jimsaruff wrote:So what's it's age; (BTW how old is our's?)
And why doesn't it have arms?
It is likely that a large galaxy like NGC 253 is very old. It may well have started forming in the early childhood of the universe, and the universe is estimated to be, if I remember correctly, 13.7 billion years old. (But you shouldn't take my word for it. Google it if you are really interested.)
If we assume, however, that the universe really is 13.7 billion years old, then a galaxy like NGC 253 is probably at least 12 billion years old, if you ask me. But you can be sure that it hasn't always looked the way it does now. For one thing, it must of course have been much, much smaller when it started forming 12 billion years ago or more. It must also have undergone violent episodes of star formation, when its shape must have been chaotic and its light must have been dominated by bright blue stars and pink emission nebulae.
Personally I'd say that if we could see NGC 253 more face on, we still wouldn't think that its arms were very regular and dominant. Not all galaxies possess large and dominant arms. Please follow this link
to see a picture by Jyri Näränen and Kalle Torstensson of almost face-on galaxy NGC 7217. The galaxy has intricate "swirling" dust structures, but it doesn't really have arms.
On the other hand, galaxies can be seen to have obvious arms even if they are seen at a high inclination. Follow this link
to see a picture of galaxy NGC 3981. Even though the galaxy is almost edge-on to us, we can clearly see its two dominant spiral arms.
Also take a look at NGC 4100 by Jack Burgess and Adam Block here
. Even though the general outline of that galaxy is very regular, the arm structure of its disk is very obvious.
As to why some galaxies have dominant arms and other don't, well, I'd say it has everything to do with how the individual galaxy grew and evolved, how many mergers it had and how messy they were, and whether they enhanced or braked the rotation of the galaxy's disk, and how big the black hole in the center of the galaxy is, and so on and so on.
Last edited by Ann on Wed Dec 21, 2011 6:11 am, edited 1 time in total.