On the instability of gaseous media.

Abstract

Introducing the assumption that all large scale astrophysical structures, e.g., gas clouds, galaxies, etc. have formed by collapse of regions of an initially more homogeneous distribution of matter according to Jeans' gravitational instability criterion, it is possible to work backwards from existing conditions to the initial conditions. It is found that gravitational instability produces a collapse in which the scale of the final structure is approximately one-sixth of its original dimensions. This agrees with observations of the interstellar gas clouds which show that the clouds are separated by approximately twice the diameter of their extremities. Given the relative velocity of neighboring clouds, it is possible from the diameter of the clouds to determine the average density of the interstellar medium. The relative velocity 3 km/sec gives roughly 10 hydrogen atoms per cm1. This agrees with the average density computed by smearing out the estimated mass of the galaxy over the estimated dimensions of the galaxy. The collapse time in such a medium is of the order of I0~ years. On the other hand, galaxies are separated by distances of the order of 100 or more times their diameters, implying that they have separated greatly since their formation. Observations imply masses of the order of 1011 Mo and internal velocities 300 km/sec for the brighter galaxies. From these is predicted a relative velocity 50 km/sec for galaxies not in clusters. The observed motions and dimensions of both spheroidal and spiral galaxies yield an initial mean density of the order of 5 X 10-26 gm/cm3 at the time the collapse of the primordial homogeneous medium began. That both types give the same initial density suggests that regions not occupied by relatively active eddies have collapsed over smaller dimensions to form spheroidal galaxies, i.e., galaxies of low mass, whereas active regions, if they collapse at all, form the more massive galaxies. Finally, the collapse time for forming a galaxy is found to be of the order of 3 X 108 years. Department of Mathematics and Astronomy, University of Utah, Salt Lake City, Utah.