Astronomisches Rechen-Institut Heidelberg

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ARI-Heidelberg Mitteilungen Serie B, No. 24

Author(s): Brosche, P.
Title: A Model for the Early Evolution of Galaxies
Source: Astron. Astrophys. 6, 240-253
Year: 1970
Abstract: It is assumed that a protogalaxy consists of several clouds whose position and velocity vectors have no preferred direction. A most probable statistical angular momentum follows from this. The galaxy loses energy through cloud collisions and therefore shrinks. This process is quasi-stationary, the virial theorem being thus constantly applicable. For a simple model (a sequence of rigidly rotating homogeneous spheroids) all the characteristic quantities, including the collision time determining the time dependence of evolution follow from the virial theorem in tensor form and from the conservation of angular momentum. Stellar formation occurs with increasing density of the gas. In this way it is possible to: a) understand the origin of the angular momentum of galaxies; b) use a process for the energy output which is still continuing -interstellar cloud collisions- and have a plausible energy source for them at the same time; c) the Hubble sequence can be regarded as an angular momentum sequence (mass being constant). This means: the random angular momenta vary around the most probable value. Galaxies with relatively small angular momentum shrink rapidly and have converted the majority of their mass into stars within 108 years, before they can reach the flat state. These galaxies are identified with elliptical nebulae. Galaxies with average angular momentum convert about 10% of their mass into stars, those with large angular momentum about 1% and the evolution time scale is up to 1010 years. Accordingly these galaxies are identified with spiral nebulae and irregular nebulae. The possible extension of the Hubble sequence on the elliptical side as far as the quasars, fits into this picture because, being objects with very little angular momentum, they can emit a great deal of energy and thus become very small; d) a consequence of this interpretation is a radius-type relation which is recognisable in the empirical material.
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