Galaxies density waves

An unknown event disturbed the equilibrium of the interstellar cloud, and it collapsed. This process may have been caused by shock waves from a supernova explosion, or by a density wave of a spiral arm of the galaxy. The gas molecules and the particles were compressed, and with increasing compression, both temperature and pressure increased. It is possible that the centrifugal forces due to the rotation of the system prevented a spherical contraction. The result was a relatively flat, rotating disc of matter, in the centre of which was the primeval sun. Analogues of the early solar system, i.e., protoplanetary discs, have been identified from the radiation emitted by T Tauri stars (Koerner, 1997). 

The principal features of the spiral structure are believed to be maintained by a pattern of density waves. This pattern rotates about the galactic center like a rigid body with an angular velocity that is lower than that of most of the material. Thus, the material of the Galaxy (i.e. both stars and gas) flows through the pattern and as it enters the density wave, the gas is strongly compressed. There are also less important elements of the spiral pattern that rotate with the material these are called material arms, as against density-wave ... 

The importance of this frequency for the resonance problem is that any force that is periodic on this time scale will produce a resonant interaction. For the more general problem of galactic structure, the points at which the perturbation frequency is the same as the epicyclic frequency are called the Lindblad resonances. These play a central role in density wave theory, which has been applied to a wide variety of astrophysical problems, including planetary ring systems, accretion disks, and the structure and evolution of spiral disk galaxies. 

A hot fluid model would be highly desirable for applications in astrophysics. As we have already mentioned, the formation of RES in the primordial plasma could be an important source of large-scale nonuniformities in density and temperature, which seeded the formation of galaxies and clusters of galaxies [4], In particular, it is conjectured that in the early universe matter was present in the form of a mixture of electrons, positrons and photons in thermal equilibrium at a temperature above me2. It is evident that the propagation of relativistic EM waves in such peculiar environment should be addressed in the framework of a hot-plasma model. 

One of the most interesting events in astronomy during the last few decades has been the discovery of many types of molecules, both ionized and neutral, in the diffuse and dense ISC that pervade the Milky Way galaxy. These ISC are at such enormous distances from the solar system that in situ probes, e.g. conventional mass spectrometers, cannot be exploited to analyse their composition. The only tool available to probe their physical conditions and chemical compositions is spectroscopy over the whole spectral range (from radio waves to gamma rays). The diffuse clouds of very low number density (about 10 cm ) consist mainly of H and H2 together with C, N and... 

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