Hubble grills a confused galaxy

By Phil Plait | August 12, 2010 6:00 am

Galaxies come in lots of shapes and sizes, but in general, we can group them into four flavors: spiral, elliptical, irregular (no real shape), and peculiar (definite shape, but weird).

We can also say lots of general things for each class: spirals are flat and have lots of gas and dust, ellipticals are spheroids with very little gas and dust, and so on.

The problem is, that pesky Universe of ours delights in throwing a monkey in the wrench. Behold NGC 4696, the confused elliptical:

hst_ngc4696

[Click to galacticate — and you need to, the above image doesn’t give you any idea of just how freaking cool this image is!]

This two-and-a-half hour exposure Hubble Space Telescope image shows incredible detail. The galaxy NGC 4696 is the diffuse glow dominating the right hand side of the image. It sits in the center of the ginormous Centaurus galaxy cluster, a sprawling city of hundreds of galaxies about 150 million light years away.

Clusters of galaxies like this sometimes have one big, fat elliptical sitting in the center. Called the central dominant (or cD*) galaxy, it generally has far more mass than any other galaxy in the cluster and has weird features (like multiple bright cores, an extended halo of stars, and lots and lots of satellite galaxies). We think these galaxies started off relatively normal, but then eat other galaxies that wander too closely — clusters are thick with galaxies, so such encounters are common. The now-heavier galaxy sinks to the center of the cluster through various forces, where it can really let itself go and eat even more galaxies. That explains the multiple cores (undigestible leftovers), their puffy halos (lots of orbital energy can be added to stars in the collision, inflating their paths), and the plenitude of little satellites (again, leftovers from previous galactic meals).

If the one word "weird" works with cDs, then NGC 4696 fits this description pretty well. Note the dark swirl apparently near the galaxy’s center, wrapping around it. That’s a trail of dust 30,000 light years long — 300 quadrillion kilometers (200 quadrillion miles) in length. It’s very rare for ellipticals to have any dust at all in them, so seeing something like this really lets you know this guy is strange.

In the super-high-res image, you can see very subtle striations in the galaxy’s innermost core. That’s from ionized hydrogen, again very rare in ellipticals. Usually, all the gas is locked up in stars, and very little is floating freely.

Also, unseen in this image, vast amounts of high-energy radiation are flooding out of the galaxy’s heart. This X-ray emission is clear in images taken by the Chandra observatory. Every big galaxy (even ours!) has a supermassive black hole in its core, with millions or even billions of times the mass of our Sun. In most galaxies that black hole isn’t actively eating matter (to stretch the gourmand analogy a little more), so we don’t detect it.

But if enough matter falls into the gaping maw, it can pile up just outside the point of no return, creating a huge disk of superheated material. Stuff that hot blasts out X-rays, and NGC 4696 is doing just that. Again, this all fits with the idea that it’s been overeating; collisions with other galaxies can dump octillions of tons of matter into that central black hole, converting the normal galaxy into an active one.

Images like this one from Hubble are gorgeous, jaw-dropping — and I haven’t talked about the myriad background galaxies! But they are critical in giving us a big picture of galaxies. It’s only by being able to get an overview of these beasts that we can hope to understand them. Like living beings, they are a complex interaction of smaller components, and if we don’t get such a long view like this one, we’re like the blind men and the elephant, only looking at one small part and making (erroneous) claims based on that.

Credit: ESA/Hubble and NASA



*A commenter below noted that the term cD is technically not short for "central dominant". I’m sorry I phrased it that way; I knew that, but the colloquialism, as I understand it, has rather taken over. My apologies for any confusion.

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (36)

Links to this Post

  1. Confused Galaxy « Waxing Apocalyptic | August 12, 2010
  2. Hubble grills a confused galaxy « Men Into Space | August 12, 2010
  1. SpaceIsFun!

    Wow, nice picture! Man, with stuff like this from Hubble, I can hardly wait for James Webb! But of course Hubble pictures like Deep Field and Pillars of Creation will always be classics that are near and dear to my heart! Keep up the awesome blog, Phil!

    On a side note, could you settle a little debate between by friend and I? We were discussing Armageddon the other day, specifically the way the astronauts got to the asteroid (by Shuttle, if you haven’t seen it, although I’m fairly sure you have!). Although we both agree they couldn’t have gotten their with a normally launched Shuttle, I think that if a Shuttle was launched with 4 SRBS into the highest orbit possible, and then, after jettisonning the SRBs and ET, rendevoused with four new SRBs and another ET that had been previously launched, it could escape earth’s gravity. I also think that if the only payload it had was a nuclear bomb, it could carry enough extra consumables to get the crew there alive. Since we couldn’t agree, we decided to take it to an expert. We chose you because one of us heard you wrote a book or something :) (Just kidding, of course, we’re both avid readers of BA.) Thanks!

  2. Brian

    Cripes. Is NGC 4696 really that much bigger than ever other galaxy in the area? It’s halo covers nearly half the image.

  3. There needs to be some house-rattling bass sound accompanying this picture to get the full effect.

    mmmmmmmmmmmmmm

    Yeah, like that.

  4. Messier Tidy Upper

    NGC 4696 isn’t confused – its just busy eating! ;-)

    That 30,000 light years long trail of dust is just the remains of its latest spiral galaxy meal.

    Reminds me of Centaurus A or NGC 5128 (http://en.wikipedia.org/wiki/Centaurus_A ) a bit only super-sized – & *how*! ;-)

    Great image – and lots of background galaxies – satellites & cluster members mostly I presume? :-)

  5. Jamie Mueller

    Is that the “Ghost of the Enterprise” galaxy at the top middle-left?

  6. Oli
  7. Pepijn

    So that white glow, is that glowing dust or something, or is that all stars?

  8. Amos Zeeberg (Discover Web Editor)

    Love the pic, but the dust looks just like schmutz on my monitor. Kind of driving me crazy.

    Phil, are the strange elements in NGC 4696 (dust, ionized hydrogen) the leftovers from when it ate a bunch of potentially non-elliptical galaxies?

  9. Pepijn (7): yes, it’s just stars, like when you go outside and see the Milky Way at night. The combined glow of billions of stars just looks like haze!

    Amos (8): Yup. Dwarfs, mostly, since they’re the most abundant, but any other spiral, or even another elliptical that may have had gas and dust when it ate other galaxies… making NGC 4696 potentially a cannibal of cannibals. :)

  10. Gentle Suggestion

    Please, Phil, consider not using the word “ginormous.”

    Gigantic is a great word.

    Enormous is a reasonably good word.

    Ginormous is a bad word.

    You’re one of the smartest people in the media today and I hate to cringe every few days because one of your posts contains something like that. It’s a tiny (+miniscule = tiniscule?) issue, I know, but choosing your vocabulary more carefully will slightly improve your life.

  11. Gentle Suggestion (10): Tell me, you understood the meaning of the word, yes? Then it worked. And using words like that (or embiggen, galacticate – which I note you didn’t mention – and others) does, in fact, improve my life because it makes me smile, and I know it makes other people smile too.

  12. Gary Ansorge

    1. SpaceIsFun!

    Asteroid velocity in the vicinity of earth would generally be traveling at around 30 km/sec, thus we would first have to go toward the asteroid, perhaps swing around the moon and sneak up behind it. At 30 km/sec, even the nuclear lightbulb(with an ISP a bit over 3000(compared to liquid H2 and O2 of 450) probably couldn’t carry enough reaction mass to do the job) and travel time from Luna orbit to earth impact would only be 12,800 seconds(about 3 hours), not nearly enough time to do everything the movie wanted to do.

    Thermonuclear fusion(like the Bussard Polywell reactor) should have an ISP of between 10,000 and 100,000. THAT could probably do the job right.

    There’s just no way Armageddon entertained a viable idea(from a technical perspective).

    Gary 7

  13. Gary Ansorge

    Phil;

    “we’re like the blind men and the elephant, ”

    Three blind men encounter an elephant on the road. One feels the elephants trunk and says” This is long and slender, like a snake.”

    The second says, while feeling the elephants side “It’s broad and tall, like a barn.”

    The third says, from the back of the elephant “I don’t know about all that, but it’s hung like an elephant.”

    ,,,and the elephant just smiled.

    Gary 7

  14. HirOwl
  15. MattF

    SpaceIsFun!: Although we both agree they couldn’t have gotten their with a normally launched Shuttle, I think that if a Shuttle was launched with 4 SRBS into the highest orbit possible, and then, after jettisonning the SRBs and ET, rendevoused with four new SRBs and another ET that had been previously launched, it could escape earth’s gravity.

    Ignoring the difficulties of such a scenario (docking reliably to a new stack, storing the fuel on orbit until needed, and so on), you’re on the right track.

    Ignoring the atmosphere, it takes exactly the square root of two times the amount of energy to achieve escape velocity from a particular altitude as to enter a circular orbit at that altitude. Of course, there were other SRBs that NASA had considered but never used — bigger ones for some applications, and filament-spun ones for launching into polar orbit from Vandenberg (they’re lighter, which is important because you need more oomph to get into polar orbit).

    But generally, if you’re looking for scientific accuracy from Armageddon, you’re going to be disappointed. Trying to rationalize realistic launches into the movie in an attempt to lend it accuracy is a bit like stopping a hurricane with a handkerchief.

    Gary Ansorge: Asteroid velocity in the vicinity of earth would generally be traveling at around 30 km/sec

    And you’re right about that. Matching velocities with the asteroid is another big problem, even assuming you can leave Earth orbit.

  16. Messier Tidy Upper

    @13. Gary Ansorge : ROTFLMAO! :-)

  17. SpaceIsFun!

    Thanks Matt, and Gary! So it’s possible that if it was able to dock with a new stack, it could make it to a NEO? That’s basically what our argument/bet was about, if there was anyway the Shuttle could possibly reach an asteroid with a crew.

  18. Rich

    I vote that blogs are very casual and as such the language can be casual. If this were a published piece, I would expect that BadGrammar.com could go after Phil.

    A noble spirit embiggens the smallest man.

  19. What’s the big fat weird galaxy at the centre of the Local Group? Is it us?

  20. Gary Ansorge

    15. Messier Tidy Upper

    Cool!

    First time I ever read that tale of the blind men was in a book of Sufi Parables. A good teaching story but so damn dry, I just had to add my twist.

    I have a great deal of respect for elephants. I would hope, if they could understand the story, they too would enjoy it.

    Glad you liked it. Now, as you smile, you can keep people wondering all day what you’re up to.

    Gary 7

  21. Tribeca Mike

    Speaking of the Milky Way, in 1960’s Tucson (before the building boom) the view of the Milky Way was quite something, even in town. For several years I thought it was merely a huge cloud bank that never went away, until the old man wised me up and completely changed my way of thinking of the universe.

    Thanks for another great photo!

  22. Isaac

    Oh, and Gentle Suggestion (10)? It’s minuscule, not miniscule.

  23. Joerg

    Astronomer grills confused Bad Astronomer:

    Our understanding of ellipticals and in particular ellipticals at the center of cool core clusters has evolved quite a bit in recent years. It is now generally understood that galaxies at the center of cool core clusters are subject to massive infall of intracluster medium and that these galaxies are gas rich, dust rich, and star forming. Other prominent examples are the cD in the Perseus cluster as shown by the fantastic work of Andy Fabian and the cD in RBS 864. The latter one looks like an elliptical but if you zoom in even shows spiral structure in the center.

    But what a great picture this is!

  24. Brian

    Gentle Suggestion@10: Ginormous has a history going back 60 years now, and it was recently added to Merriam-Webster. It’s a bit late to be trying to smother it now. Your post makes it sound that Phil only used the word from carelessness. It’s probably closer to the truth that he was quite intentional in using an informal, slangy word. (I’m given to understand that he has had some measurable experience in the field of expository writing — going so far as to successfully induce a press to commit a connected series of essays to print, even.)

  25. Justin Howell

    Phil, cD != central dominant, though that’s a common mnemonic. See Mihalas & Binney p. 297-298. The D means it’s an elliptical with an extended envelope, the c prefix means supergiant.

  26. MattF

    SpaceIsFun!: Thanks Matt, and Gary! So it’s possible that if it was able to dock with a new stack, it could make it to a NEO? That’s basically what our argument/bet was about, if there was anyway the Shuttle could possibly reach an asteroid with a crew.

    Let me be more clear. By “on the right track”, I meant that you were thinking about this better than most people — who seem to think (wrongly) that, for example, the Shuttle could go to the Moon if it needed to in its present implementation.

    There are several problems with getting the “dock to second stack” thing to work as an inbound-asteroid-reaching strategy. Some of the easiest problems show up when you try to figure out how to get exactly the change in velocity you need from the engines at your disposal. Once you turn solid rockets on, for example, they don’t shut off until they burn out. The Space Shuttle Main Engines (SSMEs), for that matter, are not really designed to be stopped and re-started. They can throttle within a certain range, but that’s about it.

    If you wanted to come home safely, you’d also have to figure out how to slow down to orbital speed before re-entry. (The Shuttle’s frame and Thermal Protection System are not designed to handle the stresses of returning from deeper space.)

    It would really be better to design a ship specifically for the task. But as far as most people I talk to seem to think, the Shuttle is a general-purpose space machine, so it should be able to handle whatever we throw at it with relatively minor tweaking.

    All that said, though, there are a few asteroids that require smaller changes in velocity than a trip to the Moon. A trip to one of them tends to be longer duration than a Moon trip, naturally. There were some in the Apollo Applications program tossing around the idea of replacing the LM in a Moon rocket stack with a module that housed all sorts of consumables, and sending an Apollo capsule to a Near-Earth Asteroid.

    There were also some interesting ideas surrounding the so-called “Shuttle-C” design.

  27. Tod R. Lauer

    Ummm, Phil, only about 25% of the secondary nuclei in first-ranked galaxies are from cannibalism. Most have nothing in them, and of those that do, 50% of the cases are just chance projections. The remaining 25% are high-velocity unbound interactions.

    Of course, modern ground work and HST shows that some amount of dust is common in ellipticals. We just didn’t think so in the old photographic days because the centers were too hard to expose right.

    Thanks for the picture…

  28. t-storm

    Embiggen is a perfectly cromulent word.

  29. Matt

    Loving that image. Has as much fun looking at the background galaxies as I did the main feature.

  30. Nigel Depledge

    Brian (24) said:

    (I’m given to understand that he has had some measurable experience in the field of expository writing — going so far as to successfully induce a press to commit a connected series of essays to print, even.)

    Wait, you mean Phil wrote a book? ;-)

  31. L1ttl3J1m

    ginourmous = gigantic x enormous, I see no problem here.

  32. Messier Tidy Upper

    @ ^ Nigel Depledge :

    Actually he’s written two ..as I think you well know! ;-)

    Personally, I can’t wait for his next one but I guess he’s going to be a bit pre-occupied with the TV show for a while.

  33. mike burkhart

    I have read in some astronomy books that there is a class of galaxy called SO or lenticler . These galaxys have a central buldge unikle an ellipitcal and a disk insted of arms like a sprial . Maybe this is no longer a classafaction that is used?

  34. @ ^ mike burkhart : The S0 classification for lenticular galaxies is still used & useful.

    Click my name for the wikipage on lenticular galaxy which has further info. :-)

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