sillyworm 2 wrote: ↑
Fri Dec 28, 2018 2:48 pm
What are the factors(if known) that decide whether a galaxy has a single arm or multiple spiral arms?
Extremely few galaxies have just a single arm. One example of such a one-armed galaxy is NGC 4725. (But if you look carefully at the picture of NGC 4725, you can see what looks like a second (short) arm sprouting from about 6 o'clock and bending upwards and to the right.)
NGC 1097. Photo:
Spitzer Space Telescope.
Not many galaxies are truly two-armed, either. NGC 1097 at left is an example of a galaxy that is fairly
Most spiral galaxies have more than two arms. NGC 1672 at right is an example of a "mostly two-armed" galaxy that is sprouting several secondary arms.
My amateur impression is that the disks of starforming galaxies rather easily break up into multiple arms. I also think that galaxies with strong bars more easily develop just two arms instead of multiple arm structures, compared with unbarred galaxies or galaxies with weak bars. Moreover, I think that large massive galaxies more easily develop an elegant spiral system than smaller, more lightweight galaxies.
Also, I think that "just the right degree of disturbance" may enhance the formation of just two spiral arms. I think M81 (at left) owes its elegant two-armed shape to interactions with M82 at right, and NGC 3077 off the frame at upper left, and small companion galaxy Holmberg IX "above" M81.
I also want to show you one of the most elegant and perfect two-armed spiral galaxy that I know of, NGC 2857. In the SDSS picture at right, you can see that there are some other galaxies in the field, and it is just possible that NGC 2857 is interacting a little bit with one or more of them.
NGC 4625 (top) and NGC 4618.
I should add that UV images taken by the GALEX telescope are particularly good at revealing outer arm structures, since many (but not all) spirals form optically faint outer arms that are comparatively bright in ultraviolet light. In the picture at left, NGC 4625 at top is revealed to sport a large and intricate set of previously undiscovered ultraviolet-bright arms.
(By the way, this GALEX picture of NGC 1365
shows NGC 1365 to be mostly two-armed, and generally lacking in outer arms.)