One-Armed Galaxy (2009 June 6)

[apodman]

http://antwrp.gsfc.nasa.gov/apod/ap090606.html

One-Armed Spiral Galaxy NGC 4725

also http://antwrp.gsfc.nasa.gov/apod/ap050901.html

also http://antwrp.gsfc.nasa.gov/apod/ap050618.html

Thank goodness for these photographs. Until now, nobody believed me about the one-armed galaxy.

Sincerely,
Dr. Richard Kimble

[orin stepanek]

(Computer simulations of the formation of single spiral arms suggest that they can be either leading or trailing arms with respect to a galaxy's overall rotation.)

Leading or trailing? Wouldn't the arms be trailing the rotation of the galaxi? I woulld think that leading arms would be against the nature of the beast!

Orin

[apodman]

Leading or trailing? Wouldn't the arms be trailing the rotation of the galaxi? I woulld think that leading arms would be against the nature of the beast!

I've seen a few simulations of the formation and maintenance of spiral galaxy arms. None of them convinced me 100% that the simulation reflects all the actual laws of physics that are in play. It's easy to make a simulation but more difficult to make one that accurately reflects physical reality. So just because a simulation says it's so doesn't convince me it's really so. As far as I'm concerned, the question of the evolution of spiral arms is still open.

[bystander]

Thank goodness for these photographs. Until now, nobody believed me about the one-armed galaxy.

Why should we believe you, Dr. Kimble, a pod man and a fugitive from justice.

[Chris Peterson]

(Computer simulations of the formation of single spiral arms suggest that they can be either leading or trailing arms with respect to a galaxy's overall rotation.)

Leading or trailing? Wouldn't the arms be trailing the rotation of the galaxi? I woulld think that leading arms would be against the nature of the beast! :?

We are easily fooled by our preconceptions. A spiral galaxy looks like some sort of pinwheel, so it is hard to see it otherwise. But actually, it is a big rotating disc, with most of its matter invisible to us (but still exerting gravitational influence). The arms are actually shock waves within the disk, not stable, gravitationally bound structures in their own right.

[lakeside]

THERE ARE TWO ADDITIONAL REMARKABLE FEATURES OF THE ONE ARMED GALAXY. ONE IS THE VERY FAINT BUT LARGE HALO SURROUNDING THE LOWER AND LEFT PORTIONS OF THE GALAXY. THE LIGHT EMITTED FROM THE HALO MUST ORIGINATE FROM STARS OF SOME SORT. ARE THEY DIFFERENT IN SOME WAY FROM WHAT IS NORMAL DO THEY OFFER A CLUE ABOUT DARK MATTER IN THIS VERY LARGE AND DIFFUSE HALO? THE SECOND FEATURE IS THE INCOMING MASS IN COLLISION WHICH PRESUMABLY FORMS THE "SPOKES" OF HOT YOUNG STARS. IS THERE A SPECTOSCOPIC CLUE AS TO THE NATURE OF THAT MASS AND IS IT DIFFERENT FROM SPIRAL ARMS IN OTHER GALAXIES??

JW

[orin stepanek]

THERE ARE TWO ADDITIONAL REMARKABLE FEATURES OF THE ONE ARMED GALAXY. ONE IS THE VERY FAINT BUT LARGE HALO SURROUNDING THE LOWER AND LEFT PORTIONS OF THE GALAXY. THE LIGHT EMITTED FROM THE HALO MUST ORIGINATE FROM STARS OF SOME SORT. ARE THEY DIFFERENT IN SOME WAY FROM WHAT IS NORMAL DO THEY OFFER A CLUE ABOUT DARK MATTER IN THIS VERY LARGE AND DIFFUSE HALO? THE SECOND FEATURE IS THE INCOMING MASS IN COLLISION WHICH PRESUMABLY FORMS THE "SPOKES" OF HOT YOUNG STARS. IS THERE A SPECTOSCOPIC CLUE AS TO THE NATURE OF THAT MASS AND IS IT DIFFERENT FROM SPIRAL ARMS IN OTHER GALAXIES??

JW

I'm wondering if the halo you speak of is the glow of stars so faint that you can't make out their individuality. Surely a galaxy that is 100,000 LY across must have billions of stars in it.

Orin

[orin stepanek]

(Computer simulations of the formation of single spiral arms suggest that they can be either leading or trailing arms with respect to a galaxy's overall rotation.)

Leading or trailing? Wouldn't the arms be trailing the rotation of the galaxi? I woulld think that leading arms would be against the nature of the beast!

We are easily fooled by our preconceptions. A spiral galaxy looks like some sort of pinwheel, so it is hard to see it otherwise. But actually, it is a big rotating disc, with most of its matter invisible to us (but still exerting gravitational influence). The arms are actually shock waves within the disk, not stable, gravitationally bound structures in their own right.

Isn't is possible to measure the rotation of a galaxy? What is rotation curve?

Orin

[Chris Peterson]

Isn't is possible to measure the rotation of a galaxy? What is rotation curve?

Yes, it is generally pretty easy to measure the rotation of galaxies by looking at Doppler shifts. The rotation curve is the plot of rotation rate as a function of distance from the center. Galaxy rotation curves provided the first major piece of evidence for dark matter.

[neufer]

Thank goodness for these photographs. Until now, nobody believed me about the one-armed galaxy.

Why should we believe you, Dr. Kimble, a pod man and a fugitive from justice.

apodman: [Holding bystander at gunpoint] It was this one-armed galaxy!

Deputy Marshal bystander: I don't care!