Calibration of Surface Brightness Fluctuations for Dwarf Galaxies in the
  Hyper Suprime-Cam $gi$ Filter System
  Surface brightness fluctuation (SBF) magnitudes are a powerful standard
candle to measure distances to semi-resolved galaxies in the local universe, a
majority of which are dwarf galaxies that have often bluer colors than bright
early-type galaxies. We present an empirical $i-$band SBF calibration in a blue
regime, $0.2 \lesssim (g-i)_0 \lesssim 0.8$ in the Hyper Suprime-Cam (HSC)
magnitude system. We measure SBF magnitudes for 12 nearby dwarf galaxies of
various morphological types with archival HSC imaging data, and use their tip
of the red giant branch (TRGB) distances to derive fluctuation - color
relations. In order to subtract contributions of fluctuations due to young
stellar populations, we use five different $g-$band magnitude masking
thresholds, $M_{g,{\rm thres}} = -3.5, -4.0, -4.5, -5.0,$ and $-5.5$ mag. We
find that the rms scatter of the linear fit to the relation is the smallest
(rms = 0.16 mag) in the case of $M_{g,{\rm thres}} = -4.0$ mag, $\overline{M_i}
= (-2.65\pm0.13)+ (1.28\pm0.24) \times (g-i)_0$. This scatter is much smaller
than those in the previous studies (rms=0.26 mag), and is closer to the value
for bright red galaxies (rms=0.12 mag). This calibration is consistent with
predictions from metal-poor simple stellar population models.
