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Fig. 1

image

Shell appearance differs depending on the line of sight. In the three-dimensional space, a shell is a part of a spherical surface centered on the galaxy. The observer in direction A sees the shell as a sharp-edged structure. The observer viewing the shell from direction B does not see any part of the shell tangentially, and so the shell appears diffuse. The two images on the right show an illustrative simulation of a radial minor merger between two spherical galaxies. Only the stars from the smaller galaxy are shown. The middle panel shows the view perpendicular to the line of collision. The view in the right panel is inclined by 35° to the line of collision.

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