The first black holes and stars
Our understanding of cosmology has expanded greatly in recent years. On the one hand, detailed observations of the cosmic microwave background have shown us a ‘baby picture’ of the universe as it was only 300 000 years after the Big Bang.
At that time, the universe was smooth to nearly one part in 100 000; lumps of matter such as galaxies, stars, and planets did not yet exist. On the other hand, technological advances in optical, X-ray, and radio telescopes have allowed us to observe the detailed properties of galaxies and quasars at unprecedented distances. These instruments currently allow us to probe the universe when it was only about one billion years old. At this time, protogalaxies were beginning to merge to form the galaxies (and clusters of galaxies) that we see today.
The last frontier of cosmology is to explore the time between these two epochs: the Dark Ages during which the first protogalaxies and quasars formed. This era has proven difficult to study. The protogalaxies are distant and extremely faint; moreover, much of their light is absorbed as it travels toward us. Tantalising clues from the Wilkinson Microwave Anisotropy Probe and the Sloan Digital Sky Survey suggest that these objects may be unlike anything we can see in the nearby universe. For example, the first protogalaxies may form through different mechanisms than our own Milky Way, and the stars inside these objects may be hundreds of times more massive than our own sun.
The Square Kilometre Array will study the detailed properties of the first luminous objects in the universe. When structure first began to form, most of the gas in the universe was smoothly distributed and electrically neutral. A special property of neutral hydrogen is that it produces weak radiation at a wavelength of 21 cm. At these early times, all of the gas in the universe would be visible through this transition.
However, once the first objects grew, their light ionized pockets of the gas around them, shutting off the 21 cm emission from these regions. Astronomers have labeled this important event as “reionisation”.
The SKA will be able to take snapshots of the 21 cm emission at many different epochs, before, during and after reionisation, yielding detailed information about the formation of the fist structures in the universe. It will provide the best measurements available of the characteristics of the first light sources in the universe.
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