They Might Be Giants: An Efficient Color-based Selection of Red Giant Stars

We present a color-based method for identifying red giants based on the Pan-STARRS grz and WISE W1 and W2 photometry. We utilize a subsample of bright stars with precise parallaxes from Gaia's second data release (DR2) to verify that the color-based selection reliably separates dwarfs from giants. The selection is conservative in the sense that contamination is small (≈30%) but not all giants are included (the selection primarily identifies K giants). The color-based selection can be applied to stars brighter than W1 ≈ 16, more than two magnitudes fainter than techniques relying on shallower 2MASS photometry. Many streams and clouds are visible in the resulting sky maps, especially when binned by Gaia DR2 proper motions, including the Sagittarius stream, the Hercules-Aquila Cloud, the Eastern Banded Structure, Monoceros, and the Virgo Overdensity. In addition to the characterization of new and known stellar streams, we expect that this method for selecting red giants will enable detailed analysis of the diffuse stellar halo to distances exceeding 100 kpc.