Today's APOD is a truly fascinating portrait of the effects of a galactic collision that I personally had never heard of before. I have found a few pictures that might shed some light on what is going on.
Ejected gas from NGC 5291 and its interloper.
Credit: P-A Duc, CEA-CNRS/NRAO/AUI/NSF/NASA.
The picture at left shows gas that has been ejected from the interaction between NGC 5291 and its interloper, the Seashell galaxy. The ejected gas forms a huge ring. Where the gas is more concentrated, dwarf galaxies have formed.
NGC 5291, the Seashell galaxy and surrounding material.
Photo: Spitzer Space Telescope.
At right is a Spitzer Space Telescope infrared image of the same region. Stars are shown as cyan-colored, while dust is red. The larger galaxy, NGC 5291, contains some dust, which can be seen in the Spitzer image as a faint red ring around its center. In today's APOD, the dust is very obvious. (The dwarf galaxies that have formed in the ejected gas stream are very red from warm dust.)
But the Seashell galaxy is "all blue" in the Spitzer image. It contains no concentrated pockets of dust at all. Virtually all dust and gas has been driven out of the Seashell galaxy.
To me, the most amazing object in today's APOD is the Seashell galaxy. This small galaxy contains no gas or dust, is all yellow and has an amazing, elegantly curving and "sharp-edged" shape, almost like a cosmic cutout. It is so different from the typical yellow elliptical "blob". But because of its yellow color and perfectly smooth "brightness profile", it is also so different from a typical spiral galaxy.
NGC 5216 and 5218.
Photo: Martin Winder, Dietmar Hager.
I can't resist comparing The Seashell galaxy to NGC 5218, one of the two interacting galaxies in what is known as Keenan's system. NGC 5218 has bluish arcs surrounding a yellow bulge. The arcs are sharp-edged and smooth. They contain no star clusters or dust patches at all. The bluish arcs are the remains of previous star formation. Their smooth shape and lack of dust shows that no new stars are forming there today. The stars in the arc are likely about one to two billion years old, and their shape is likely caused by a mixture of normal interactions of components in a spiral galaxy and interactions with NGC 5216.
But the stars in the Seashell galaxy must be much older. The Seashell must have given up all star formation several billion years ago. Note that the entire galaxy is the same yellow color. How can its strange shape be explained?
I think the Seashell is an example of strong tidal forces acting on a relatively small and lightweight elliptical galaxy. The fantastic shape of it must be caused by gravity acting differently on different parts of it. In fact, the Seashell shows a ring-like structure which is not unlike the huge gas ring emanating from the interaction between NGC 5291 and the Seashell, and the "ring" of the Seashell is oriented in more or less the same direction as the huge gas ring.
What a truly fantastic APOD!