  Massive galaxies display extended light profiles that can reach several
hundreds of kilo parsecs. These stellar halos provide a fossil record of galaxy
assembly histories. Using data that is both wide (~100 square degree) and deep
(i>28.5 mag/arcsec^2 in i-band), we present a systematic study of the stellar
halos of a sample of more than 3000 galaxies at 0.3 < z < 0.5 with $\log
M_{\star}/M_{\odot} > 11.4$. Our study is based on high-quality (0.6 arcsec
seeing) imaging data from the Hyper Suprime-Cam (HSC) Subaru Strategic Program
(SSP), which enables us to individually estimate surface mass density profiles
to 100 kpc without stacking. As in previous work, we find that more massive
galaxies exhibit more extended outer profiles. When this extended light is not
properly accounted for as a result of shallow imaging or inadequate profile
modeling, the derived stellar mass function can be significantly underestimated
at the highest masses. Across our sample, the ellipticity of outer light
profiles increases substantially as we probe larger radii. We show for the
first time that these ellipticity gradients steepen dramatically as a function
of galaxy mass, but we detect no mass-dependence in outer color gradients. Our
results support the two-phase formation scenario for massive galaxies in which
outer envelopes are built up at late times from a series of merging events. We
provide surface mass surface mass density profiles in a convenient tabulated
format to facilitate comparisons with predictions from numerical simulations of
galaxy formation.
