Mapping the Galactic Halo IV. Finding Distant Giants Reliably with the Washington System

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Abstract
We critically examine the use of the Washington photometric system (with the 51 filter) for identifying distant halo giants. While this is the most powerful photometric technique for isolating G and K giant stars, spectroscopic follow-up of giant candidates is vital. There are two situations in which interlopers outnumber genuine giants in the diagnostic M-51/M-T2 plot, and are indistinguishable photometrically from the giants. (1) In deep surveys covering tens of square degrees, very metal-poor halo dwarfs are a significant contaminant. An example is our survey of the outer halo (Morrison et al. 2000, Dohm-Palmer et al. 2000), where these metal-poor dwarfs dominate the number of photometric giant candidates at magnitudes fainter than V = 18 and cannot be isolated photometrically. (2) In deep surveys of smaller areas with low photometric precision, most objects in the giant region of the color-color plot are dwarfs whose photometric errors have moved them there. Color errors in M-51 and M-T2 need to be smaller than 0.03 mag to avoid this problem. An example of a survey whose photometric errors place the giant identifications under question is the survey for extra-tidal giants around the Carina dwarf spheroidal of Majewski et al. (2000a). Accurate photometry and spectroscopic follow-up of giant candidates are essential when using the Washington system to identify the rare outer-halo giants.

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