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

TL;DR

This paper uses a simple galaxy formation model to accurately reproduce the observed distributions of luminosity, size, and distance of Milky Way satellites, predicting the total satellite count and properties for upcoming surveys.

Contribution

The study introduces a forward-modeling approach with a regulator-type model that matches observed satellite distributions without needing orphan galaxy models, and predicts satellite counts for future surveys.

Findings

Model reproduces observed satellite luminosity and size distributions.

Predicts about 440 satellites with specific properties within 300 kpc.

Shows a flattening in the stellar mass-halo mass relation at low masses.

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 $M_{\rm peak}$ for the model satellites is not described by a power law, but has a clear flattening of $M_\star-M_{\rm peak}$ scaling at $M_{\rm peak}<10^8\,M_\odot$ imprinted by reionization. As a result, the fraction of low mass haloes ($M_{\rm peak} < 10^8\, M_\odot$) hosting galaxies with $M_V<0$ is predicted to be 50% at $M_{\rm peak} \sim 3.6 \times 10^7\,M_\odot$. 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 $440^{+201}_{-147}$ (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.