Floppy disk

Bad Astronomy
The entire universe in blog form
June 8 2006 11:14 PM

Floppy disk


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?

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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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.