Where Did the Outskirts Go? Outer Stellar Haloes as a Sensitive Probe of
  Supernova Feedback
  A recent comparison by Merritt et al. 2020 of simulated and observed Milky
Way-mass galaxies has identified a significant tension between the outskirts
($r>20\kpc$) of the stellar halos in simulated and observed galaxies. Using
observations from the Dragonfly telescope and simulated galaxies from the
Illustris-TNG100 project, Merritt et al. 2020 finds that the outskirts of
stellar halos in simulated galaxies have surface densities $1-2$ dex higher
than observed galaxies. In this paper, we compare two suites of 6 simulated
Milky Way-like galaxies, each drawn from the same initial conditions, simulated
with the same hydrodynamical code, and with similar total stellar masses, but
with two different models for feedback from supernovae. We find that the MUGS
simulations, which use an older ``delayed-cooling'' model for feedback, also
produce too much stellar mass in the outskirts of the halo, with median surface
densities well above observational constraints. The MUGS2 simulations, which
instead use a new, physically-motivated ``superbubble'' model for stellar
feedback, have $1-2$ dex lower outer stellar halo masses and surface densities,
and generally match the median surface density and the diversity of structure
seen in observed stellar halos. We conclude that there is no ``missing
outskirts'' problem in cosmological simulations, provided that supernova
feedback is modelled in a way that allows it to efficiently regulate star
formation in the low-mass progenitor environments of stellar halo outskirts
