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The Atacama Large Millimeter Array: Implications of a Potential Descope 2 Technical Performance Specifications ALMA is being designed according to three level-1 technical performance requirements that were specified in the original proposal.1 REQUIREMENT 1: The ability to detect spectral line emission from CO or [CII] in a normal galaxy like the Milky Way at z = 3 in less than 24 hours of observation. Millimeter emission lines are formed by the cool gas from which stars are born. Studying them in distant galaxies is one of the most promising ways to understand the evolution of galaxies. So far, more than 30 galaxies have been detected in this manner beyond a redshift of z = 2. However, these detections have been limited to luminous objects that are unrepresentative of normal galaxy formation. ALMA will make it possible to detect star-forming gas in normal galaxies like the Milky Way. 1 See Annex B of the ALMA Agreement, dated June 11, 2002, between the European Southern Observatory and the National Science Foundation. Annex B is reprinted in Appendix B of this report.
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The Atacama Large Millimeter Array: Implications of a Potential Descope The rotational lines of carbon monoxide are well understood and can be observed through Earth’s atmosphere. The committee finds that a 60-antenna ALMA just meets Requirement 1 and that 50- and 40-element arrays would take 36 and 56 hours, respectively, to detect a galaxy similar to the Milky Way at z = 3 and so would fail the benchmark. In practice the smaller number of antennas would limit the number of objects that could be studied using carbon monoxide. A forbidden line of singly ionized carbon, [CII], is more intense than the carbon monoxide line, but since there is only one strong [CII] emission line it will not be observable from many redshifts since it will be shifted to a frequency for which Earth’s atmosphere is too opaque, even above the superb ALMA site. For example, while it may be detectable from z = 3 or somewhat higher, [CII] would not be detectable in the range 2.2 < z < 2.6 due to atmospheric absorption. If selection is limited to only those redshifts up to z = 3 at which the atmosphere would be transparent, then a high signal-to-noise detection of [CII] is still possible in 24 hours, even with a 40-element array. The committee notes, however, that even with this consideration, the implicit scientific goal of tracing star formation in galaxies up to z = 3 will not be met, because, unlike the case with carbon monoxide, the [CII] line does not directly trace the cold molecular component of the galaxies in a well-understood or systematic way. REQUIREMENT 2: The ability to image the gas kinematics in protostars and protoplanetary disks around young stars at a distance of 150 pc (roughly the distance to star-forming clouds in Ophiuchus or Corona Australis), enabling one to study their physical, chemical, and magnetic field structures and to detect the gaps created by planets undergoing formation in the disks. Elucidating the evolution of young gas and dust disks and planets has always been one of ALMA’s strongest science drivers. However, progress requires high-quality images, excellent sensitivity, and the highest angular and spectral resolution possible. In order to assemble a large enough sample (20-30) of disks for study
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The Atacama Large Millimeter Array: Implications of a Potential Descope over a range of inclinations and developmental stages, ALMA is required to observe these objects out to a distance of 150 parsecs (pc). The disks themselves could be as large as several arcseconds, but the interesting science is on a smaller angular scale. Disks are expected to have central holes as large as 30 milliarcseconds. The regions associated with planet formation in our solar system are less than a few tenths of an arcsecond in angular size. In the planet-forming regions, disks should exhibit gaps, tidally swept out by bloated, Jupiter-mass planets. To resolve gaps will require the limiting resolution of 6 milliarcseconds at the highest observing frequency (900 GHz). Reducing the number of antennas will not affect the angular resolution, because their maximum spacing will remain unchanged. However, it will lead to a serious degradation in the quality of the images on account of the poorer coverage of the Fourier transform plane. Figure 1 presents a simulation of a face-on disk-gap-planetring system seen in continuum (dust) emission with 60 antennas from 50 pc (with 600-hour observing2). The structure is clearly visible, and even the central star can be seen. Although there is one system at this distance, TW Hya, the chances that the TW Hya disk has a gap with a bloated Jupiter are small. To understand disk evolution and giant planet formation and migration, a statistical sample of a large number of young disks must be observed. Such a sample can be obtained only by observing regions of star formation that lie at about 150 pc. Repeating the simulation at this distance, with a three-times-smaller angular size and only 10 percent of the flux, leads to an extremely blurred image. It is the view of the committee that by working hard with real data, it would be possible to recover from a 60-antenna array the principal desired features, holes and gaps. However, this benchmark will be very difficult to achieve if the array size is reduced to 50, and achieving the benchmark may not be possible with a 40-antenna 2 Although not unprecedented, 600 hours is an extremely long observation time.
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The Atacama Large Millimeter Array: Implications of a Potential Descope FIGURE 1 Simulation performed by Sebastian Wolf and image processed by Mark Holdaway for a face-on gaseous disk orbiting a young star at a distance of 50 pc, with 600-h observing on a 64-antenna array. The dark ring is the gap in the gas and dust, and the white circle in the gap (top) is a bloated Jupiter at 5 AU. array on account of the strong scaling of the image fidelity and the sensitivity with antenna number. Although imaging the gaps in continuum emission from dust is significant, observations in lines of the gas in the disks will be just as important in order to constrain the physical structure of the disk, the evolving chemistry, the location of the frost line, where water changes to ice, the origin of the outflowing winds, and the gas kinematics. It is the study of the gas that brings understanding of how planets form. With 60 antennas, measurement of the kinematics and chemistry at high spatial resolution takes 3 days of observing for just one disk found in the nearest region of star and planet formation. The required time increases by factors of 1.4 and 2.3 for a 50- and a 40-antenna array, respectively, just to recover the same sensitivity. In addition, the image quality will be degraded unless the antenna configuration is changed and yet
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The Atacama Large Millimeter Array: Implications of a Potential Descope more observing time is expended. Factoring in the number of sources that must be studied in order to understand the evolution of gas and dust into planets, it is clear that smaller arrays become increasingly impractical. Whether imaging the gas or the dust, it is possible, in principle, to build up an image by changing the antenna configuration and integrating for much longer times. In the case of ALMA, the improvements will be limited by calibration and pointing errors and the fact that sources under study are dynamical and thus change. Combining images in this manner under varying atmospheric conditions is extremely challenging technically. REQUIREMENT 3: The ability to provide precise images at an angular resolution of 0.1″. Here the term “precise image” means accurately representing the sky brightness at all points where the brightness is greater than 0.1 percent of the peak image brightness. This requirement applies to all sources visible to ALMA that transit at an elevation greater than 20 degrees. Much of ALMA’s work will concern extended sources where the extremely high angular resolution needed to meet Requirement 2 is less crucial. The challenge will be to create images with high dynamic range so that faint components can be seen in the presence of bright components. Reducing the number of antennas would reduce the number of different baselines that could be utilized in a single pointing and would distribute the power over the raw map. Some of the lost information could be recovered through image processing, although calibration uncertainties and noise set a fundamental limit to how well the image can be reconstructed. Based on its scrutiny of image simulations made for differing array configurations, the committee concluded that Requirement 3 can be met with a 50- and a 60-antenna array, if very long integration times and a change in the telescope configuration are allowed. The capability with a 40-antenna array is unclear and needs further study. However, the committee is not confident that multi-configuration data could be processed through automated
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The Atacama Large Millimeter Array: Implications of a Potential Descope pipelines and still achieve adequate quality. This would jeopardize meeting the requirement that ALMA be accessible to a large community. The committee concludes that two of the three level-1 requirements, involving sensitivity and high-contrast imaging of protostellar disks, will not be met with either a 40- or a 50-antenna array. It is not clear if the third requirement, on dynamic range, can be met with a 40-antenna array even if extremely long integrations are allowed for.
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