High-redshift quasars and their host galaxies II: multiphase gas and stellar kinematics

TL;DR

This study uses high-resolution simulations to explore the complex gas phases, kinematics, and feedback processes in a $z ightrsim 6$ quasar host galaxy, revealing insights into gas dynamics, outflows, and black hole growth.

Contribution

It provides a detailed analysis of gas phase distribution, morphology, and outflows in a $z ightrsim 6$ quasar using advanced simulations, highlighting the role of molecular gas and the importance of resolution.

Findings

Obscuration mainly due to molecular gas in the disc.

Molecular outflows likely caused by gas lifted near the MBH.

Resolutions below ~100 pc are unreliable for MBH mass estimates.

Abstract

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.