This is fantasy, but I hope for some sensible comment from all you Cosmic Engineers out there.
The Sombrero Galaxy is a spectacular sight, seen almost edge on, with a thick rim of dust. In fact a recent pic taken in the Infrared shows that almost all its dust is concentrated in the rim. http://apod.nasa.gov/apod/ap050511.html
Could it be, could it just be, that this is the residue of a Galactic RingWorld?
In Niven's stories, the Ringworld stays intact.
In Iain M.Banks' "Consider Phlebas" he imagines a smaller version, an Orbital, that he then destroys, using the weapon 'grid-fire',a function of the energy between the universes in the Multiverse. I said this was fantasy!
The fragments of the orbital are dust, that immediately spread out and disperse under radial momentum of the spinning Orbital.
But a Galactic RingWorld is on such a different scale. How fast would it need to spin for One G at its inner surface? Not much, I suspect in relation to either the Orbital or RingWorld. So less radial momentum to spread it out, and the distances are galactic, not mere star-system, so it would take much, much longer to dissipate. In other words, could the Sombrero have been the site of a Galactic RingWorld, constructed out of all the dust that it originally contained, that has suffered a catastrophic disintegration?