One of the ongoing debates in astronomy has centered on the formation of galaxies. On one side, people argue that galaxies form much like stars, through the collapse of giant clouds of material that spin up (and in some cases flatten out) as they take on galaxy form. On the other hand, there are those who argue that large galaxies form through the aggregation of small systems that merged over time into today's giant galaxies. Deciding between these two formations scenarios has hampered by a lack of evidence. Today, we don't see giant galaxies in their first stages of formation (although we do see ongoing galactic mergers at all scales.) We also don't see small galaxies forming either. Without definitive evidence, all we have is theories getting flung about in the wind. Today, we larger and larger telescopes, we are able to catch the first glimmers of faint light from young galaxies at the beginning of the universe. In looking at these early systems we find the answer may not be either/or, but rather galaxies form through both these mechanisms.
Stephan's Quintet - Interacting Galaxies. Click here for original source URL.
Using infrared sensitive telescopes, including Hubble Space Telescope and the ground-based set of Very Large Telescopes in Chile, astronomers have taken advantage of finite speed of light to look back in time and observe galaxies in the first few billion years of the universe. As near as we can tell, giant elliptical galaxies were among some of the first to form. These early giant systems likely didn't have time to form via the accretion of numerous smaller systems, and it is believed that in rare instances, early in the universe, large galaxies formed through the collapse of a single cloud of material into a single large galaxy.
At the same time, side-by-side with these singular large systems, we see much more numerous examples of small, irregular, star bursting baby galaxies. It is believed that in general, the gas that pervaded the early universe fragmented into clouds of a variety of different sizes. These fragments formed a variety of different sized, generally small, irregular galaxies that over time came together to form the structures we see today. Billions of years ago there were proportionally more of these small, star-forming galaxies so we know they have been "mopped up" by mergers since then.
Galaxy formation is an ongoing process. While most of the smallest galaxies are formed, the extraordinarily rare Green Peas discovered by the Galaxy Zoo project may be an exception. These systems shine bright in the green due to their vast amounts of star formation. These systems can be 1/1000 times the size of the Milky Way and contain as much (or more) star formation. Looking around the sky we can also find numerous examples of giant spirals colliding and merging. As these mergers run their course, many of the collisions will result in giant elliptical galaxies. In between these smallest and largest systems, galaxies like our own Milky Way consume a steady diet of small systems. These minor mergers can lead to warps, bars, rings, and other structures.
While out understanding of galaxy formation is improving, it is still far from complete. We know for instance that dark matter halos likely formed first and luminous matter formed structures within this dark matter scaffolding. We also don't know the details of how galaxies such as our own were able to form with detailed structures, or the origins of the gravitational density waves that most likely cause arms to form in some, but not all, disk galaxies. Questions still await, and the forthcoming James Webb Space Telescope may be our best hope for answering these remaining questions. The Hubble Space Telescope is of modest size and cannot go much further than 12-13 billion years back in time. The 6.5-meter James Webbspace Telescope has infrared capability and great sensitivity that will let it see the highly redshifted light from the first stars and galaxies to form in the universe. This era is called "first light."