by Ann » Tue Jul 03, 2012 9:09 pm
I want to return to the idea of "ripples in a pond". Astrophtographer David Malin was, or so I think, the first person to notice that large elliptical galaxies are surrounded by faint shells. David Malin interpreted these shells as signs of mergers. When a smaller galaxy fell into a larger galaxy, the larger galaxy would respond by expanding. But as the galaxy rebounded and contracted again, shells of stars would remain outside the galaxy, like debris washed up on a shore.
The Cartwheel Galaxy. Photo: Hubble
The Cartwheel galaxy was hit by another galaxy, and the Cartwheel responded by taking on this strange shape. As the Cartwheel rebounds and contracts again, remnants of its outer shell will likely remain.
When large ellipticals have many outer shells, that is probably a sign that they have undergone many mergers.
Ann
[float=left][img2]http://images.travelpod.com/users/wanderingdivers/2.1298391034.ripples-on-the-lake.jpg[/img2][c][size=80]Photo: Wanderingdivers[/size][/c][/float]I want to return to the idea of "ripples in a pond". Astrophtographer David Malin was, or so I think, the first person to notice that large elliptical galaxies are surrounded by faint shells. David Malin interpreted these shells as signs of mergers. When a smaller galaxy fell into a larger galaxy, the larger galaxy would respond by expanding. But as the galaxy rebounded and contracted again, shells of stars would remain outside the galaxy, like debris washed up on a shore.
[float=right][img]http://www.shannonthunderbird.com/cartwheel-galaxy.png[/img][c][size=80]The Cartwheel Galaxy. Photo: Hubble[/size][/c][/float] The Cartwheel galaxy was hit by another galaxy, and the Cartwheel responded by taking on this strange shape. As the Cartwheel rebounds and contracts again, remnants of its outer shell will likely remain.
When large ellipticals have many outer shells, that is probably a sign that they have undergone many mergers.
Ann