sentences:  This collision produces a density wave in the disk that leads to a characteristic ring-like appearance.Such an event would have happened at least 2–3 billion years in the past, and may have resembled the processes that form polar-ring galaxies.However, there is no sign of any second galaxy that would have acted as the "bullet", and the likely older core of Hoag's Object has a very low velocity relative to the ring, making the typical formation hypothesis quite unlikely.