  Galactic stellar halos have long been considered to contain fossil
information on early dynamical and chemical evolution of galaxies. We propose
that the surface brightness distributions of old stellar halos contain the
influence of reionization on early formation histories of galaxies. By assuming
that reionization significantly suppresses star formation in small subgalactic
clumps virialized after reionization redshift ($z_{\rm reion}$), we first
numerically investigate how structural and kinematical properties of stellar
halos formed from merging of these subgalactic clumps depend on $z_{\rm
reion}$. We then discuss what observable properties of galactic stellar halos
offer us the fossil records of reionization influence on hierarchical formation
of halos based on the current results of numerical simulations. We particularly
suggest that both the half-light radius of stellar halos and the slope of their
surface brightness profile contain useful information on when star formation in
subgalactic clumps were significantly influenced by reionization. By using the
simulated surface brightness distributions of galactic stellar halos for models
with different $z_{\rm reion}$, we also discuss how wide-field imaging studies
of extragalactic halos will help us to elucidate the epoch of cosmic
reionization.
