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!  

Advertisement

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.

TODAY IN SLATE

The World

How Canada’s Shooting Tragedies Have Shaped Its Gun Control Politics

Where Ebola Lives Between Outbreaks

Gunman Killed Inside Canadian Parliament; Soldier Shot at National Monument Dies

Sleater-Kinney Was Once America’s Best Rock Band

Can it be again?

Paul Farmer: Up to 90 Percent of Ebola Patients Should Survive

Is he right?

Science

“I’m Not a Scientist” Is No Excuse

Politicians brag about their ignorance while making ignorant decisions.

Technology

Driving in Circles

The autonomous Google car may never actually happen.

In Praise of 13th Grade: Why a Fifth Year of High School Is a Great Idea 

PowerPoint Is the Worst, and Now It’s the Latest Way to Hack Into Your Computer

  News & Politics
The World
Oct. 22 2014 6:30 PM The Tragedies That Have Shaped Canada's Gun Politics
  Business
Continuously Operating
Oct. 22 2014 2:38 PM Crack Open an Old One A highly unscientific evaluation of Germany’s oldest breweries.
  Life
Gentleman Scholar
Oct. 22 2014 5:54 PM May I Offer to Sharpen My Friends’ Knives? Or would that be rude?
  Double X
The XX Factor
Oct. 22 2014 4:27 PM Three Ways Your Text Messages Change After You Get Married
  Slate Plus
Tv Club
Oct. 22 2014 5:27 PM The Slate Walking Dead Podcast A spoiler-filled discussion of Episodes 1 and 2.
  Arts
Brow Beat
Oct. 22 2014 4:10 PM Skinny Mark Wahlberg Goes for an Oscar: The First Trailer for The Gambler
  Technology
Future Tense
Oct. 22 2014 5:33 PM One More Reason Not to Use PowerPoint: It’s The Gateway for a Serious Windows Vulnerability
  Health & Science
Wild Things
Oct. 22 2014 2:42 PM Orcas, Via Drone, for the First Time Ever
  Sports
Sports Nut
Oct. 20 2014 5:09 PM Keepaway, on Three. Ready—Break! On his record-breaking touchdown pass, Peyton Manning couldn’t even leave the celebration to chance.