How Galaxies Grow
As the baby galaxies formed, gravity pulled many together. If two with black holes merged, the black holes may have merged and sucked stars into their nether regions, increasing their mass and making the stars around them spin more rapidly. It appears it is possible many baby galaxies with black holes could have merged into one. In such a case, some of the black holes would have merged, but others would have been thrown out into space.
From views from the Hubble Space Telescope of the very early universe, it appears that merging of the young galaxies was like a rush of lemmings to the sea. Some astronomers refer to the phenomenon as a ‘conveyor belt’ bringing young galaxies together.
At the end of this formative process, which may have seen numerous mergers, the dark matter halo became more massive and bloated, and the normal matter quickly formed new stars. If the stars formed very quickly, and used all the hydrogen available, the result was an elliptical galaxy.
If the star formation was slow, due to the gas molecules colliding with each other as they fall toward the core, then the conservation of angular momentum forms a disk, and a spiral galaxy is created.
In fact, the mergers continue even to the present day. Telescopes reveal galactic collisions throughout the universe. Our galaxy and our neighbor Andromeda are hurtling at each other at astronomical speed. In 10 gigayears, the two will collide, and perhaps create a single elliptical.
It appears that ellipticals developed the most massive black holes. Our Milky Way, for instance, has a relatively small black hole of only about 4 million solar masses. Ellipticals have black holes with masses of hundreds of million solar masses, or more. This may explain why star formation in ellipticals occurred so quickly. The stars near the black hole’s discontinuity were immediately swallowed up, so star formation in the outer regions proceeded quickly as the gas swirled around the black hole.
Once galaxies fully form, they evolve and cluster. We will discuss that next.