starsurfer wrote: Ann wrote:
What you describe as haloes in these particular galaxies are actually examples of tidal shells.
So do you think that the chaotic nature of the spiral pattern of flocculent galaxies as well as
their extended tidal shells can be products of mergers and collisions?
Isn't it generally accepted that the spiral structure in galaxies is a result of density waves? I think collisions would affect density waves and therefore the overall structure. You can easily see this in the visual appearance of interacting and peculiar galaxies.
M51, a grand design galaxy with long arms, and companion NGC 5195. Photo: Bill Snyder.
NGC 2775, a flocculent galaxy with patchy, broken spiral fragments. Photo: Adam Block.
It is certainly true that interactions can enhance the spiral structure of disk galaxies. M51, the Whirlpool galaxy, is the poster child for this sort of enhanced spiral structure through interactions with another galaxy. Bill Snyder's picture also shows large tidal shells surrounding both M51 and its companion, NGC 5195. These shells are clearly also caused by the mutual interactions.
But take a look at Adam Block's picture of NGC 2775. Go to this page to see a full size image
of it. Here you can also see a huge tidal shell surrounding NGC 2775.
NGC 2775 could be considered a poster child for flocculent galaxies. Its spiral structure is extremely broken up and patchy.
But NGC 2775 doesn't have any sort of companion that is in any way comparable to the Whirlpool Galaxy's NGC 5195. Adam Block's full size image shows two small blue galaxies at upper right of NGC 2775; they could be at the same distance as NGC 2775 and possibly be interacting with it, but being small and blue, they are quite clearly not very massive. A small yellowish galaxy is visible in the lower left corner of the full size image. Being yellowish, it is a bit more massive than the small blue ones, but no tidal bridge can be seen between this galaxy and NGC 2775.
I'm wondering if NGC 2775 could have interacted rather violently with another galaxy and perhaps merged with it. That would explain the huge tidal shell. It could also possibly explain NGC 2775's extremely patchy and almost disintegrated spiral structure.
My point is that interactions can both enhance, disrupt and perhaps destroy a galaxy's spiral structure.