  Here we explore the infrared (IR) properties of the progenitors of high-z
quasar host galaxies. Adopting the cosmological, data constrained semi-analytic
model GAMETE/QSOdust, we simulate several independent merger histories of a
luminous quasar at z ~ 6, following black hole growth and baryonic evolution in
all its progenitor galaxies. We find that a fraction of progenitor galaxies
(about 0.4 objects per single luminous quasar) at 6.5 < z < 8 has an IR
luminosity of L_IR > 10^13 Lsun (hyper-luminous IR galaxies; HyLIRGs). HyLIRGs
progenitors reside in the most massive halos, with dark matter (DM) masses of
M_DM ~ 10^12.5 - 10^13 Msun. These systems can be easily observed in their ~ 1
mm-continuum emission in a few seconds of integration time with the Atacama
Large Millimeter/submillimeter Array (ALMA), and at least 40% of them host
nuclear BH activity that is potentially observable in the soft and hard X-ray
band. Our findings are in line with recent observations of exceptional massive
DM halos hosting HyLIRGs at z ~ 7, suggesting that z ~ 6 luminous quasars are
indeed the signposts of these observed rare peaks in the high-z cosmic density
field, and that massive IR-luminous galaxies at higher z are their natural
ancestors.
