Forward-modelling the Luminosity, Distance, and Size distributions of the Milky Way Satellites

Abstract

We use \texttt{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 {\it distribution} of stellar half-mass radii, $r_{1/2}$, of the MW satellites and the overall $r_{1/2}-M_{\star }$ 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 $M_{\star }$ and peak halo mass $M_{\mathrm{peak}}$ for the model satellites is not described by a power law, but has a clear flattening of $M_{\star }-M_{\mathrm{peak}}$ scaling at $M_{\mathrm{peak}}<10^8{M}_{\odot }$ imprinted by reionization. As a result, the fraction of low mass haloes ($M_{\mathrm{peak}}<10^8{M}_{\odot }$) hosting galaxies with $M_V<0$ is predicted to be 50% at $M_{\mathrm{peak}}\sim 3.6\times 10^7{M}_{\odot }$. 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 $440_{-147}^{+201}$ (68\% 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.

MNRAS in press, 28 pages, 22 figures. Code to reproduce figures is available at https://github.com/kibokov/grumpy_mw_forward_model

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