galaxy’s center, their gas is stripped away where it becomes part of the bigger galaxy. Furthermore, the gas gets stirred up, which heats it enormously. Eventually, the smaller galaxies fall to the core and are fully absorbed, building up the mass of the gourmand galaxy’s heart (which is why it looks bright in the visible light Hubble image).

NGC 1132 is also quite huge, bigger than the Milky Way (which has eaten its share of smaller galaxies over time too), and sits in a vast cloud of dark matter – normally invisible, but in this case revealing itself by its gravitational effect on the stars in the elliptical itself (actually, to be honest, I’m guessing how they found it: there are many ways to detect dark matter, but the venerable method is to measure the velocities of the stars in the galaxy, which are affected by dark matter; stars move faster when more dark matter is present in a halo around the galaxy). The amount of dark matter detected is way more than usually seen for a single galaxy; more evidence that this galaxy has been busily gobbling up companions — and their dark matter, too.

The final nail: the galaxy is surrounded by zillions of globular clusters. These are collections of stars, sometimes with a million or more stars in them, packed into a tight ball shape sometimes only a few light years across. Giant ellipticals tend to have lots of globulars, which survive the acts of galactic cannibalism, changing allegiance from their original galaxy to the bigger one (globulars are pretty but fickle). The Milky Way has over 100 globulars around it, another indication that we too are the big guy on the block, but at the cost of dozens of other galaxies’ existence.

It’s amazing to me that just by critically examining one or two images of a galaxy we can see so much; so much of the galaxy’s present, so much of its past. And we can be confident of our interpretation! It just takes practice… and of course centuries of slow but solid scientific progress to bring us to the point where we can understand the Universe we live in.

And appreciate its beauty too. NGC 1132 is pretty.


Comments (24)

Links to this Post

  1. Tangled Bank #98! « PodBlack Blog | February 6, 2008
  2. Glob smacked | Bad Astronomy | Discover Magazine | August 5, 2008
  1. Mjbutah

    It is pretty….I love Hubble pictures. There is always so much going on, the longer you look the more you see.

  2. cameron

    Is it me or does it look like there’s a good deal of gravitational lensing going on too?

  3. Quiet_Desperation

    Check it, babies

    *sigh* Do we need to stage an intervention, Phil? ūüėČ

    I guess it’s pretty. I’m a barred spiral man myself.

  4. Pat

    I’d like to know the distribution of dark matter, whether or not its in the typical shell. Goes towards collisionless: with so many other galaxies that might have had their own dark matter shells, would there be density waves or eddies? Did it gobble the galaxies and the dark matter?

  5. Quiet_Desperation

    gravitational lensing

    Orientation doesn’t seem right. I think it’s just edge on galaxies.

  6. Thanny

    It seems plausible to me that all globular clusters are probably the cores of consumed dwarf galaxies – the stars that were able to stay together due to their mutual gravity, while the rest was ripped away by tidal forces.

    If that view is accurate, then there’s pretty much no way to know if a globular is stolen from a larger merged galaxy, or comes straight from the original dwarf.

  7. What gets me is just how many other entire galaxies are in that image.

  8. Pat

    I’m curious: has anybody put forth the idea that maybe ellipticals are the way they are because they ended up stripped of their dark matter after plummeting towards the center of a cluster? It /looks like/ most clusters are shells of dark matter as well, and if a standard spiral were stripped of the confinement provided by dark matter, wouldn’t its higher energy plasma, stars, and dust be flung outward, leaving a proper Newtonian-orbiting spheroid of stars?

    Just wontering if it is the collisions or the situation that first makes an elliptical, rather than age.

  9. Ben

    After staring at the pretty picture, your posting and the image made me think. Can we assume that, due to the ginormous (that’s not a real word, I know, but it should be when it comes to cool things like that one) amount of X-rays that NGC 1132 will be a completeley dead galaxy, without any form of life?

    I repeatedly read that, because of the high density of stars in our galactic core and the resulting amount of cosmic rays all planets in the core will most likely be barren. So, if this is true, should we assume that smaller galaxies are better cribs for emerging life?

    Those questions may sound a little strange since we are talking about galaxies millions of lightyears away, but, well, it’s the same approach I mostly take when it comes to space: When we go out one day, where should we knock?


  10. Joules


    That relates to a recent article in astronomy magazine (their latest mag features galaxies). Someone wrote that ellipticals are the end of the road for a spiral galaxy like our milky way. It makes sense that in its own destruction it may drag its neighbors along with it. It’s so fun to watch the pieces of a puzzle fit together.

  11. Kim

    quote “Maybe the supermassive black hole in the core of NGC 1132 is furiously gobbling down matter, and spewing out vast amounts of energy in the process.”
    Could you explain this statement? I thought nothing could escape a black hole. How does it spew energy out?
    I’m not doubting the statement at all. I’m a newbie regarding astronomy and the physics relating to it and am looking to learn.

  12. Sespetoxri

    Kim, just google ‘Hawking Radiation’ and most everything you get will explain what Phil is talking about.

  13. Sespetoxri

    Actually, in retrospect he’s not really talking about Hawking Radiation, but rather the heating of gas in the accretion disk around a black hole. This will explain that bit for you –

    Sorry for the confusion!

  14. Gary Ansorge

    I just finished reading Rare Earth by Peter D. Ward and Donald Brownlee. Their contention is that, due to a lot of chaotic behavior in the universe, the probability of COMPLEX life managing to rise on any planet was nearly zero, though those though little bacteria could probably survive most planetary extinction events.

    THAT would go a long way toward explaining why we haven’t been visited yet or may never be visited.

    GAry 7

  15. John Macleod

    In a stunning image like this (I blew it up big) how can you tell the globular clusters from galaxies. Many galaxy structures are obvious, you can see the spiral or see that you are looking at a galaxy edge on, but I can’t tell which are the clusters.

  16. The elliptical galaxy is certainly cool, but there’s some interesting stuff in the big picture…

    This guy seems to have formed a ring (near the bottom, centre):

    And this one just seems to be having issues (above and right of centre of the main elliptical):

  17. Matt

    If you look at the Xray composite pic, down in the lower left hand corner you can see a really cool galaxy collision.

    Make sure to embiggen!

  18. Matt

    Also, on the right side, about 2/3 down, look for the “eye of sauron”

  19. Buzz Parsec

    Ben and Matt –

    Galaxies become ginormous when they embiggen themselves by devouring bazillions of smaller galaxies. Just wanted to help with the technical vocabulary… :-)

  20. I wondered about the lower left thing and almost posted that oen too… I couldn’t decide if it was a collision, or a lucky superposition of two galaxies separated by a huge distance that just happen to be nearly in line.

  21. Ben

    “Ben and Matt –

    Galaxies become ginormous when they embiggen themselves by devouring bazillions of smaller galaxies. Just wanted to help with the technical vocabulary‚Ķ :-)”

    Thanks for clearing that up, Buzz. ūüėÄ

  22. Matt

    on the possible collision, I am not sure exactly either, but it looked like interaction to me.

    That’s what i love about picutres like this, you can really look them over and find so many cool things that I have no idea what they are.

    Of course I am analyzing this by squinting at my laptop screen.


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