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theses

We present four studies of radial migration of stars in thickened galactic disks and its relation to the possible internal formation of thick disks. This is an effect caused by transient spirals during which stars that corotate with a spiral gain or lose angular momentum and migrate radially while remaining on nearly circular orbits. The overall angular momentum distribution and surface density of the disk remains unchanged however. Our tools are collisionless N-body simulations that each simulate a single disk galaxy. First, we study the extent to which radial migration is affected by the vertical motion of stars. We find that rms angular momentum changes are reduced by it, but rather gradually, and the maximum changes are almost as large for thick disk stars as for those in a thin disk. Radial migration is also weaker in disks with greater velocity dispersions and under spirals of lower amplitude or greater number of arms. In barred simulations, it is slightly stronger outside the bar than in comparable cases with just spirals, but the cumulative effect of multiple spiral events still dominates. In the second study, we determine that vertical actions, and not vertical energies, of stars are conserved on average during radial migration. Then, we test the hypothesis that bars suppress the outwards radial migration from the inner galactic region that is capable of building a thick disk in the outer regions. If this works, it can explain why the Milky Way thick disk primarily contains old stars. First, however, we check whether realistic thick disks form in our simulations by this proposed outwards migration mechanism. We find that they do not, and attribute it to our particle addition prescription and the not strong enough spiral activity. Nevertheless, we go on to test the above hypothesis, and indeed find that bars significantly reduce this outwards migration. Although small and slow variations of the bar properties allow some trapped particles to escape, new stars that would form inside the bar would be born deep in its potential well, and would remain trapped.

ETD_4978

Ph.D.

Includes bibliographical references

by Michael Solway

doi:10.7282/T3R20ZDK

ETD doctoral

The author owns the copyright to this work.