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The Atacama Large Millimeter Array: Implications of a Potential Descope 4 The Threshold for Transformational Science ALMA can produce transformational science in many areas. Examples of three such areas follow: The early universe is particularly accessible to ALMA because dust emission in the infrared from young galaxies gets red-shifted into the submillimeter band. Galaxies have already been observed emitting at a time when the universe was less than a billion years old. Millimeter emission from gas and dust already has been detected in extreme objects to redshift 6.4, as far back as optical emission has been detected. Thus ALMA should be a key instrument for understanding the formation of the very first galaxies. ALMA will be the first telescope capable of detailed imaging of the gas/dust disks out of which planets form around young stars, and capable of distinguishing the subtle dynamical and chemical processes at work. Rapid strides in these fields have been made in recent years using single submillimeter dishes and millimeter interferometers. However, these instruments lack the sensitivity and resolution to perform as well as ALMA, even if it is descoped to 40 antennas, as Table 1 makes clear. There is no doubt that a detailed understanding of planet formation will be one of ALMA’s greatest scientific legacies.
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The Atacama Large Millimeter Array: Implications of a Potential Descope Galaxy formation is a rich and complex sequence of steps in which objects build up by mergers and accretion; these processes also trigger the star formation that produces most of the observable light in the universe. Immediately prior to star formation, gas collects in massive clouds that can be observed at millimeter wavelengths. Some of the light produced by star formation in young galaxies emerges in the optical and ultraviolet bands, but much is radiated into submillimeter wavelengths. Star formation also powers the synchrotron emission seen at decimetric radio wavelengths. Thus, understanding the birth and evolution of galaxies requires integration of results from a suite of new facilities: the giant segmented mirror telescope in the optical, the James Webb Space Telescope in the near infrared, ALMA in the millimeter and submillimeter, and the extended Very Large Array in the radio. Collectively these facilities will revolutionize our understanding of galaxies. All of them are needed to produce a comprehensive description. The committee notes that the most important discoveries from major observatories are usually unanticipated. It is reasonable to expect that the same will be true for ALMA. Even if the array is reduced to 40 antennas, ALMA’s performance in terms of image fidelity, imaging speed, single field sensitivity, and mosaiced image speed surpasses the performance of the best competitor instruments by factors of 6, 18, 4, and 3, respectively, thus opening up much discovery space. Further, these factors ignore the transparency of the atmosphere at the telescope sites, which greatly increases ALMA’s gains at wavelengths near 1 millimeter and allows ALMA to dominate at submillimeter wavelengths where the other interferometers (CARMA [Combined Array for Research in Millimeter-wave Astronomy] and Plateau de Beure) cannot even operate. Finally, ALMA’s unmatched ability to achieve angular resolution of 0.1 arcsecond or better is crucial for comparison to the results of observations made at other wavelengths.
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The Atacama Large Millimeter Array: Implications of a Potential Descope The committee concludes that despite not achieving the level-1 requirements, a descoped array with 50 or 40 antennas would still be capable of producing transformational results, particularly in advancing understanding of the youngest galaxies in the universe, how the majority of galaxies evolved, and the structure of protoplanetary disks, and would warrant continued support by the United States.
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