Forward-modelling the luminosity, distance, and size distributions of the Milky Way satellites

We use GRUMPY, a simple regulator-type model for dwarf galaxy formation and evolution, to forward model the dwarf galaxy satellite population of the Milky Way (MW) using the Caterpillar zoom-in simulation suite. We show that luminosity and distance distributions of the model satellites are consistent with the distributions measured in the DES, PS1, and SDSS surveys, even without including a model for the orphan galaxies. We also show that our model for dwarf galaxy sizes can simultaneously reproduce the observed distribution of stellar half-mass radii, r(1/2), of the MW satellites and the overall r(1/2)-M-* relation exhibited by observed dwarf galaxies. The model predicts that some of the observed faint stellar systems with r(1/2) < 10 pc are ultra-faint dwarf galaxies. Scaling of the stellar mass M-* and peak halo mass M-peak for the model satellites is not described by a power law, but has a clear flattening of M-*-M-peak scaling at M-peak < 10 M-circle dot imprinted by reionization. As a result, the fraction of low mass haloes (M-peak < 10<^>8, M-circle dot) hosting galaxies with M-V < 0 is predicted to be 50 per cent at M-peak similar to 3.6 x 10(7) M-circle dot. We find that such high fraction at that halo mass helps to reproduce the number of dwarf galaxies discovered recently in the HSC-SSP survey. Using the model we forecast that there should be the total of 440(+201)(-147) (68 per cent confidence interval) MW satellites with M-V < 0 and r(1/2) > 10 pc within 300 kpc and make specific predictions for the HSC-SSP, DELVE-WIDE, and LSST surveys.