The Colorful Cosmos

Cosmic Shell Games

NGC 474 and NGC 470

NGC 474 is a large elliptical galaxy 100 million light years away in Pisces. It forms an interacting pair with NGC 470 the spiral galaxy to the right and is classified in the Peculiar Galaxy catalog as ARP 227 and peculiar it is.

NGC 474 has an unusual and complex series of shell layers and tidal streams that surround the central part of the galaxy and its core.

The origin of the structure has been somewhat of a mystery, however it was recently postulated that the tidal features come from the accretion of a spiral galaxy that collided around 1.3 billion years ago and merged around 900 million years ago.

Volume 660, April 2022 of AANDA in the Extragalactic Astronomy section theorizes that based on the phase-space wrapping model by Quinn the formation of the shells began by a nearly radial minor or intermediate merger of the two galaxies. The large galaxy disrupted the smaller secondary galaxy by tidal forces causing the release of stars which moved out at various radial velocities. The orbital period and position was then determined by pericentric velocities. In effect then the shells are density waves made of stars near the epicenters of their orbits and expand over time.