Notes on early Science with ALMA. 1) Goals The purpose of this excercise is to define realistic goals for the early phases of ALMA scientific operations, before ALMA is "sufficiently" complete. We must also define the term "sufficiently" complete, since no input definition has been given yet. 1) Definition & Requirements 1.a) Early Science is NOT Science Commissioning. 1.b) Early Science involves the community. This implies Early Science will go through a phase of "Call for proposal", although perhaps in some restricted form. 1.c) Early Science must show the unique ALMA capabilities 1.d) Early Science must show these capabilities to ALL astronomers, not only mm experts 1.e) Early Science must not require the most delicate capabilities which will ultimately build into ALMA. 1.f) Early Science must be used to provide feedback to ALMA on operations from the User community. 2) Science Commissioning Since "Early Science" is not "Science Commissioning", let us define more precisely what is "Science Commissioning". 2.a) Science Commissioning must demonstrate the BASIC ALMA capabilities 2.b) Science Commissioning must test systematically the expected operating modes of ALMA 2.c) Science Commissioning must have a verifiable output 2.d) Science Commissioning involves experts only 2.e) Science Commissioning can be a phased activity overlapping with Early Science. When more ALMA capabilities become available, they first go through a phase of Science Commissioning, before being offered for Early Science 3) What are the "Unique Capabilities" of ALMA in "Early Science" 3.a) Long baselines on a good site. By 2007, ALMA will be equipped up to 4 km baselines. This is much more than any array so far, and on a much better site 3.b) Southern Sky This is ONLY interesting if there are unique objects not visible from the Northern Sky. Repeating the same sort of science that Northern arrays will have been doing on Northern sources is not worth. Unique objects include - The Galactic center - Centaurus A - the Magellanic Clouds - transient objects (comets) 3.c) Sensitivity First year of operation: factor 2-3 better than other arrays in continuum, but only slightly more sensitive in spectral line. =09 Second year of operation: Gain a factor 2 in sensitivity. Third year of operation: Another 50 % improvement. Getting to better than 50 % of the ultimate sensitivity. 3.d) Polarization This may have become possible on other arrays, but certainly not be exploited for many years at that time 3.e) Frequency coverage 340 GHz and above will be relatively unique, given the site characteristics. However, it is possible that in the first year (or so) of operation, the receivers may not be equipped with such high frequency bands. 3.f) Wide Bandwidth Besides offering the best continuum sensitivity, wide bandwidth is a major advantage for any spectral line survey. 3.g) Imaging Quality As soon as the number of antennas will exceed 12, the imaging quality will be better than that of any other instrument. 3.h) Fast Wide Field imaging As soon as the number of antennas will exceed 12, the imaging speed will surpass by a large amount that of other instruments. The imaging quality may depend on the nature of the field being imaged. 3.i) Accurate Calibration This is a goal. The better site will help, but there is no guarantee=85 4) What is the duration of "Early Science" 4.a) Early Science starts as soon as 6 to 8 antennas have been commissioned. Reasoning:=09 - less than 6 antennas is useless=09 - commissioning is a delicate problem. A learning curve is required - commissioning additional antennas requires sub-array capabilities, or shared time with "Early Science". If sub-array mode is used, a minimum of 8 antennas (6 in main array, 2 + new commissioned antenna is sub-array) will be necessary. - Science commissioning requires 6 antennas to be operational to ensure most modes are being tested 4.b) Early Science ends at the latest when 32 antennas have been commissioned. (We may decide to end it before=85) Reasoning: - The sensitivity is now half of the final one - The imaging quality has become close to final 4.c) Early Science phases can be applied separately to different frequency bands. 5) What "Early Science" must demonstrate 5.a) Nice images for the public This is a 600 M$ project=85 5.b) Nice images for the astronomy community This remains the best way to convince the average optical astronomer=85 5.c) A range of scientific applications >From solar system physics to the high redshift universe 5.c) Some unique scientific results 6) What should we avoid in "Early Science" =09 6.a) Doing just the same thing in the South than we have been for 10 years in the North 6.b) Spending 3 months obtaining one result the first year, while it could be obtained in a day once ALMA is completed. With 6 antennas, ALMA will 100 times slower than when fully completed. This is a factor to remember. This probably implies that any experiment in the "Early Science" phase should not exceed 1 week of observing time. Very extensive surveys should be avoided. 6.c) Imposing requirements which cannot be met. 7) ALMA Sensitivity The ALMA sensitivity can be derived from Memo 393. The expecte Tsys is Tsys (nu) =3D 15 + (nu/100 GHz) K below 300 GHz Tsys (nu) =3D 1200 K for band 9 Under appropriate observing conditions. The effective sensitivity can be derived from the usual noise equation, using an antenna gain of 40 Jy/K (average, including typical phase noise and quantization noise). Alternatively, one can use Table 6 of memo 372, which gives the single baseline sensitivity sigma_b (column 5) and use the (changing) number of antennas to get the point source sensitivity. No more complex procedure is currently justified given the uncertainties (receiver, number of antennas, observing frequency, atmosphere, =85) 8) Steps The goal of the project is to define more precisely the ALMA configuration (configurations, frequency bands, number of antennas (fixed or changing) in this Early Science phase. Here are some possible assumptions/options - Fixed setup: - Use 8 antennas for 1 year. - Two frequency bands (3 and 6) - Variable array configuration (from compact to 4 km) - separate additional antenna commissioning - Variable setup: - Add antennas as they come (1 per month) - Two frequency bands (3 and 6) - Less variable array configuration (to allow for concurrent antenna commissioning) - Variable frequency setup - After 1 year, add another band or two (7 and 9) - More ambitious goals: - more than 2 frequency bands ? This is obviously desirable, but may cause delays. An initial assessment of what science can be done without the highest frequencies is worthwhile. - Earlier start: - An early start will unavoidably restrict the capabilities. A suitable tradeoff should be found (e.g. limited early start, sort of "Science Demonstration" followed by a more thorough commissioning).