APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

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APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by APOD Robot » Thu Apr 16, 2015 4:12 am

Image 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, sporting a noticably more traditional spiral galaxy look, is a more distant background galaxy.

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by Ann » Thu Apr 16, 2015 4:35 am

Oh, that's a lovely image! :D

And NGC 4725 is such an interesting galaxy, with its single arm. The galaxy is very elegant, too. Its harmonious morphology (quite apart from its single arm) suggests that this is a relatively "settled-down" galaxy, with a large old population of yellow stars and limited amounts of star formation, harmoniously spread out in the galaxy so as not to upset the galactic equilibrium. A galaxy that is somewhat similar to NGC 4725 when it comes to stellar populations and galactic equlibrium is M81. NGC 488 is also somewhat similar to NGC 4725 in those respects (it looks bluer in this image than it really is).

So, fascinating! NGC 4725 is located in the well-groomed constellation of Corona Berenices, but the galaxy is rather well-groomed itself!

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by daddyo » Thu Apr 16, 2015 5:55 am

Leading or trailing arms? That seems so counterintuitive... But what does a brain know.

Maybe there's inner and outer shells of matter that have significantly different angular momentums, causing this effect, possibly from previous galactic mergers.

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by Craine » Thu Apr 16, 2015 12:27 pm

There seems to be a few bright spots to the bottom-right of the NGC 4725, with some more to the right and upwards
I am guessing here, but is this perhaps a dwarf-galaxy being gobbled up, with a tidal stream forming a small arm?

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by Dad is watching » Thu Apr 16, 2015 1:09 pm

My first impression was that the central part appears to be an older elliptical galaxy that has 'caught' a smaller galaxy, where the smaller galaxy has been ripped apart and is in a downward spiral (gravitationally speaking), leaving the one arm as it is drawn out in the process. It also made me wonder if a second lesser galaxy is caught in a similar fashion, would that result in a second arm? Could that be the arm creation process? Could we count the number of galaxies that a larger galaxy has consumed by counting the number of arms? :?:

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by Chris Peterson » Thu Apr 16, 2015 2:16 pm

daddyo wrote:Leading or trailing arms? That seems so counterintuitive... But what does a brain know.

Maybe there's inner and outer shells of matter that have significantly different angular momentums, causing this effect, possibly from previous galactic mergers.
Keep in mind that the spiral structure is largely illusory. That is, it doesn't represent structure that is winding or unwinding. Stars in the central region orbit at speeds which vary with distance from the center- slow at the very center, speeding up as the radial distance increases. But by the time we are far enough from the core to see spiral structure, the rotational velocity of all stars is about the same (because of the effects of dark matter). So the entire system moves in unison, like a pinwheel. And like a pinwheel, the direction of rotation has nothing to do with the direction of the fixed spiral structure.

Within that orbital motion, the spiral structure may be slowly moving either forward or backward in relation to the nearly fixed orbital speed of the stars that comprise it, rather like you might see density variations moving forward or backwards through lines of cars caught in a traffic jam.
Chris

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by Boomer12k » Thu Apr 16, 2015 3:57 pm

Nice one, Martin!

I am wondering if....Eliptical Galaxy first....in falling material, or Satellite Galaxy falling into it, and thus merger. I wonder if...second arms would be from a further second merger or in falling material, at another time, or angle of entry, but the link shows they are forming from all around the hub, so I may be wrong. In-falling material from the same arm, but at different angles. Since Older Central Population of Stars in hub... Plus inner part looks very "elliptical".
Left side to us, looks like a non absorbed mass. Whole thing looks like an accretion disk for a black hole...looks like some material has not fallen inward, "odd galaxy" link, shows some in-falling material has reached the central area, and are starting to form "arms", as material is drawn inward from different angles, as I commented above.

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by Chris Peterson » Thu Apr 16, 2015 4:06 pm

Boomer12k wrote:I am wondering if....Eliptical Galaxy first....in falling material, or Satellite Galaxy falling into it, and thus merger. I wonder if...second arms would be from a further second merger or in falling material, at another time, or angle of entry, but the link shows they are forming from all around the hub, so I may be wrong. In-falling material from the same arm, but at different angles. Since Older Central Population of Stars in hub... Plus inner part looks very "elliptical".
Most evidence suggests that large galaxies all start out as spirals, and that large ellipticals are end-stage products. Many spiral galaxies must have absorbed collisions without losing their essential structure, a fact which remains an unsolved problem for galaxy modelers. Almost certainly, however, dark matter is responsible for the dominant forces and processes.
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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by daddyo » Thu Apr 16, 2015 8:23 pm

Chris Peterson wrote:
daddyo wrote:Leading or trailing arms? That seems so counterintuitive... But what does a brain know.

Maybe there's inner and outer shells of matter that have significantly different angular momentums, causing this effect, possibly from previous galactic mergers.
Keep in mind that the spiral structure is largely illusory. That is, it doesn't represent structure that is winding or unwinding. Stars in the central region orbit at speeds which vary with distance from the center- slow at the very center, speeding up as the radial distance increases. But by the time we are far enough from the core to see spiral structure, the rotational velocity of all stars is about the same (because of the effects of dark matter). So the entire system moves in unison, like a pinwheel. And like a pinwheel, the direction of rotation has nothing to do with the direction of the fixed spiral structure.

Within that orbital motion, the spiral structure may be slowly moving either forward or backward in relation to the nearly fixed orbital speed of the stars that comprise it, rather like you might see density variations moving forward or backwards through lines of cars caught in a traffic jam.
This neat simulation seems to show a relationship between spiral arm structure direction and rotation direction: http://svs.gsfc.nasa.gov/vis/a010000/a0 ... ppletv.m4v

It's interesting how it appears the core bar is counter-rotating to the spiral arms though, it might be a video aliasing effect near the center since it's spinning so fast. But if you watch it evolve a few times it really looks like two opposite angular momentum structures, inner and outer. The article also says "There is a steady shift toward greater organization to the present time as the disorganized motions dissipate and rotation speeds increase. These galaxies are gradually settling into well-behaved disks." Maybe ellipticals are no longer thought to be the end product, if the simulation's right that is. I've always wondered how ellipticals form, for some reason thought they were just small galaxies and didn't have enough material to form interesting structures, or they were disks but got their arms ripped off from collisions.

Taken from http://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=11087

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by DavidLeodis » Fri Apr 17, 2015 11:13 am

It's a great picture :).

On enlarging it there are what seem to be a number of background galaxies. There is however a pair of galaxies (or perhaps a split galaxy if there can be such a thing) that intrigues me just above and a left of the very bright star above NGC 4725. The pair seem to form a mirror image, with each having a short trail. I would be interested to hear of any information about that pair.

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by starsurfer » Sun Apr 19, 2015 12:52 pm

Truly stunning image with great clarity in the details! I presume Ha data was taken as the nebulae are really clearly visible. It is possible that the appearance of NGC 4725 might partly be due to a possible interaction with the peculiar galaxy NGC 4747, which is also known as Arp 159. Both can be seen together in this image by Bob Franke.

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Re: APOD: One-Armed Spiral Galaxy NGC 4725 (2015 Apr 16)

Post by bettereye » Wed Apr 22, 2015 7:08 pm

I clearly see the second arm .. much shorter than the long one, but it is there, and I suspect its missing part is merged with the long one.