This is a very interesting galaxy indeed. But shells around elliptical galaxies are not uncommon, although they are rarely as complex as the shells and streams around NGC 474.
In his book
A View of the Universe (1993), David Malin wrote about how he discovered shell-like features in the outer envelope of galaxy Centaurus A during a meeting with astronomer John Graham:
John offered this plate as a suitable object for demonstrating unsharp masking and we very quickly discovered that the faint outer envelope of the galaxy embraces a series of incomplete arcs or shells, illustrated in Fig. 8.30.
...
The improved photographic emulsions of the last two decades and the photographic techniques they have triggered have shown that a surprisingly large number of otherwise normal-looking ellipticals had shell-like structures of the kind found in Cen A.
Obviously some of the features seen in NGC 474 are tidal tails and
stellar streams similar to the ones found around the Milky Way, but personally I believe, in my complete amateur way, that ripples have a lot to do with the shells:
APOD Robot wrote:
Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant.
Ripples are like splashes when a rock falls into a pond. When it comes to a galaxy, it is easy to imagine that a density wave ripples through the galaxy, reaches a maximum size, and withdraws again. But it might leave arcs and shells of stars behind in a way similar to how
a wave may wash pebbles onto a beach.
The ultimate ripple galaxy may be the
Cartwheel galaxy. The Cartwheel galaxy is of course not an elliptical galaxy, but it might become one in the future. And then it might sport some magnificent shells. Maybe it will look like
NGC 1344, an elliptical galaxy that just might have been a Cartwheel Galaxy in the past.
Ann
This is a very interesting galaxy indeed. But shells around elliptical galaxies are not uncommon, although they are rarely as complex as the shells and streams around NGC 474.
In his book [i][url=http://i.ebayimg.com/t/View-Universe-David-Malin-1993-Hardcover/00/$T2eC16FHJGwFFZV5dNEpBR(!GksH9w~~_35.JPG]A View of the Universe[/url][/i] (1993), David Malin wrote about how he discovered shell-like features in the outer envelope of galaxy Centaurus A during a meeting with astronomer John Graham:
[quote]John offered this plate as a suitable object for demonstrating unsharp masking and we very quickly discovered that the faint outer envelope of the galaxy embraces a series of incomplete arcs or shells, illustrated in Fig. 8.30.
...
The improved photographic emulsions of the last two decades and the photographic techniques they have triggered have shown that a surprisingly large number of otherwise normal-looking ellipticals had shell-like structures of the kind found in Cen A. [/quote]
Obviously some of the features seen in NGC 474 are tidal tails and [url=http://news.rpi.edu/luwakkey/2842]stellar streams similar to the ones found around the Milky Way[/url], but personally I believe, in my complete amateur way, that ripples have a lot to do with the shells:
[quote]APOD Robot wrote:
Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple though the galactic giant.[/quote]
Ripples are like splashes when a rock falls into a pond. When it comes to a galaxy, it is easy to imagine that a density wave ripples through the galaxy, reaches a maximum size, and withdraws again. But it might leave arcs and shells of stars behind in a way similar to how [url=http://lakehuron.ca/uploads/images/black-streaks-magnetite.jpg]a wave may wash pebbles onto a beach[/url].
The ultimate ripple galaxy may be the [url=http://upload.wikimedia.org/wikipedia/commons/2/26/Cartwheel_Galaxy.jpg]Cartwheel galaxy[/url]. The Cartwheel galaxy is of course not an elliptical galaxy, but it might become one in the future. And then it might sport some magnificent shells. Maybe it will look like [url=http://www.aao.gov.au/images/image/ngc1344.jpg]NGC 1344[/url], an elliptical galaxy that just might have been a Cartwheel Galaxy in the past.
Ann