Comments from Jason Kalirai (SAC) on Chapter 4 (Milky Way) #620
Description
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overall, this is a really great effort. I like the organization of a broad set of Milky Way science cases into three bins; populations that are only found within the plane, a wide range of astrometry applications, and mapping of structures in the halo. Although this doesn't capture all of the science cases in the Science Book, it is a reasonable approach to represent a significant fraction of Milky Way science in driving cadence choices. The one component that I see as missing, which is briefly acknowledged by the authors, is the static science. The overall uniformity and depth through a survey of the Milky Way plane will inform a number of areas in Galactic structure and statistics from studies bulk populations (e.g., IMF). At least one example of such a science case will be important to develop, so we can look at the trade space b/w optimization for variability studies vs static science. For example, one of the conclusions from section 4.2 is that the co-added depth is a lower priority than other measurements; I'm sure this will be cause tension with the static science needs. My comments below are mostly just endorsements of things said in the chapter.
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the authors correctly point out some of the limitations of the baseline strategy in minion_1016, which allocates most of the inner plane observations over a short time period (through the special survey). This is very bad for temporal studies (i.e., proper motions). One thing that will be interesting to review after we have stellar population simulations (see below) is what the special survey looks like vs a simple extension of the primary survey to the plane.
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the specific analysis of metrics to drive cadence choices requires relevant simulations of Milky Way populations. This has been lacking for a number of years now but discussions are starting up again. Simulations with different levels (and types) of crowding would greatly inform the conclusions on how to set and evaluate metrics. The group is discussing over email how to move this forward, and that will help refine the summary. This is important for assessing 1.) photometric sensitivity, 2.) completeness, 3.) star/galaxy separation, and 4.) proper motions (i.e., astrometry of quasars/galaxies to define a zero motion frame of reference).
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section 4.4 – would be good to get a specific tie in to the WFIRST opportunity of 2000 sq deg high latitude survey at high resolution for halo mapping w/ LSST. In this case, the optical-IR baseline would aid in the characterization of the stellar populations also.
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section 4.5 – important to develop this further.