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

Abstract

We predict the spatial distribution and number of Milky Way dwarf galaxies to be discovered in the DES and LSST surveys, by completeness correcting the observed SDSS 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 Exploring the Local Volume In Simulations (ELVIS). 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. Inclusive of all toy models and simulations, at 90% confidence we predict a total of 37-114 L $\gtrsim 10^3$L${}_{\odot }$ dwarfs and 131-782 L $\lesssim 10^3$L${}_{\odot }$ dwarfs within 300 kpc. These numbers of L $\gtrsim 10^3$L${}_{\odot }$ 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 $r_{\mathrm{limit}}$ of 25.8 mag, we predict: 3-13 L $\gtrsim 10^3$L${}_{\odot }$ and 9-99 L $\lesssim 10^3$L${}_{\odot }$ dwarfs for DES, and 18-53 L $\gtrsim 10^3$L${}_{\odot }$ and 53-307 L $\lesssim 10^3$L${}_{\odot }$ dwarfs for LSST. These enormous predicted ranges ensure a coming decade of near-field excitement with these next generation surveys.

4 pages, 3 figures, 2 tables, submitted to the Astrophysical Journal Letters; comments welcome

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