The nature and origins of the low surface brightness outskirts of
  massive, central galaxies in Subaru HSC
  We explore the stellar mass density and colour profiles of 118 low redshift,
massive, central galaxies, selected to have assembled 90 percent of their
stellar mass 6 Gyr ago, finding evidence of the minor merger activity expected
to be the driver behind the size growth of quiescent galaxies. We use imaging
data in the $g, r, i, z, y$ bands from the Subaru Hyper Suprime-Cam survey and
perform SED fitting to construct spatially well-resolved radial profiles in
colour and stellar mass surface density. Our visual morphological
classification reveals that $\sim 42$ percent of our sample displays tidal
features, similar to previous studies, $\sim 43$ percent of the remaining
sample display a diffuse stellar halo and only $\sim 14$ percent display no
features, down to a limiting $\mu_{r\mathrm{-band}}$ $\sim$ 28 mag
arcsec$^{-2}$. We find good agreement between the stacked colour profiles of
our sample to those derived from previous studies and an expected smooth,
declining stellar mass surface density profile in the central regions (< 3
R$_{\mathrm{e}}$). However, we also see a flattening of the profile ($\Sigma_*
\sim 10^{7.5}$ M$_\odot$ kpc$^{-2}$) in the outskirts (up to 10
R$_{\mathrm{e}}$), which is revealed by our method of specifically targeting
tidal/accretion features. We find similar levels of tidal features and
behaviour in the stellar mass surface density profiles in a younger comparison
sample, however a lack of diffuse haloes. We also apply stacking techniques,
similar to those in previous studies, finding such procedures wash out tidal
features and thereby produces smooth declining profiles. The stellar material
in the outskirts contributes on average $\sim 10^{10}$ M$_\odot$ or a few
percent of the total stellar mass and has similar colours to SDSS satellites of
similar stellar mass.
