Elliptical galaxies are my least favorite DSO to image, since typically they are featureless. But about 10% of elliptical galaxies have faint shell structures. I find these shell structures to be fascinating.
The poster contains 10 examples of elliptical galaxies with faint shell structures. Next to each example is the object name and its distance from us, in millions of light years. The objects are shown to scale.
Shell structures are difficult to see and can easily be “lost” through overprocessing. Ironically, they can also be created, as artifacts, by overprocessing. When shell structures are present, image processing should be applied conservatively.
Shell structures are pretty much impossible to detect through the eyepiece, instead requiring photographic images to detect them. As such, they were discovered only recently, in 1980. They can easily be confused with star streams or loops.
Shells often look like faint concentric arcs in our two dimensional images but are believed to be three dimensional shells. These shell structures are usually massive, sometimes spanning hundreds of thousands of light years.
The shell structures are thought to be generated through compressive waves from galaxy mergers. Here is an interesting description by Dr. Ronald Buta from the 2011 paper, Galaxy Morphology:
“The explanation of shell/ripple galaxies is one of the great success stories in galactic dynamics. Shells are thought to be remnants of a minor merger between a massive elliptical and a lower mass disk-like galaxy. The main requirements are that the disk-shaped galaxy be "cold", or lack any random motions, and that the potential of the elliptical galaxy should be rigid, meaning the elliptical is much more massive than its companion. The smaller galaxy's stars fall into the center of the galaxy and phase wrap, or form alternating outward-moving density waves made of the disk galaxy's particles near the maximum excursions of their largely radial orbits in the rigid potential. Many, but not all, of the main properties of shells can be explained by this model.”If you would like to read more about any of these objects, each of the objects in the poster has previously been uploaded and described individually on Astrobin. They all reside in my
Astrobin Elliptical Shells Collection.
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