1) High-redshift Galaxy Detection: The sensitivity of an array is primarily controlled by three major factors: total collecting area, the noise performance of the receivers, and atmospheric transparency and phase stability. The detection requirement for high-redshift galaxies therefore has a direct impact on ALMA’s collecting area, polarization and frequency requirements, and site.

Contemporary millimeter arrays have collecting areas between 500 and 1000 m2 and can detect CO emission from the brightest high-redshift galaxies (which are amplified by gravitational lensing) in one to two days of observations; signals from normal, unlensed objects will be typically 20-30 times fainter. Current millimeter-wave receiver technology has approached fundamental quantum limits, and the noise level of ALMA’s detectors cannot be reduced beyond this point by much more than a factor of 2; an important additional factor of in sensitivity is gained by requiring that ALMA support front-end instrumentation capable of measuring both states of polarization. The proposed ALMA site will minimize the noise contributions of the atmosphere, so that the remaining factor of 7-10 in sensitivity can only be gained by increasing the collecting area by a similar amount. Hence, ALMA must have at least 7000 m2 in collecting area.

The molecular spectral lines which generally serve as diagnostics of the gas content and dynamics of galaxies early in the history of the Universe have frequencies that are fixed in the rest frame of the galaxy, but will be observed at frequencies that depend upon redshift. Since galaxies are found at every redshift (i.e., age) ALMA should ideally provide access to all atmospheric windows from 30-950 GHz, so that galaxies of all ages may be studied. Initially, however, the array will support observations in the four highest-priority frequency bands. Additional capabilities may be added in the operational phase of ALMA. Since the redshift of the galaxies will initially be essentially unknown, the instantaneous bandwidths of the receivers and the correlator should also be as large as possible; this will also maximize the continuum sensitivity of the array.

2) Protoplanetary/Protostellar Disks: These requirements have impacts on ALMA’s operating frequency, baseline size, frequency resolution, and polarization. Theoretical calculations indicate that the gaps created by Jupiter-mass objects in protoplanetary disks will be ~1 AU in extent. Combined with the distance to the nearest star-forming regions (60-150 pc), the requirement that such gaps be resolvable in protoplanetary disks implies an angular resolution of 0.010″ or better. This can be achieved by combining high-frequency (f > 650 GHz) observations with array baselines of at least 10 km. Proper study of the kinematics of the disk images further requires that spectroscopy be carried out at velocity reso-



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