With the signing of the bilateral ALMA agreement on February 25, 2003, the ALMA project has formally entered its construction phase. It seems appropriate to take this opportunity to summarize the scientific goals of ALMA as well as key aspects of its technical design and critical milestones in its construction schedule.
ALMA will be a multi-purpose observatory that will allow study of a wide range of objects and scientific goals. The following examples of the types of science that will be possible with ALMA are taken from the ALMA Project Plan:
In a Canadian context, anyone who has used SCUBA on the JCMT should be thinking about using ALMA. Observations of dust continuum emission at angular resolutions of 1¢¢ or better should be straight-forward with ALMA.
ALMA will be made up of an array of 64 12 m antennas with surface accuracies of 25 mm rms, sufficient to allow observations at wavelengths as short as 350 mm (900 GHz). The array will be re-configurable in a continuous way from a compact array with a diameter of approximately 150 m to a large array with a diameter of about 5 km. A still larger array with a maximum baseline of roughly 14 km is also planned. Rather than operate in several present configurations (as the VLA does), ALMA is planned to change configurations in a semi-continuous way, with a few antennas in the inner part of the current configuration moved out to the next stations of the larger array. The array design resembles a multi-arm spiral.
ALMA will be equipped with receivers operating in four frequency bands: 84-116 GHz, 211-275 GHz, 275-370 GHz, and 602-720 GHz. (The corresponding wavelength ranges are 3.6-2.6 mm, 1.4-1.1 mm, 1.1-0.8 mm and 0.5-0.4 mm.) All frequency bands will be dual polarization with a bandwidth of 8 GHz in each polarization. Each antenna will also be equipped with a water vapor radiometer which monitors the water line in the Earth's atmosphere at 183 GHz. These radiometers will be used to correct errors in phase introduced by differences and variations in the water vapor column above different antennas, effectively providing ``adaptive optics'' at these radio frequencies. The antennas will also be capable of ``fast switching'' which can also correct for phase errors.
ALMA will be capable of unprecedented angular resolution at millimeter wavelengths. For example, observations around 1 mm can easily match the 0.1¢¢ angular resolution that can be obtained with the Hubble Space telescope. One important fact to keep in mind about ALMA is that, with an array that is being reconfigured semi-continuously, it will be possible for observers to choose the resolution that best suits their science goals. For example, at a wavelength of 1 mm, resolutions between 1.4¢¢ and 0.02¢¢ will be obtainable with ALMA. The spectral resolution of ALMA will also be very good, with resolutions of 0.1-1 km/s typically achievable. The total spectral bandwidth of ALMA will be much better than current arrays; having 8 GHz of bandwidth is important for observations of wide galaxy lines at high frequencies and should allow efficient searches for radio redshifts using redshifted CO lines for the first time.
The vast majority of observations with ALMA will be dynamically scheduled to match observing programs with the required atmospheric transmission and phase stability. One key feature of the ALMA software is there will be a science pipeline which will calibrate and image the data automatically. Thus astronomers will not need to be an expert in millimeter interferometry to use ALMA successfully.
ALMA is formed of two equal partners, North America and Europe. The North American partner is composed of the U.S.A. and Canada, while the European partnership consists of ESO and Spain. The total cost of ALMA is $552M in U.S. dollars. Funding from both partners is now secure. Canada's contribution to ALMA directly is $20M U.S. dollars. Most of this money will be spent producing the 3 mm receiver bands for all 64 antennas, with additional money spent on site infrastructure in Chile and various software efforts. The majority of the ALMA funds come from NRC, with a smaller contribution from CFI. Our participation in ALMA is part of the North American Partnership for Radio Astronomy (NAPRA) agreement. As part of the NAPRA agreement, we will provide both money for ALMA and also build the new correlator for the Expanded Very Large Array (EVLA). I personally view Canada's efforts to build the EVLA correlator as very much part of our ALMA efforts; we are using the unique expertise in correlator design at DRAO to build a very interesting instrument and one that is crucial to the success of the EVLA. In exchange for this effort and our direct ALMA construction effort, we are gaining access to ALMA for all Canadian astronomers and also ensuring continued access for Canadians to other NRAO instruments like the EVLA and the Green Bank Telescope (GBT).
The ALMA Project Plan defines a list of milestones and deliverables which must be met during ALMA construction. Evaluation of the prototype antennas at the ALMA test facility was to have started in the 4th quarter (Q4) of 2002; however, this has been delayed to Q1 2003 due to a number of factors. The first major milestone is the start of the initial phase of civil works in Chile in Q4 2003. Milestones in 2005 include having the central backend system ready to install at the array site, completion of the initial phase of civil works in Chile, and having the first production antenna, together with a back end and front end (i.e. receivers) available in Chile at the Operation Support Facility by Q4 2005.
Because it is made up of an array of antennas, it will be possible to do science with ALMA before the array is fully complete. The next milestone is the start of ``Early Science'' operations in Q3 2007. Early Science will be real observing with ALMA, in the sense that astronomers will write proposals, design the ``scheduling blocks'' that will be used by the scheduler to observe their sources, and receive calibrated data and images from the ALMA Pipeline. However, the observing modes will be limited initially (for example, polarization will not be available immediately), there will likely be between 6 and 10 antennas in the array, and all four receiver bands may not be available initially. Science observations will continue with ALMA simultaneously with construction of the full array, with new capabilities being made available to the community as they are tested. The final ALMA milestones are the completion of construction by the end of 2011 and the start of full science operations by the beginning of 2012.