High-redshift quasars and their host galaxies II: multiphase gas and
  stellar kinematics
  Observations of $z \gtrsim 6$ quasars provide information on the early phases
of the most massive black holes (MBHs) and galaxies. Current observations at
sub-mm wavelengths trace cold and warm gas, and future observations will extend
information to other gas phases and the stellar properties. The goal of this
study is to examine the gas life cycle in a $z \gtrsim 6$ quasar: from
accretion from the halo to the galaxy and all the way into the MBH, to how star
formation and the MBH itself affect the gas properties. Using a very-high
resolution cosmological zoom-in simulation of a $z=7$ quasar including
state-of-the-art non-equilibrium chemistry, MBH formation, growth and feedback,
we investigate the distribution of the different gas phases in the interstellar
medium across cosmic time. We assess the morphological evolution of the quasar
host using different tracers (star- or gas-based) and the thermodynamic
distribution of the MBH accretion-driven outflows, finding that obscuration in
the disc is mainly due to molecular gas, with the atomic component contributing
at larger scales and/or above/below the disc plane. Moreover, our results also
show that molecular outflows, if present, are more likely the result of gas
being lifted near the MBH than production within the wind because of thermal
instabilities. Finally, we also discuss how different gas phases can be
employed to dynamically constrain the MBH mass, and argue that resolutions
below $\sim 100$ pc yield unreliable estimates because of the strong
contribution of the nuclear stellar component to the potential at larger
scales.
