Starship Asterisk*

One Armed Spiral Galaxy NGC 4725

Explanation: While most spiral galaxies, including our own Milky Way, have two or more spiral arms, NGC 4725 has only one. In this sharp color composite image, the solo spira mirabilis seems to wind from a prominent ring of bluish, newborn star clusters and red tinted star forming regions. The odd galaxy also sports obscuring dust lanes a yellowish central bar structure composed of an older population of stars. NGC 4725 is over 100 thousand light-years across and lies 41 million light-years away in the well-groomed constellation Coma Berenices. 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. Also included in the frame, a more traditional looking spiral appears as a smaller background galaxy.

<< Previous APOD

This Day in APOD

Next APOD >>

[/b]

Maybe the Ring Galaxy would be a good name for it!

The "smaller background galaxy" is NGC 4712, in case anyone was wondering.

Rob

Great...!!! Another Gimp Galaxy for Captain Spongefoot , Mr. Mortar, Lady J , Ms Lee Dean The Kinder Gartner Queen, The Kinder Gartners and KdeV to fix with it's load of Dihydrogen Monoxide generators aboard. Loaded and ready for action , Thank you.

Always fascinating, the different phenomena the Universe holds...

I looks like it was in a collision...

How about..."THE COIL GALAXY" for a name?

:---[===] *

APOD Robot wrote: One Armed Spiral Galaxy NGC 4725

Explanation: While most spiral galaxies, including our own Milky Way, have two or more spiral arms, NGC 4725 has only one. In this sharp color composite image, the solo spira mirabilis seems to wind from a prominent ring of bluish, newborn star clusters and red tinted star forming regions. The odd galaxy also sports obscuring dust lanes a yellowish central bar structure composed of an older population of stars. NGC 4725 is over 100 thousand light-years across and lies 41 million light-years away in the well-groomed constellation Coma Berenices.

[img3="SBa galaxies can easy windup with "one arm""]http://upload.wikimedia.org/wikipedia/c ... ngFork.jpg[/img3]

One Armed Galaxy.

rstevenson wrote:The "smaller background galaxy" is NGC 4712, in case anyone was wondering.

Rob

Hey, thanks, Rob; I was wondering that! Funny it wasn't mentioned in the caption/blurb...

Ann wrote:>>>
What other one-armed spirals do you know of, Art, apart from NGC 4725? You mean NGC 772?

Ann

I'm somewhat confused.
When I looked at images of NGC 772/Arp 78, as in this Apod, I see TWO arms, originating very near each other.

This astrophotographer says
NGC 772 is both huge and distant.  Despite its distance of 106 million light years in the constellation Aries, NGC 772 is twice the diameter of our Milky Way Galaxy, allowing us to see detailed structure.  The asymmetric spiral arms are distorted by interaction with the dwarf elliptical galaxy NGC 770 at the 1:00 position.  Stars stewn outward from NGC 770 appear as a fine haze extending away from the pair. 

I find the shape of NGC 772 resembles the nautilus shell, as illustrated to the right, so I have called it the Nautilus Galaxy.. 

Margarita

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.

In the case of NGC 4725 (also known by the telephone number UGC 7989) it is known that the spiral arm is trailing. For those that like the details, here is data of neutral hydrogen in the galaxy. The nice thing of the hydrogen is that one can measure the rotation easily. This rotation combined with where the dust lanes are (which tell what is the near and what is the far side of the galaxy disk) tell that the spiral arm must be trailing.

Rense Boomsma

Ann wrote:
Well, NGC 4314 isn't one-armed, but instead it is a perfect example of a highly evolved but still recognizable grand design two-armed spiral.

I never said that it was. But it wouldn't take much Wild-Root Cream-Oil to get that cowlick down into a single arm.

Ann wrote:
What other one-armed spirals do you know of, Art,
apart from NGC 4725?

Well, NGC 4725 / UGC 7989 isn't really one-armed, but instead, it is a perfect example of a highly evolved but still recognizable grand design two-armed SBa spiral.

http://asterisk.apod.com/viewtopic.php? ... 62#p200212

http://en.wikipedia.org/wiki/Fearless_Fosdick

one arm, two arms… I have two but cannot create stars… I feel so inadequate… I reckon NGC 4725 is quite busy what with so few arms with which to work with… perhaps a comb WOULD help!

renseb wrote:

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.

In the case of NGC 4725 (also known by the telephone number UGC 7989) it is known that the spiral arm is trailing. For those that like the details, here is data of neutral hydrogen in the galaxy. The nice thing of the hydrogen is that one can measure the rotation easily. This rotation combined with where the dust lanes are (which tell what is the near and what is the far side of the galaxy disk) tell that the spiral arm must be trailing.

Rense Boomsma

Thanks for the authoritative information, Dr Boomsma!

As I can't send you private message, may I use this format to ask if you would write a little on the Wikipedia page
http://en.wikipedia.org/wiki/NGC_6946

about the findings that I read about in Universe Today?
Margarita

One thing I find interested in a little knot in the arm of the galaxy, at about the 5:30 position. I can't quite tell if it's a small region of sub-rotation, or another galaxy in the background. Be kind of neat, though, to actually find a galaxy with a notable eddy spiral in one of its arms.

I'd say that's a background galaxy, Tara. If you search for more images of NGC 4725, you can find some which make it more apparent because the photographers used different processing or imaging techniques.

Ann wrote:

Tara_Li wrote:One thing I find interested in a little knot in the arm of the galaxy, at about the 5:30 position. I can't quite tell if it's a small region of sub-rotation, or another galaxy in the background. Be kind of neat, though, to actually find a galaxy with a notable eddy spiral in one of its arms.

I agree with Geckzilla that it is probably a background galaxy, but in my humble amateur opionon, it could actually be an eddy in the arm of NGC 4725.

There exists one definite example of a smallish spiral galaxy that clearly sits in a tidal feature connecting two galaxies. Take a look at this Hubble image of NGC 6050 and IC 1179. This interacting pair is also known as Arp 272. As you can see, there is most definitely a third galaxy involved in this cosmic dance.

Eddies can appear in fluid dynamics but not, I think, when the dynamism of a system is driven by gravity. Any seeming eddy in an image of a galaxy will either be a background galaxy or a much smaller colliding galaxy, as in the image you linked to Ann. In that case I don't think eddy is quite the right word. An eddy is "a current ... running contrary to the main current; especially : a circular current : whirlpool" according to one on-line dictionary. It's that contrariness that can't spontaneously appear in gravitational systems. And if it does come about because of a galaxy collision, it is really just an independant gravitational system only temporarily operating near the larger system. It will pass through and be disrupted as it does so, so it wouldn't be right to call it an eddy of the larger system.

Rob

rstevenson wrote:Eddies can appear in fluid dynamics but not, I think, when the dynamism of a system is driven by gravity. Any seeming eddy in an image of a galaxy will either be a background galaxy or a much smaller colliding galaxy, as in the image you linked to Ann. In that case I don't think eddy is quite the right word. An eddy is "a current ... running contrary to the main current; especially : a circular current : whirlpool" according to one on-line dictionary. It's that contrariness that can't spontaneously appear in gravitational systems. And if it does come about because of a galaxy collision, it is really just an independant gravitational system only temporarily operating near the larger system. It will pass through and be disrupted as it does so, so it wouldn't be right to call it an eddy of the larger system.

Except that aren't planets supposed to collapse from eddies in the dust clouds around the stars? Sure, there's some magnetic effects in play, but the primary force in the protoplanetary disk is gravity. "Fluid" is kind of a fluid term, really...

Tara_Li wrote:

rstevenson wrote:Eddies can appear in fluid dynamics but not, I think, when the dynamism of a system is driven by gravity. ...

Except that aren't planets supposed to collapse from eddies in the dust clouds around the stars? Sure, there's some magnetic effects in play, but the primary force in the protoplanetary disk is gravity. "Fluid" is kind of a fluid term, really...

Yes, but that's because the gas and dust in a newly forming stellar system can form local aglomerations which gravitationally coalesce into planets. But to get an eddy on a galactic scale you'd need to figure out some way to get entire stellar systems - a great many of them in a group, if they're to appear to us as an eddy - to loop around as they orbit the galactic center of gravity. In other words, they'd need a local center of gravity around which to orbit, just like planets orbit around their star. Conceivably a black hole could collect a few stellar systems around itself, and drag them around the galaxy as it orbits. But I don't think that is gravitationally stable in the long run. It would make for an interesting modelling exercise though.

Rob

Tara_Li wrote:Except that aren't planets supposed to collapse from eddies in the dust clouds around the stars? Sure, there's some magnetic effects in play, but the primary force in the protoplanetary disk is gravity. "Fluid" is kind of a fluid term, really...

No, the primary force in the protoplanetary disc isn't gravity. The density of the disc is high enough that you have fluid dynamic mechanisms in play. That's what allows the formation of eddies, and it's what allows the system to collapse, heating up and creating the central star in the first place. Once you have accretion and the remaining dust and gas is blown away by the protostar, the system becomes almost completely gravity dominated. Outside of occasional collisions and near misses, there is very little energy or momentum transfer, and the system is essentially stable.