The committee emphasizes that its recommended activities have the capacity to find the unexpected and the versatility to engage in follow-up observations. For example, WFIRST and LSST as recommended here would open up the time domain to reveal remarkable surprises and enable the creation of massive databases that will be mined for decades. It would be unprecedented in the history of astronomy if the gravitational radiation window being opened up by LISA does not reveal new, enigmatic sources. Most of the observing time on GSMT, IXO, and ACTA would not be allocated according to a preordained strategy; rather, individuals and teams would compete for time to explore new scientific approaches and pursue recent discoveries. The broadly based and balanced suite of facilities that are recommended is flexible and resilient enough to make and exploit the many unanticipated and thrilling discoveries that are sure to come during the coming decade. Many of the most fundamental advances in astronomy and astrophysics have resulted from theoretical discoveries that could not have been anticipated in any planning exercise—the theory of inflation is one example—but the recommended Theoretical and Computational Network program and augmentations in individual investigator grants programs at NSF and NASA will help to enable such discoveries.


Understanding the dramatic evolution of galaxies over cosmic time through observations is a key part of the committee’s recommended science program. Following the growth of cosmic structure and learning empirically how the dark and luminous matter are connected is a major science goal for GSMT, which, with its superb spectroscopic reach, would be able to measure redshifts and thus infer distances all the way from our local neighborhood to the epoch of reionization and monitor the buildup of mass and the rise and fall of star formation at visual wavelengths. Meanwhile CCAT would provide the submillimeter perspective on the history of star formation over cosmic time. (See Figure 7.2 for an illustration of the complementarity.) The “fossil record” of how our Milky Way galaxy was assembled can be traced by studying resolved stellar populations with LSST and JWST, and by using the adaptive optics capability on GSMT. GSMT would also be able to perform exquisite spectroscopy of the most ancient, nearby stars. In the next decade, large-scale numerical simulations of the formation and evolution of galaxies should achieve the spatial resolution and physical realism necessary to interpret these observations successfully and to tell the story of how our galaxy was born.

Our understanding of star formation under a wide variety of physical conditions will benefit from extensive surveys of the giant molecular clouds within which stars form. ALMA will and CCAT would be major tools for this exploration.

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