Weighing in at 1042 kilograms and measuring 1021 meters across, galaxies are perhaps the most awe-inspiring objects known to mankind. They are also the only places in an otherwise dark and unforgiving universe where stars and planets are able to form. In the past five to ten years we have made enormous progress in understanding when galaxies came into being and how they changed and evolved over the course of cosmic time. For the first time, we have a rudimentary idea of what our own Milky Way looked like in the distant past, and we can now simulate Milky Way–like galaxies inside powerful computers. As we are starting to understand what happened in our galaxy's past, we are now turning to the question of why it happened. Untangling the complex physical processes that shape galaxies is extremely difficult, and will require continued advances in computers and information from powerful new telescopes coming online in the next decade.
If a forest is a collection of trees, and a city a collection of buildings, then the universe is a collection of galaxies. This is apparent when we look at the Ultra Deep Field, a remarkable image obtained with the Hubble Space Telescope that shows the faintest light humanity has yet detected (see Figure 1).1 The blackness of space is punctuated by little blobs of light, each comprising a gravitationally bound system of tens of billions of stars. Galaxies contain nearly all the stars and planets in the universe, play host to the supermassive black holes in their centers, and serve as signposts delineating the large-scale cosmic web of dark matter structure. How galaxies were formed is a central question in astronomy. And because galaxies live at the intersection of the study of the structure of the universe as a whole and of the properties of the dark matter, gas, stars, and planets within them, the question is interwoven with many other fields of astronomy. Furthermore, understanding galaxy formation also means understanding our own galaxy, the Milky Way, and therefore our own cosmic history.
This is a young field of research. One hundred years ago, astronomers were trying to measure the extent . . .
- 1Steven V. W. Beckwith et al., “The Hubble Ultra Deep Field,” The Astronomical Journal 132 (2006): 1729.