Spiral galaxies scale

Briefly, astronomers have two basic methods for estimating distance scales for spiral galaxies that are independent of the observation of special standard candles such as supernovae. 

The sun is located near the edge of the arm O. It may be noted that this picture of the spiral structure in the vicinity of the sun is based on both optical and radio observations, whereas the large scale structure of the interstellar gas (Fig. 4) is based only on observations of the X21 cm hydrogen line and kinematic distances, which become highly uncertain towards the galactic center and anticenter. Kinematic distances are obtained from a model for the rotation of the Galaxy due to Schmidt. This model relates the circular orbital velocity of the gas to the distance from the galactic center (see Fig. 3). 

The astrophysical problem of justifying on theoretical grounds the morphology of galaxies (spiral and eUiptical, with their different content in stars and gas), their chemical evolution (initial rapid enrichment of metals, i.e., any element heavier than hydrogen and helium), and, finally, the attempt to trace a classification based on different physical aspects of the evolution, has been tackled by employing the approach of cooperative systems. In these models a scenario is proposed where the large-scale dynamics are related to the local microscopic interactions. At the same time a macroscopic description (e.g., the interplay of various phases, the metallicity) is derived by means of few (stochastic) variables. 

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. 

We present preliminary results on the intrinsic shapes of s[nral galaxy disks, based on a K -band imaging surrey. The sample galaxies were selected to be face-on on the basis of their HI linewidths and their luminosities. We fit two-dimensional bulge-disk modds that minimize the impact of spiral structure on the apparent shape. We find that for the majority of galaxies the intrinsic disk ellipticity at a few scale lengths is 0.35 and briefly discuss implications for the diape of the dark matter halos and for the intrinsic scatter in the TuUy-Fisher relation. 

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