Formation and rotation of disc galaxies with haloes

Abstract

We consider a picture in which disc galaxies formed from collapsing gas in extended haloes of dark material. It is assumed that these systems were produced by hierarchical clustering and were endowed with their angular momenta by tidal torques. In order to test the theory, we have developed some models that determine the structure of the haloes in terms of the observable properties of the discs. A comparison of the models with a sample of 25 galaxies leads to the following conclusions: (1) Collapse factors of 10 or more and halo-to-disc mass ratios of five or more are required to explain the rapid rotation of the discs. (2) The halo rotation curves, computed on the assumption of detailed conservation of angular momentum in the discs, are fairly fiat. (3) Most discs are globally unstable to the formation of non-axisymmetric perturbations unless their mass-to-light ratios are significantly smaller than current estimates. (4) There is a tentative indication that the mass and luminosity distributions in the bulge regions of galaxies are not closely related. (5) The parts of galactic discs that are unstable to fragmentation at 10⁴ K correspond well with the parts that are optically visible; smaller discs would be completely stabilized by rotation.