Too Many, Too Few, or Just Right? The Predicted Number and Distribution of Milky Way Dwarf Galaxies

We predict the spatial distribution and number of Milky Way dwarf galaxies to be discovered in the Dark Energy Survey (DES) and Large Synoptic Survey Telescope (LSST) surveys, by completeness correcting the observed Sloan Digital Sky Survey dwarf population. We apply most massive in the past, earliest forming, and earliest infall toy models to a set of dark matter-only simulated Milky Way/M31 halo pairs from the Exploring the Local Volume In Simulations project. Inclusive of all toy models and simulations, at 90% confidence we predict a total of 37–114 L 103 L dwarfs and 131–782 L 103 L dwarfs within 300 kpc. These numbers of L 103 L dwarfs are dramatically lower than previous predictions, owing primarily to our use of updated detection limits and the decreasing number of SDSS dwarfs discovered per sky area. For an effective rlimit of 25.8 mag, we predict 3–13 L 103 L and 9–99 L 103 L dwarfs for DES, and 18–53 L 103 L and 53–307 L 103 L dwarfs for LSST. We also show that the observed spatial distribution of Milky Way dwarfs in the LSST-era will discriminate between the earliest infall and other simplified models for how dwarf galaxies populate dark matter subhalos.