On the Formation of Molecular Clumps in QSO Outflows

TL;DR

This paper investigates the origin of cold molecular clumps in quasar outflows, showing that dust destruction and instabilities make stable clump formation unlikely, challenging existing interpretations of molecular outflows.

Contribution

The study combines analytical and 3D numerical simulations to explore the formation and stability of molecular clumps in QSO outflows, highlighting the difficulty of their sustained existence.

Findings

Dust grains are rapidly destroyed within ~10,000 years.

Stable dense clumps only form under artificially enhanced radiation fields.

Molecular outflows are difficult to interpret due to dust destruction and transient clumps.

Abstract

We study the origin of the cold molecular clumps in quasar outflows, recently detected in CO and HCN emission. We first describe the physical properties of such radiation-driven outflows and show that a transition from a momentum- to an energy-driven flow must occur at a radial distance of R ~ 0.25 kpc. During this transition, the shell of swept up material fragments due to Rayleigh-Taylor instabilities, but these clumps contain little mass and are likely to be rapidly ablated by the hot gas in which they are immersed. We then explore an alternative scenario in which clumps form from thermal instabilities at R >~ 1 kpc, possibly containing enough dust to catalyze molecule formation. We investigate this processes with 3D two-fluid (gas+dust) numerical simulations of a kpc^3 patch of the outflow, including atomic and dust cooling, thermal conduction, dust sputtering, and photoionization from the QSO radiation field. In all cases, dust grains are rapidly destroyed in ~10,000 years; and while some cold clumps form at later times, they are present only as transient features, which disappear as cooling becomes more widespread. In fact, we only find a stable two-phase medium with dense clumps if we artificially enhance the QSO radiation field by a factor 100. This result, together with the complete destruction of dust grains, renders the interpretation of molecular outflows a very challenging problem.