In a recently-accepted paper, Adam Smercina, I and our collaborators have discovered that the satellite galaxies of Milky Way-like galaxies vary much more than previously anticipated. We conducted a deep g-band survey of the satellites around the MW-mass central galaxy M94 (NGC 4736), out to 150 kpc in galactocentric radius, with Subaru’s Hyper Suprime-Cam. Despite our expectation of discovering ~10 ‘classical’ satellites (scaling from the MW and 3 other nearby Milky-Way like galaxies), we discovered only two – both with M* < 10⁶ M_Sun. Via extensive artificial galaxy testing, we have determined that our survey is >85% complete down to a stellar mass limit of ~4×10⁵ M_Sun. Thus, M94 very likely only has two low-mass satellites within 150 kpc.

In order to schematically illustrate the implications of this finding for galaxy formation, we explore simple halo occupation models for painting galaxies onto dark matter halos and subhalos. If we adopt a ‘standard’ halo occupation model, following an extrapolation of e.g., Behroozi et al. 2013, we expect M94-like systems (with 2 satellites within 150kpc) <0.2% of the time. Furthermore, none of these modeled systems have a most massive satellite less massive than10⁶ M_Sun.

A significantly more stochastic halo occupation model that increases the scatter in the SMHM relation all the way up to M_peak > 10¹⁰ M_Sun produces M94-like systems much more frequently, >4% of the time, fits the luminosity function of all Milky-Way like satellite systems better and produces a few systems with low-mass dominant satellites.

We suggest that the considerable variation in the number (and character) of satellites of central galaxies like the Milky Way contains important information about how galaxies might populate dark matter halos, and in particular appears consistent with a framework in which there is large scatter between dark matter subhalo mass and the mass of the satellite galaxy living in that halo.

Satellite stellar mass functions and statistics for M94 and other nearby galaxies and EAGLE halos, assuming two halo occupation models. Left: Satellite mass functions for nearby galaxies: M94 (orange), the MW (blue), M31 (red), M81 (green), and M101 (purple). Also shown are the median (black line) and 50% (dark gray), 90% (gray), and 99% (light gray) confidence intervals for simulated satellite mass functions for MW-mass galaxies taken from the dark matter in the EAGLE hydrodynamical simulation.