Survey Telescope (WFIRST) and the International X-ray Observatory (IXO), and the masses of the black holes can be measured using a GSMT.

Simulations show that the first galaxies were likely relatively small and that the giant galaxies observed today grew by successive mergers. Observations of mergers should be possible using JWST, ALMA, WFIRST, and GSMT. As galaxies merge it is likely that their black holes merge as well. The proposed Laser Interferometer Space Antenna (LISA) mission will search for the signatures of these processes by scanning the skies for the bursts of gravitational waves produced during these early mergers when the black holes are relatively small. (LISA will not be sensitive to the mergers of more massive black holes.) An important part of the strategy is to search for associated flashes of electromagnetic radiation that are expected as part of these events. The proposed Large Synoptic Survey Telescope (LSST) will be ideally suited to this task and, working with a GSMT, should make it possible to pinpoint and date the sites of black hole merger events.

In summary, this survey committee recommends improving understanding of the history of the universe by observing how the first galaxies and black holes form and grow. To do so requires that current capabilities be supplemented with the priority ground- and space-based activities identified in this survey; see Box 7.1.

New Worlds: Seeking Nearby, Habitable Planets

The search for exoplanets is one of the most exciting subjects in all of astronomy, and one of the most dynamic, with major new results emerging even as this report was being written. As described in Chapter 2, an unexpectedly wide variety of types and arrangements of planets have been identified—even a few systems with some resemblance to our solar system. What has not been found yet is an Earth-like planet, that is, a terrestrial body with an atmosphere, signs of water and oxygen, and the potential to harbor life. This survey is recommending a program to explore the diversity and properties of planetary systems around other stars, and to prepare for the long-term goal of discovering and investigating nearby, habitable planets. This program is likely to be informed by theoretical calculations and numerical simulations.

Locating another Earth-like planet that is close enough for detailed study is a major challenge, requiring many steps and choices along the way. The optimum strategy depends strongly on the fraction of stars with Earth-like planets orbiting them. If the fraction is close to 100 percent, then astronomers will not need to look far to find an Earth-like planet, but if Earth-like planets are rare, then a much larger search extending to more distant stars will be necessary. With this information in hand, ambitious planning can begin to find, image, and study the atmospheres of those Earth-like planets that are closest to our own. Equally important to the characterization of an Earth-like planet is to understand such planets as a class.

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