A Lonely Giant: The Sparse Satellite Population of M94 Challenges Galaxy
  Formation
  The dwarf satellites of `giant' Milky Way (MW)-mass galaxies are our primary
probes of low-mass dark matter halos. The number and velocities of the
satellite galaxies of the MW and M31 initially puzzled galaxy formation
theorists, but are now reproduced well by many models. Yet, are the MW's and
M31's satellites representative? Were galaxy formation models `overfit'? These
questions motivate deep searches for satellite galaxies outside the Local
Group. We present a deep survey of the `classical' satellites
($M_{\star}$$\geqslant$4$\times$10$^5 M_{\odot}$) of the MW-mass galaxy M94 out
to 150 kpc projected distance. We find $only\ two$ satellites, each with
$M_{\star}{\sim}10^6 M_{\odot}$, compared with 6-12 such satellites in the four
other MW-mass systems with comparable data (MW, M31, M81, M101). Using a
`standard' prescription for occupying dark matter halos (taken from the fully
hydrodynamical EAGLE simulation) with galaxies, we find that such a sparse
satellite population occurs in $<$0.2% of MW-mass systems $-$ a $<$1%
probability among a sample of five (known systems + M94). In order to produce
an M94-like system more frequently we make satellite galaxy formation much more
stochastic than is currently predicted by dramatically increasing the slope and
scatter of the stellar mass-halo mass (SMHM) relation. Surprisingly, the SMHM
relation must be altered even for halos masses up to 10$^{11}M_{\odot} -$
significantly above the mass scales predicted to have increased scatter from
current hydrodynamical models. The sparse satellite population of this `lonely
giant' thus advocates for an important modification to ideas of how the
satellites around MW-mass galaxies form.
