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Galaxy Formation

The iconic, multi-coloured image of the Cosmic Microwave Background (CMB) appears to show a very young Universe mottled with nascent structure. In fact, the image is misleading; the 300,000 year-old Universe it shows was incredibly smooth, with a mass-energy density that varied by only one part in 100,000.

We know that such small variations could not have collapsed gravitationally into denser structures and eventually into galaxies. How then, did our cosmos evolve from a virtually featureless void to the Universe we see today, filled with billions of galaxies containing billions of stars?

According to the widely accepted Lambda Cold Dark Matter (ΛCDM) model, in the very early Universe, variations in the density of dark matter were more pronounced than variations in normal, or baryonic, matter. Clouds, or halos, of dark matter were concentrated enough that they collapsed under their own gravitational attraction and, in turn, attracted baryonic matter.

The dark and baryonic matter gathered into the first structures in the cosmos, dwarf galaxies. These galaxies were the “building blocks” of the cosmos, merging to form larger disk galaxies, such as our own Milky Way Galaxy, which in turn merge to form large, relatively featureless elliptical galaxies.

There is strong evidence in support of this “bottom-up” formation of galaxies. When we look back at the early Universe, we see—in contrast to the relatively well formed spirals and ellipticals in the contemporaneous cosmos—distorted galaxies that appear to be in the process of merging or appear to have formed from a merger recently. Closer, we see dwarf galaxies around large galaxies like ours that appear to be galactic laggards, last to merge. And, we see faint streams, filaments and tails of gas associated with galaxies, the remnants of past mergers.

Many questions regarding galaxy formation remain to be solved and are the focus of research by U of T astronomers:

  • What is the nature of the spiral arms of galaxies?
  • What role does magnetism play in the formation and evolution of galaxies?
  • How do supermassive black holes develop in the centres of disk galaxies?

At U of T Astro:

Recent research:


An image of Stephan’s Quintet which includes four, gravitationally bound galaxies that will likely merge. (The fifth, in the upper left, is much closer to us than the others and not gravitationally bound to them.) Credit: NASA, ESA, and the Hubble SM4 ERO Team.