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