What is the most likely cause of the structural change from spiral galaxies to a large elliptical galaxy by 5.5 billion years?

A large elliptical galaxy can form when two spiral galaxies interact and merge. Their mutual gravitational pull brings them together, and the collision disrupts the spirals’ organized disks and arms, leaving a more rounded structure. Rotation alone would not turn spiral galaxies into a large elliptical galaxy, and mass loss or stellar cooling would not explain the dramatic structural change.

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