mowj-e cagâli (#)
Fr.: onde de densité
A wave phenomenon in which the density fluctuations of a physical quantity propagates in a compressible medium. For example, the → spiral arms of a → galaxy are believed to be due to a density wave which results from the natural instability of the → galactic disk caused by its own gravitational force. A common example of a density wave concerns traffic flow. A slow-moving vehicle on a narrow two-lane road causes a high density of cars to pile up behind it. As it moves down the highway the "traffic density wave" moves slowly too. But the density wave of cars does not keep the same cars in it. Instead, the first cars leave the density wave when they pass the slow vehicle and continue on at a more normal speed and new ones are added as they approach the density wave from behind. Moreover, the speed with which the density wave moves is lower than the average speed of the traffic and that the density wave can persist well after its original cause is gone. See → density wave theory.
density wave theory
negare-ye mowj-e cagâli
Fr.: théorie des ondes de densité
One possible explanation for → spiral arms,
first put forward by B. Lindblad in about 1925 and developed later by
C.C. Lin and F. H. Shu. According to this theory, spiral arms are not material
structures, but regions of somewhat enhanced density, created by
→ density waves. Density waves are perturbations amplified by
the self-gravity of
the → galactic disk. The perturbation results from natural
non-asymmetry in the disk and enhanced by environmental processes, such as galaxy encounters.
Density waves rotate around the → galactic center and periodically
compress the disk material upon their passage. If the spiral arms were
rigid structures rotating like a pinwheel,
the → differential rotation
of the galaxy would wind up the arms completely in a relatively
short time (with respect to the age of the galaxy), → winding problem.
Inside the region defined by the → corotation radius,
density waves rotate more slowly than the galaxy's stars and gas; outside that
region they rotate faster.