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In my recently completed dissertation study of the role of cloud collisions on evolution of disk galaxies, I have found that they cause bulge formation. I have performed more than a dozen runs using N-body code with a self-gravitating ``zero-thickness" disk which contains stars and/or gas clouds, and is embedded inside a spherical halo. I have developed a new elaborate cloud collision routine, allowing clouds to either coalesce, fragment, or undergo star formation. The type of cloud collision depends on the pair's relative distance, and consequently their overlapping volume. Relative speed and total mass of a colliding pair determine the outcome of a collision.
Bulge formation occurs in all runs with a low galaxy halo/disk mass ratio ($\hbox{H/D}<3$) and with colliding clouds in the disk. If no cloud collisions are allowed, no bulge forms. A low H/D allows clouds to coalesce more readily via cloud collisions into bigger clouds. These bigger clouds then act as very effective scattering agents, throwing stars and some other clouds out of the disk plane. The scattered particles make the disk thicker, and most of them end up, under the influence of the halo potential, around the nucleus creating a bulge. Longer lasting, higher cloud collision rates, especially those between giant molecular clouds (GMCs), cause creation of bigger bulges. Lower H/Ds and larger initial cloud radii produce higher GMC-GMC collision rates.
In a run with increased number of particles by a factor of 4, I obtain similar effect of higher collision rates and bigger bulge, without any need for larger initial clouds.
