Floppy disk

By Phil Plait | June 8, 2006 11:14 pm

The Milky Way Galaxy is a spiral, a magnificent pinwheel floating in space. But it’s flat! It only looks like a pinwheel when you see it full-on, what astronomers call "face on". What would it look like from the side?

It would look pretty much like NGC 5866, pictured above in a new Hubble release. But not exactly: 5866 is about 2/3 as big as the Milky Way, for one. For another, the Milky Way has spiral arms, and NGC 5866 is what’s called an S0 galaxy– it’s disk-shaped, but has no arms. Face on, it would look like a relatively featureless disk.

Which strikes me as weird, actually. Spirals form when a disk galaxy undergoes some sort of shock to the system, like passing near another galaxy, or actually eating another galaxy. The gravitational interaction sets up patterns of waves which circle the disk, making the spiral arms.

It’s weird, because NGC 5866 appears to have been disturbed in a similar way. Look at this zoomed image of the dust lane across the middle:

See how the edges warp a bit? The blue light is from the stars in the galaxy, and define the flat disk. But the dust, near the edges, is bent. This is usually the sign of a disturbance like I described above. But this galaxy has no arms! That, to me is odd, and I cannot explain it. Worse yet, the stars being blue means they are young. Stars form in giant gas clouds when the clouds collapse. Clouds collapse when… you guessed it, the galaxy is disturbed. So there is a ton of evidence this galaxy has recently undergone some sort of gravitational encounter, but it has no arms. Weird.

Another thing: see how the central part is reddish? That might be due to the dust in the disk of the galaxy scattering away and/or absorbing the blue light from the central stars, or it might be because the central bulge is itself red. Which is it? Either, or both?

And a final question. I’m not a galaxy expert, so I’m not sure how you can tell if an edge-on galaxy like NGC 5866 is an S0. If it’s not tilted, even a little, how can you tell if it has spiral arms or not? I can guess: radio telescopes can map the clouds in the galaxy. If the distribution of clouds is smooth, then it’s an S0. But if they clump up in some places and not in others, then that would indicate they are piling up in the spiral arms, and the galaxy is not an S0.

So I personally have a few questions for which I’d like answers. But not all is lost! Bill Keel, the astronomer who took this image of NGC 5866 with Hubble, happens to be a friend of mine (and a frequent poster on the Bad Astronomy/Universe Today bulletin board). I’ll ask him about this, and if he can give me a coherent answer, I’ll see what I can do about posting it here. Or maybe he’ll show up in the comments section…

This’ll be fun, to see how close (or how far off) I was in my guesses! Maybe Bill can set me straight. Or tell me how brilliant my observations are.

CATEGORIZED UNDER: Astronomy, Cool stuff, NASA, Science

Comments (25)

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  1. Brooke | July 22, 2007
  1. I can’t wait to see how this turns out. :)

  2. HvP

    One thing I’ve always loved about you and this site is your “kid in a candy store” attitude to new things you’ve yet to figure out. 😛

    This photo is so cool. I’m just pleased with myself if I can take a reasonably attractive photo of a landscape or sunset. It’s awe inspiring that those talented among us can take such fantastic photos of entire galaxies millions of lightyears away!

  3. Nigel Depledge

    You’ve done it again, Phil. Thanks for drawing our attention to another superb astrophotograph, and for sharing your excitement about the science it contains and the questions it poses.

  4. Evolving Squid

    Maybe it doesn’t have arms… yet.

    Perhaps it recently underwent an interaction of some sort that will form arms, but the arm formation is in process and the arms haven’t really sorted themselves out yet.

  5. So there is a ton of evidence this galaxy has recently undergone some sort of gravitational encounter, but it has no arms.

    How is it going to operate its digital watch now?

  6. Evolving Squid

    How is it going to operate its digital watch now?

    Remember, we see it as it was 44 million (I think) years ago. So it probably has one of those red LED watches that eats batteries.

  7. CR

    I’m glad there is a zoomable image at the Hubble site… the streaks in the background of the photo looked vaguely like more galaxies, and the zoom function allowed me to confirm that. (Leave it to me to seem more intrigued by the background stuff than the subject in the photo! Honestly, of course, it’s all cool!)

  8. Troy

    I’m glad you sometimes have GOOD astronomy here. Very interesting.

  9. icemith

    Blake S. I thought one needed HANDS to operate a digital watch, you know the bits at arm’s end, called fingers, or digits. Oh, you mean those digital bits? Have I painted myself into a corner here? Anybody else got the time?

    I do wonder though if we cannot discern spiral arms in NGC 5866 because of the foreshortening effect caused by the huge distance between object and subject, a factor well known in photography, we are using a pretty big telephoto lens! We would be looking at maybe three or four cross-sections of those arms, so the expected spaces between the arms would be backed up with another arm section and then probably another after that.

    If radar or doppler could be used at this distance to give reasonable evidence, well and good, but I won’t be around in 88,000,014 years to confirm or deny the proposition. You are wondering why it will take the extra fourteen years? Have you ever known any government to set up committees, debate, approve, vote finance, do it all again in the next administration, build the instruments, and then have recriminations and inquiries for such things as who authorised the truckload of $250 hammers. Yeah, it will take 14 years.

    But then again there may be other evidence learned astronomers can determine for sure that there are/are not spiral arms. I’m sure there is ample evidence out there of both types.

    Um, do globular clusters have arms too?


  10. Nigel Depledge

    Icesmith said:
    “… I thought one needed HANDS to operate a digital watch, you know the bits at arm’s end, called fingers, or digits. Oh, you mean those digital bits? …”

    ‘Fraid you missed the pop-culture reference there. It’s from the Hitch-Hiker’s Guide to the Galaxy. After Ford and Arthur are rescued by the infinite-improbability-drive ship, before normality is restored, Arthur comments (inter alia): “My left arm’s floating away. How am I going to operate my digital watch now?”

  11. Spiral galaxy classification comes from a few different things. The two main things are (a) how open and well-defined the spiral arms are, and (b) the ratio of the size of the bulge to the size of the disk.

    “Later”-type spiral galaxies (Sc, as opposed to Sa) have more open and well defined spiral arms. S0 galaxies are even “earlier” than Sa galaxies. (The terms “later” and “earlier” are unfortunate, and nowadays refer only to whether it’s further left or further right on the Hubble Tuning Fork Diagram.)

    Also, “later”-type spiral galaxies have a smaller bulge compared to “earlier” type spiral galaxies. Sa galaxies like the Sombrero Galaxy have big bulges. The Milky Way is an Sbc galaxy, so has a smaller bulge. (Actually, the MW is an SBbc, because it does have something of a central bar.)

    That’s how you can tell that this galaxy is an S0 or Sa, as opposed to a later-type galaxy. It’s got that big prominent bulge. A type Sb or thereabouts galaxy seen edge on would look more like NGC 891.


  12. Eric

    Hmm… The funny thing is how they can prove that the galaxy doesn’t have arms… Since we are looking at the disk on the narrow side, I don’t see how thye can prove that it doesn’t have arms.

  13. icemith

    Nigel, Nigel, Nigel…. Yes I did miss that reference, don’t even remember it in the original. But if you picture the scene, and one’s left arm is drifting away, and most people wear their watch on their left arm, what’s the point? Or was his a digital fob watch?

    Possible? Or only merely probable?


  14. One thing I’ve always loved about you and this site is your “kid in a candy store” attitude to new things you’ve yet to figure out.

    This serves as a good counterargument to those creationists and other anti-science types who claim that scientists ignore or dismiss evidence contrary to their preconceived notions, or that scientists are just spouting the party line in order not to jeopardize the untold millions they get in grants.

    It seems that not a year goes by in which I don’t read a science news article that quotes a scientist as saying, “This discovery overturns our theories! I have no idea what’s going on! This is so-o-o cool!”

    By the way, in the photo above, it looks as though the dust is all in a flat area, but the stars are in a lens-shaped distribution. It looks as though the thickness of the bright area is about a third of its width. Is that a photographic effect, or are there stars 10,000 ly from the plane of the disk?

  15. Nigel Depledge writes:

    Arthur comments (inter alia): “My left arm’s floating away. How am I going to operate my digital watch now?

    You may remember that this joke was set up by an earlier line, in which the narrator tells of “a backwater planet in the outer western spiral arm of the galaxy, whose ape-descended inhabitants are so amazingly primitive that they still think digital watches are a pretty neat idea.”

    Not too long ago, the BBC adapted the later Hitchhiker’s Guide books for radio. One change is that the narrator now says, “they still think custom ringtones are a pretty neat idea.”

  16. icemith

    Jeez, I missed that one too.

    Maybe I should not get out as much.


  17. icemith

    Also, just wondering, is there any evidence of a mechanism in space whereby a large scale reflection of nearby (relatively so) objects like another galaxy, and this would show a different angle. If light is bent slightly around objects, due to gravity, maybe we could also have that different view. Who knows if some of the galaxies we see are not reflections.

    I’m reminded of this question because I use the view in a nearby store window when reversing into a parking space. If available, it does give a different view.


  18. icemith

    Ooops, second last paragraph, “Who knows if some of the galaxies we see now, are only reflections.’ is what I meant to say. Sorry ’bout that.

    And would Gravity rule the idea out anyway?


  19. Irishman

    icemith, not so much reflection, but there is refraction – images bent around large gravitational objects. So yes, we do have pictures with the same object shown in multiple places. Phil’s even posted some here. However, it doesn’t give us a different angle view on the object. I think it’s the geometry of the distant background object being in the right place for the optics to work out means the viewing angle won’t be significantly different.

  20. icemith

    Thanks Irishman, for the detail. I assumed there may not be evidence of large angle reflections, or small ones for that matter. Refraction I had considered, but there again, it is only small angles that are involved.

    On a different tack now, what about the common depiction of the curved space ie. the saddle? Is our ‘line of sight’ confined to the curvature of space– our ‘straight’ view is actually curved. In other words, can we not really see across that curve? Only along the curved plane, as though it was a light pipe?

    If so it would imply that light is reflected off that boundary. Of course the angles would remain small, and what would the boundary be made of? Gravity again?

    Ivan. (obviously I’m trying to wear out my Question Mark key, don’t you think?)

  21. SFwriter

    “May I ask a question?”

    “What is it?”

    “It’s an interogative statement designed to elicit information, but that’s not important right now!”

  22. icemith

    If Mark asked that, I would have to question Mark “Why?”

    ( But was Mark in “Airplane!” with Leslie Nielsen? )


  23. Could it be an evolving Galaxy in transition?

    A spiral galaxyin the process of forming that’s somehow appearing at an earlier developmental stage than it should be based on its distance?

    An interacting pair of galaxies which we can’t disentangle yet?

    An Sa,SB, SBc type spiral masquerading as an SO type spiral? Could the central-bulge-size-spiral type law be wrong at least in this case?

    Whichever turns out to be the case a great image and great set of questions. Love to hear what BA’s friend finally says.

    Finally, stars like galaxies are termed ‘later’ or ‘earlier’ regarding spectral type with no (or very tenuous?) evolutionary connection – for example Vega (type A0) is termed an earlier spectral type than Altair (A7), Tau Ceti (G8) is of “later” spectral type than our Sun (G2) and in the whole system spectral types O-F are generally termed ‘early’ and F-M (or now T~L?) described as ‘late.’ As I understand it anyhow.

    Stars and galaxies do evolve and transform from one type to another but in complex ways and not just from “early” to “late” … Galaxies often change (esp. via merger) from “late” spiral types to “early” ellipticals. (Our Milky Way will merge with M31 Andromda and undergo this shift.)The same can occur with stars too. Eg. the star that became Supernova 1987A moved from being a “late” type red supergiant to an “early” type blue supergiant star before blowing itself apart.

    Misleading word use perhaps? But still useful as a descriptive term if you understand the underlying concepts?


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