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A recent comparison by Merritt et al. 2020 of simulated and observed MilkyWay-mass galaxies has identified a significant tension between the outskirts($r>20\kpc$) of the stellar halos in simulated and observed galaxies. Usingobservations from the Dragonfly telescope and simulated galaxies from theIllustris-TNG100 project, Merritt et al. 2020 finds that the outskirts ofstellar halos in simulated galaxies have surface densities $1-2$ dex higherthan observed galaxies. In this paper, we compare two suites of 15 simulatedMilky Way-like galaxies, each drawn from the same initial conditions, simulatedwith the same hydrodynamical code, but with two different models for feedbackfrom supernovae. We find that the MUGS simulations, which use an older``delayed-cooling'' model for feedback, also produce too much stellar mass inthe outskirts of the halo, with median surface densities well aboveobservational constraints. The MUGS2 simulations, which instead use a new,physically-motivated ``superbubble'' model for stellar feedback, have $1-2$ dexlower outer stellar halo masses and surface densities. The MUGS2 simulationsgenerally match both the median surface density profile as well as the scatterin stellar halo surface density profiles seen in observed stellar halos. Weconclude that there is no ``missing outskirts'' problem in cosmologicalsimulations, provided that supernova feedback is modelled in a way that allowsit to efficiently regulate star formation in the low-mass progenitorenvironments of stellar halo outskirts.

Where Did the Outskirts Go? Outer Stellar Haloes as a Sensitive Probe of Supernova Feedback