this survey would address this goal. For example, a submillimeter survey telescope such as CCAT (formerly the Cornell-Caltech Atacama Telescope) would be capable of identifying the dusty young galaxies that ALMA plans to study in detail. The 20- to 40-meter optical telescopes, known collectively as Giant Segmented Mirror Telescopes (GSMTs), that are planned for construction over the coming decade would render within spectroscopic reach the most distant objects imaged by JWST. A GSMT would allow scientists to determine the mass of the first galaxies and to follow the buildup of the first heavy elements made inside stars. As well as discovering how infant galaxies grow, astronomers would also understand how they shine and affect their surroundings through outflows of gas and ultraviolet radiation.

A major challenge to JWST and GSMT is to understand how and why the birth rate of stars grew, peaked when the universe was a few billion years old, and has now declined to only a few percent of its peak value. The star-formation history of the universe can also be tracked by gamma-ray observations made with the proposed Atmospheric Čerenkov Telescope Array (ACTA): as high-energy gamma rays from the distant universe are converted into electrons and positrons, they can indicate how much star formation there has been along the way.

The era when the strong ultraviolet radiation from the first stars ionizes the surrounding hydrogen atoms into protons and electrons is known as the epoch of reionization, which can be studied directly using sensitive radio telescopes. These should determine when reionization occurred, and they would inform the design of a proposed new telescope that would measure how the cavities of ionized hydrogen created by the light from the first generations of stars, galaxies, and black holes expand into the surrounding gas. In the long term, realization of the full potential of this approach would require in the following decade a detailed mapping of the transition in the early universe from protogalactic lumps of gas and dark matter into the first objects, a goal of the proposed worldwide effort to construct the low-frequency Square Kilometer Array (SKA-low) as discussed in the subsection “Radio, Millimeter, and Submillimeter” under “OIR and RMS on the Ground” in Chapter 3. Studies of the intergalactic medium, which accounts for most of the baryons in the universe, at more recent times could be transformed by an advanced UV-optical space telescope to succeed the Hubble Space Telescope (HST), equipped with a high-resolution UV spectrograph.

Galaxies are composed not just of stars orbiting dense concentrations of dark matter. They also contain gas and central, massive black holes. When the gas flows rapidly onto a central black hole, it radiates powerfully and a quasar is formed. Meanwhile the black hole rapidly puts on weight. It is already known from observations that these black holes can grow very soon after the galaxies form. However, the manner in which this happens is still a mystery. These accreting black holes can be seen back to the earliest times using the proposed space-based Wide-Field Infrared



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