The stellar halo of isolated central galaxies in the Hyper Suprime-Cam
  imaging survey
  We study the faint stellar halo of isolated central galaxies, by stacking
galaxy images in the HSC survey and accounting for the residual sky background
sampled with random points. The surface brightness profiles in HSC $r$-band are
measured for a wide range of galaxy stellar masses
($9.2<\log_{10}M_\ast/M_\odot<11.4$) and out to 120 kpc. Failing to account for
the stellar halo below the noise level of individual images will lead to
underestimates of the total luminosity by $\leq 15\%$. Splitting galaxies
according to the concentration parameter of their light distributions, we find
that the surface brightness profiles of low concentration galaxies drop faster
between 20 and 100 kpc than those of high concentration galaxies. Albeit the
large galaxy-to-galaxy scatter, we find a strong self-similarity of the stellar
halo profiles. They show unified forms once the projected distance is scaled by
the halo virial radius. The colour of galaxies is redder in the centre and
bluer outside, with high concentration galaxies having redder and more
flattened colour profiles. There are indications of a colour minimum, beyond
which the colour of the outer stellar halo turns red again. This colour
minimum, however, is very sensitive to the completeness in masking satellite
galaxies. We also examine the effect of the extended PSF in the measurement of
the stellar halo, which is particularly important for low mass or low
concentration galaxies. The PSF-corrected surface brightness profile can be
measured down to $\sim$31 $\mathrm{mag}/\mathrm{arcsec}^2$ at 3-$\sigma$
significance. PSF also slightly flattens the measured colour profiles.
