by Ann » Sun Nov 20, 2016 6:07 am
Good work, NASA, ESA, Wendy Freedman et al., and Judy!
NGC 4414. Photo: Adam Block.
NGC 3521. Photo: NGC 3521. Photo: R Jay GaBany, David Martinez-Delgado et al.
I have to post my favorite photo of NGC 4414, which is by Adam Block. His photo, at left, shows the enormous starry halo surrounding this galaxy. Other flocculent galaxies also sport stellar huge halos, such as NGC 3521 at right.
NGC 3923. Photo: David Malin.
I think it was David Malin who first discovered extended shells around galaxies, but he discovered shells surrounding ellipticals. The explanation he gave, if I understood him correctly, was that elliptical galaxies have undergone many mergers, which have all sent some sort of shock waves through the galaxy. In response, tidal waves of stars have flown outwards and then stopped and receded, not entirely unlike waves washing up on a beach and then receding. But not all the stars that were washed out have receded afterwards. Some of them have stayed in their new position, marking the edge(s) of the shell(s).
I find it interesting to think of flocculent spirals in a similar way. These spirals have undergone some sort of upheavals, which have stirred up and broken their spiral symmetry, and at the same time waves of stars have washed out from them and then (mostly) receded.
The shells of NGC 3923 seem to have been lapping very gently, as has the huge shell of NGC 4414. NGC 3521, by contrast, still seems to be torn by forces ravaging it.
And of course, as I said, in flocculent galaxies the same forces that made the shells would have broken up the spiral arms, making them short and disordered.
Ann
Good work, NASA, ESA, Wendy Freedman et al., and Judy!
[float=left][img2]http://www.adamblockphotos.com/uploads/4/4/2/5/44258693/8023651.jpg?812[/img2][c][size=85]NGC 4414. Photo: Adam Block.[/size][/c][/float] [float=right][img2]http://www.dailygalaxy.com/.a/6a00d8341bf7f753ef017ee42f1239970d-600wi[/img2][c][size=80]NGC 3521. Photo: NGC 3521. Photo: R Jay GaBany, David Martinez-Delgado et al.[/size][/c][/float]
I have to post my favorite photo of NGC 4414, which is by Adam Block. His photo, at left, shows the enormous starry halo surrounding this galaxy. Other flocculent galaxies also sport stellar huge halos, such as NGC 3521 at right.
[float=left][img2]https://inspirehep.net/record/1288745/files/N3923.png[/img2][c][size=85]NGC 3923. Photo: David Malin.[/size][/c][/float] [float=right][img2]http://l7.alamy.com/zooms/947a0b637a51441196be90646befb0f7/the-setting-sun-illuminating-a-receding-wave-on-windansea-beach-la-e47t67.jpg[/img2][/float]
I think it was David Malin who first discovered extended shells around galaxies, but he discovered shells surrounding ellipticals. The explanation he gave, if I understood him correctly, was that elliptical galaxies have undergone many mergers, which have all sent some sort of shock waves through the galaxy. In response, tidal waves of stars have flown outwards and then stopped and receded, not entirely unlike waves washing up on a beach and then receding. But not all the stars that were washed out have receded afterwards. Some of them have stayed in their new position, marking the edge(s) of the shell(s).
I find it interesting to think of flocculent spirals in a similar way. These spirals have undergone some sort of upheavals, which have stirred up and broken their spiral symmetry, and at the same time waves of stars have washed out from them and then (mostly) receded.
The shells of NGC 3923 seem to have been lapping very gently, as has the huge shell of NGC 4414. NGC 3521, by contrast, still seems to be torn by forces ravaging it.
And of course, as I said, in flocculent galaxies the same forces that made the shells would have broken up the spiral arms, making them short and disordered.
Ann