[CraigS]

Originally Posted by renormalised

I wouldn't necessarily agree with their determination they've made about rotation curves. There are errors there, but not that fundamental that they can't determine an overall pattern to the rotation curves in galaxies.

I think they're saying that the mass density distribution across the arm tends to effect the acceleration of any matter moving transversely. Until better mass density info is available, this data must be considered to have a high error factor.

When they measure the rotation curves of galaxies, they don't take it off the movements of the individual stars, they measure the curves for the whole mass of the galaxies. They measure the rotation curves using the Doppler shift of emission lines (across the EM spectrum) from gases such as CO, neutral hydrogen etc. These are generally in large concentrations scattered within the galaxy, but are associated with the spiral arms in particular. Trying to determine the rotation curve from individual stars would be nigh on impossible and would give you all sorts of answers to start with.

Because of the slowness of the movements over time ? (Timescale issue here ?). Also there'd be a huge amount of data to gather and process which so far, doesn't appear to have been attempted.

What they also tend to forget is that the stars that form in the spiral arms are generally moving rather slowly w.r.t. to rest of the arm's contents when they're born. Most of the stars that delineate spiral arms are large and don't last all that long in any case. They do move with respect to the z axis of the galaxy, but not very far in their lifetimes. It's the smaller stars, like the sun, that get scattered all over the place by gravitational interactions.

Yep. And they go on to do some basic calcs using our Sun as an example, in order to quantify the accelerations (order of magnitude stuff), along the various axes. Conclusion is a sinusoidal, variable acceleration resulting in oscillation transversely across the arm. "The Intra-Arm gravitational effects are strong enough to enable Arm Stability and Persistence. In fact, they are strong enough to enable Arm-genesis."

You get exceptions, like M67, but the only reason why it's lasted as long as it has (about 4-5 billion years) is it's on the outer edges of the galaxy (in terms of its distance above the galactic plane) and has not been unduly affected by it's surroundings. Most OC's only last a few hundred million years at the very most before they get torn apart by gravitational interactions with the surroundings as well as their internal dynamics.

Then about the density waves....what is this insistence on them being planar. Spiral density waves occur when you get rotation of a gas/dust cloud where pockets of that gas/dust develop an over density due to the average velocities of the particles in those regions becoming lower than the average for the cloud. The planar nature of the wave would only be assumed in the direction perpendicular to its direction of travel....the z axis. The density waves are basically like a shock wave....a "wall" if you will. They don't sit coplanar with the spiral arms, if they did they wouldn't produce any stars or compress the gases. There'd be no spiral arms. Their direction of travel is coplanar, but their shock geometry isn't. The wavefront is perpendicular to the long axis of the arms (that which runs along the arms). All waves, including the spiral density waves themselves come from some initial direction. They're assuming a wave coming from an external source, by the sounds of their interpretation. Spiral density waves are driven internally, once the initial internal conditions are met to act as the seed for the over density.

Ok. I'm not sure why, either...

They've also seem to have forgotten that once the wave has been established, it's the movement of material in and out of the wave, not the other way around, that creates the spiral patterns.

I think because of the net transverse accelerations, they say that the material tends to hang around (oscillating about the transverse axis) and thereby forms the arm (see my above comment).

The wave moves, but at a velocity much slower than the overall rotation of the galaxies contents. Once it sets up it maintains the spiral pattern and that's all, it doesn't race out to sweep up material to form stars etc, and then deposits them into a spiral arm. Remember, these phenomena are just over densities in the general bulk of the galaxies, not some solid wall of material that has built up in one place and remains there.

Ok. It must remain there for a while though, otherwise we wouldn't see a spiral arm at all, (courtesy of the persistence of the material)? Eventually, it gets flung out ... and goes on its merry way.

The contents of the arms are free to move on in whatever direction they happen to be moving....the identity of the contents is not immutable and it changes constantly through time. In the end, the reason why most existing theories don't consider Keplerian factors is that the observations thus far have ruled them out, for the most part. The bulk of the material within spiral galaxies doesn't obey Keplerian orbital mechanics in general. If it did, it would show in the observations.

I think they're challenging their fellow scientists from the premise that the Keplarian/Newtonian physics may not have been followed thru, in sufficient detail, in order for them to justify dismissing it altogether. (Ie: they've never seen this considered in any papers). And I think they lead the reader to the conclusion that perhaps the observations are explainable, upon deeper deliberations of the modelling.

Interesting !

Cheers