Cloud Collisions and Bulge Formation in Disk Galaxies 23

Summary

ABSTRACT

I present results of several simulations of a flat self-gravitating disk composed of gas clouds and stars, to show that cloud collisions cause bulge formation.

THE CODE

I have performed a dozen runs using N-body code with a self-gravitating “zerothickness” disk containing stars and/or gas clouds, and imbedded inside a spherical halo. Clouds could either collide or not. I have developed a new elaborate cloud collision routine, allowing clouds to either coalesce, fragment, or undergo star formation. The type of cloud collisions depends on their relative distance, while the relative speed and total mass of a colliding pair determine the outcome of a collision.

RESULTS

Bulge formation occurs in all runs with a low halo/disk mass ratio (H/D < 3) containing colliding clouds. A low H/D allows clouds to coalesce more often 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 halo potential, around the nucleus creating a bulge. Longer lasting, higher cloud collision rates, especially those between giant molecular clouds (GMC-GMC collisions), cause creation of bigger bulges. Lower H/Ds and larger initial cloud radii produce higher GMC-GMC collision rates, as seen in Figure 1 (clouds and stars, and bigger clouds).