Is it not conceivable that a type 3 civilization is keeping this galaxy shipshape and Bristol fashion.

I like thinking so.

The galactic components (the stars, clusters and dust and molecular clouds in the disk) do revolve about the center in elliptical orbits), but the arms are actually stationary. The revolving objects interact gravitationally and their orbits are dynamically coupled forming spiral shaped arcs where these objects are traveling in roughly parallel lines. Check out this illustration, this is impossible to describe verbally.

These rotating objects pass through the areas of compression Where they briefly interact gravitationally. In the big clouds moving through, the gases are compressed, creating new stars. The burst of hot, bright, but very short-lived (only several million years) blue giants that briefly flares up as the clouds move through the compression zone light up the arms, making them visible from extragalactic distances. They pass out of the arm, but soon fade away, while the longer-lived normal stars like our sun, survive too faint to be seen at all.

Wiki has an excellent article on this, with wonderful illustrations and animations. http://en.wikipedia.org/wiki/Density_wave_theory

Originally, astronomers had the idea that the arms of a spiral galaxy were material. However, if this were the case, then the arms would become more and more tightly wound, since the matter nearer to the center of the galaxy rotates faster than the matter at the edge of the galaxy. The arms would become indistinguishable from the rest of the galaxy after only a few orbits. This is called the winding problem.

Lin and Shu proposed in 1964 that the arms were not material in nature, but instead made up of areas of greater density, similar to a traffic jam on a highway. The cars move through the traffic jam: the density of cars increases in the middle of it. The traffic jam itself, however, does not move (or not a great deal, in comparison to the cars). In the galaxy, stars, gas, dust, and other components move through the density waves, are compressed, and then move out of them.

More specifically, the density wave theory argues that the “gravitational attraction between stars at different radii” prevents the so-called winding problem, and actually maintains the spiral pattern.

So the disk material moves through the stationary arm, not the other way round, and as it does so is briefly illuminated by the biggest stars that form there briefly, along with billions more like our sun which are too faint to make up a major part of the galaxy’s luminosity. The big giants quickly die as they move out of the arm, going supernova. This all happens at incredibly long time scales, so all we see is an instant snapshot of the process. We only see one supernova per galaxy per century, but in the lifetime of the galaxy, they are going off like camera flashes in the bleachers at a night game. It takes our sun 200 million years to circle the galaxy, its made about 25 trips since it formed, and it has about the same number to go before it fades away to a cinder.

The Galaxy is not only alive, it is evolving. It is a cauldron of forces, gravitational, acoustic, shock waves, radiation, constantly being enriched by heavy elements cooked in the stars and exploding out into the interstellar medium. It is surrounded by a cloud of orbiting ancient globular clusters with hundreds of thousands of stars, and it interacts gravitationally with satellite galaxies, with great tides sweeping it, and occasionally it even collides with other passing galaxies.

And at the center of it all is colossal black hole of several million solar masses.