Fossil records of cosmic reionization in galactic stellar halos

Abstract

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.