  Recent comparisons between the stellar haloes of simulated Milky Way-mass
galaxies and observations of similar objects have identified a significant
tension between the masses of simulated and observed stellar haloes. Simulated
stellar haloes appear to have both total masses and surface density profiles
1-2 dex higher than observed galaxies. In this paper, we compare two suites of
15 simulated Milky Way-like galaxies, each drawn from the same initial
conditions and simulated with the same hydrodynamical code, but with two
different models for feedback from supernovae. We find that the MUGS
simulations, which use an older "delayed-cooling" model for feedback, suffer
from the same problems as other simulations examined in the literature, with
median surface densities well above observational constraints. The MUGS2
simulations, which instead use a new, physically-motivated superbubble model
for stellar feedback, have significantly reduced stellar halo masses and
surface densities (~25 times lower on average compared to MUGS stellar haloes),
and generally match both the median surface density as well as the diversity of
structure seen in observed stellar haloes. We examine how feedback produces
differences in the assembly of the stellar halo, both through in-situ stars
scattered out of the disc by high-redshift mergers and in ex-situ stars
stripped from accreted haloes. We conclude that there is no "missing outskirts"
problem in cosmological simulations, so long as supernova feedback is modelled
in a way that allows it to efficiently regulate star formation in the
progenitor environments of stellar haloes.
