The Outer Shells of Centaurus A

What causes the surrounding shells in peculiar galaxy Cen A? Last month a fascinating image of peculiar galaxy Centaurus A was released , processed to highlight a faint blue arc indicating an ongoing collision with a smaller galaxy. Another interesting feature of Cen A , however, is the surrounding system of shells , better visible here in this recently released wider pan from the four meter Blanco telescope at Cerro Tololo Inter-American Observatory . Faint shells around galaxies are not unusual and considered by themselves as evidence of a previous galaxy merger , analogous to water ripples on a pond. An unexpected attribute of these shells is the abundance of gas, which should become separated from existing stars during the collision .