A missing outskirts problem? Comparisons between stellar halos in the
  Dragonfly Nearby Galaxies Survey and the TNG100 simulation
  Low surface brightness galactic stellar halos provide a challenging but
promising path towards unraveling the past assembly histories of individual
galaxies. Here, we present detailed comparisons between the stellar halos of
Milky Way-mass disk galaxies observed as part of the Dragonfly Nearby Galaxies
Survey (DNGS) and stellar mass-matched galaxies in the TNG100 run of the
IllustrisTNG project. We produce stellar mass maps as well as mock $g$ and
$r$-band images for randomly oriented simulated galaxies, convolving the latter
with the Dragonfly PSF and taking care to match the background noise, surface
brightness limits and spatial resolution of DNGS. We measure azimuthally
averaged stellar mass density and surface brightness profiles, and find that
the DNGS galaxies generally have less stellar mass (or light) at large radii
(>20 kpc) compared to their mass-matched TNG100 counterparts, and that
simulated galaxies with similar surface density profiles tend to have low
accreted mass fractions for their stellar mass. We explore potential solutions
to this apparent "missing outskirts problem" by implementing several ad-hoc
adjustments within TNG100 at the stellar particle level. Although we are unable
to identify any single adjustment that fully reconciles the differences between
the observed and simulated galaxy outskirts, we find that artificially delaying
the disruption of satellite galaxies and reducing the spatial extent of in-situ
stellar populations result in improved matches between the outer profile shapes
and stellar halo masses, respectively. Further insight can be achieved with
higher resolution simulations that are able to better resolve satellite
accretion, and with larger samples of observed galaxies.
