The formation of galaxies hosting z ∼ 6 quasars

We investigate the formation and properties of galaxies hosting z similar to 6 quasars in the gigaparsec scale cosmological hydrodynamical simulation MassiveBlack, which includes a self-consistent model for star formation, black hole accretion and associated feedback. We select the host galaxies based on the sample of quasars in MassiveBlack brighter than the Sloan Digital Sky Survey magnitude limit in the redshift range 5.5 ?z? 6.5. We find that quasar hosts in the simulation are compact gas-rich systems with high star formation rates (SFRs) of SFR similar to 102103 M? yr-1 consistent with observed properties. We show that the star-forming gas in these galaxies predominantly originates from high-density cold streams which efficiently penetrate the halo and grow the galaxy at the centre. The ratio of molecular to total cold gas mass in these galaxies is Mmol/Mcold similar to 0.1, much larger than local galaxies of similar masses, indicating that star formation in high-redshift quasar host galaxies is more efficient than their local counterparts. We show that MassiveBlack predicts a deviation from the local MBHs and MBHM* relations, implying that black holes are relatively more massive for a given stellar host at these redshifts.