The infrared-luminous progenitors of high-z quasars

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 LIR > 1013 L☉ (hyper-luminous IR galaxies; HyLIRGs). HyLIRGs progenitors reside in the most massive haloes, with dark matter (DM) masses of MDM ∼ 1012.5-1013 M☉. 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, and at least 40 per cent of them host nuclear black hole 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 haloes 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.