Stacked fluxes of all samples in the observed-frame soft- and hard-band vs. redshift. Horizontal error bars indicate the width of the redshift bins, the solid line indicates the observed soft-band flux of the low-z LBG analog VV 114 from Grimes et al. (2006). VV 114 has an IR-derived SFR of 48 M_⊙   yr^-1 (Soifer et al. 1989). The horizontal dashed line indicates the soft-band on-axis detection limit of the Chandra 4 Ms mosaic as reported by Xue et al. (2011).