Starship Asterisk*

Virgo Cluster Galaxy NGC 4731

Explanation: Barred spiral galaxy NGC 4731 lies some 65 million light-years away. The lovely island universe resides in the large Virgo cluster of galaxies. Colors in this well-composed, cosmic portrait, highlight plentiful, young, bluish star clusters along the galaxy's sweeping spiral arms. Its broad arms are distorted by gravitational interaction with a fellow Virgo cluster member, giant elliptical galaxy NGC 4697. NGC 4697 is beyond this frame above and to the left, but a smaller irregular galaxy NGC 4731A can be seen near the bottom in impressive detail with its own young blue star clusters. Of course, the individual, colorful, spiky stars in the scene are much closer, within our own Milky Way galaxy. NGC 4731 itself is well over 100,000 light-years across.

<< Previous APOD

Discuss Any APOD

Next APOD >>

[/b]

Maybe it should be named..."The Propeller Galaxy"...just a thought.

Or "The Tilde Galaxy."

I thought of it as the tadpole galaxy

I'm not quite sure this is an example of "an island universe"...though I'd be interested in seeing a picture.

owlice wrote:Or "The Tilde Galaxy."

Just goes to show - Even when you're bent out of shape, you can still look N-I-C-E

I for one think that NGC 4731 represents an early stage in the formation of a spiral galaxy. My basic idea is that you have two galaxies pass close to and disrupt each other (like the Antenna galaxies are doing). The result is that even their central cores get stretched out like silly putty. After they pass, there seems to be an effect whereby a lot of the mass gets pulled into the area between the galaxies, creating a gravitational field that halts their outward post-collision progression. It is as if their central objects are being drained to make a new central object. See the APOD for NGC 1300 (http://antwrp.gsfc.nasa.gov/apod/ap060827.html).

My interpretation of NCG 1300 is that each of its arms are the remains of a galaxy. The bright areas at the transitions to the bar are the remains of the central objects of the old galaxy, and the bar is a newly forming central object and halo for the combined galaxy. NCG 4536 (seen in http://www.atlasoftheuniverse.com/galgrps/virii.html) then is the next state of spiral galaxy formation, as the arms wrap around the new central object. Similarly, NGC 4731 is an earlier stage then NGC 1300, where the arms are still very loose and the new central object and halo are fairly young. (This is not to discount tidal effects from a nearby galaxy, which certainly are influencing the shape of the arms and the evolution of NCG 4731.)

It would be interesting to see if people can find pictures that represent transition stages between the initial collision and the formation of a strong bar. I also wonder what would cause such a phenomenon, and suspect that dark matter is the culprit. So maybe we can learn more given that a real-universe trajectory (or set of trajectories) for spiral galaxy formation from the remains of a collision can be identified.

Charles Ive's "The Unanswered Question" (mark).

If NGC 4731 has had a gravitational distortion from NGC 4697 you would not get the effect you can see. If they passed by each other, that would be a rarity indeed because of the vast distances between galaxies. If it is gravitational distortion, (assumption), then why are they no other distorted interactions among other galaxies in the Virgo Clusters? There are other galaxies that are closer than 4731 & 4697 but show no signs of distortion. And this also applies to other superclusters.

tesla wrote:If NGC 4731 has had a gravitational distortion from NGC 4697 you would not get the effect you can see. If they passed by each other, that would be a rarity indeed because of the vast distances between galaxies.

The Virgo cluster consists of a large number of galaxies that are interacting gravitationally, and are orbiting one another in a complex fashion. Repeated close passes and occasional collisions are to be expected; there is nothing particularly rare about them. And in fact, what we are seeing looks very much like what is seen in hundreds of other tidally distorted galaxies.

If it is gravitational distortion, (assumption), then why are they no other distorted interactions among other galaxies in the Virgo Clusters?

There are.

There are other galaxies that are closer than 4731 & 4697 but show no signs of distortion. And this also applies to other superclusters.

What galaxies are closer? You need to consider not only the current distance, but the distance over the last few hundred million years.

tesla wrote:If NGC 4731 has had a gravitational distortion from NGC 4697 you would not get the effect you can see.

On that point, I agree with you. That is one reason for my speculation above. (Of course, the shape itself is also more or less in accord with my speculation to begin with.)

If they passed by each other, that would be a rarity indeed because of the vast distances between galaxies.

However, on this point I must agree with Chris. Millions of light years matter little over the course of billions of years. In fact, it is now believed that most large galaxies are the result of multiple collisions involving smaller galaxies. Even my speculation is that the openness of the spiral arms of NGC 4731 is because that galaxy is the result of a recent collision between two smaller galaxies of similar size.