  Using the 4 Ms Chandra Deep Field-South (CDF-S) survey, we have identified a
sample of 6845 X-ray undetected galaxies that dominates the unresolved ~ 20-25%
of the 6-8 keV cosmic X-ray background (XRB). This sample was constructed by
applying mass and color cuts to sources from a parent catalog based on
GOODS-South HST z-band imaging of the central 6'-radius area of the 4 Ms CDF-S.
The stacked 6-8 keV detection is significant at the 3.9 sigma level, but the
stacked emission was not detected in the 4-6 keV band which indicates the
existence of an underlying population of highly obscured active galactic nuclei
(AGNs). Further examinations of these 6845 galaxies indicate that the galaxies
on the top of the blue cloud and with redshifts of 1 < z < 3, magnitudes of 25
< z_850 < 28, and stellar masses of 2E8 < M_star/M_sun < 2E9 make the majority
contributions to the unresolved 6-8 keV XRB. Such a population is seemingly
surprising given that the majority of the X-ray detected AGNs reside in massive
(> ~1E10 M_sun) galaxies. We discuss constraints upon this underlying AGN
population, supporting evidence for relatively low-mass galaxies hosting highly
obscured AGNs, and prospects for further boosting the stacked signal.
