AGN-driven outflows in clumpy media: multiphase structure and scaling relations (2407.17593v2)

Abstract: Small-scale winds driven from accretion discs surrounding active galactic nuclei (AGN) are expected to launch kpc-scale outflows into their host galaxies. However, the ways in which the structure of the interstellar medium (ISM) affects the multiphase content and impact of the outflow remains uncertain. We present a series of numerical experiments featuring a realistic small-scale AGN wind with velocity $5\times 10^3-10^4\ \rm{km/s}$ interacting with an isolated galaxy disc with a manually-controlled clumpy ISM, followed at sub-pc resolution. Our simulations are performed with AREPO and probe a wide range of AGN luminosities ($L=10^{43-47}\ \rm{erg/s}$) and ISM substructures. In homogeneous discs, the AGN wind sweeps up an outflowing, cooling shell, where the emerging cold phase dominates the mass and kinetic energy budgets, reaching a momentum flux $\dot{p} \approx 7\ L/c$. However, when the ISM is clumpy, outflow properties are profoundly different. They contain small, long-lived ($> 5\ \rm{Myr}$), cold ($T<10^{4.5}\ \rm{K}$) cloudlets entrained in the faster, hot outflow phase, which are only present in the outflow if radiative cooling is included in the simulation. While the cold phase dominates the mass of the outflow, most of the kinetic luminosity is now carried by a tenuous, hot phase with $T > 10^7 \ \rm K$. While the hot phases reaches momentum fluxes $\dot{p} \approx (1 - 5)\ L/c$, energy-driven bubbles couple to the cold phase inefficiently, producing modest momentum fluxes $\dot{p} < L/c$ in the fast-outflowing cold gas. These low momentum fluxes could lead to the outflows being misclassified as momentum-driven using common observational diagnostics. We also show predictions for scaling relations between outflow properties and AGN luminosity and discuss the challenges in constraining outflow driving mechanisms and kinetic coupling efficiencies using observed quantities.

• The paper demonstrates that AGN outflows evolve into multiphase biconical structures in clumpy media due to radiative cooling at phase boundaries.
• The simulation using Arepo reveals that while the cold gas carries most of the mass, the hot phase holds the bulk of the kinetic energy.
• The study highlights that variations in AGN luminosity and ISM inhomogeneity lead to scatter in scaling relations and challenges in observational diagnostics.

Insights Into AGN-Driven Outflows in Clumpy Media: A Numerical Investigation

The paper "AGN-driven outflows in clumpy media: multiphase structure and scaling relations" by S. R. Ward et al. presents a comprehensive numerical paper of active galactic nuclei (AGN) outflows interacting with a clumpy interstellar medium (ISM). The research deploys advanced simulations to investigate the multiscale and multiphase structures of outflows, and it discusses the implications of these structures on observational diagnostics.

Multiscale Outflow Structures

The paper starts with setting up a realistic numerical experiment using small-scale AGN winds with velocities ranging between $5000 \-- 10,000$ km/s. These winds interact with an isolated galaxy disc configured with a clumpy ISM at sub-pc resolution. The simulation employs the Arepo code, which allows for a detailed resolution of the wind-ISM interactions. A significant finding is that in clumpy discs, outflows evolve into biconical structures with striking disparities in phase distribution and energetics compared to outflows in homogeneous media.

The outflows in a clumpy ISM give rise to a multiphase structure where the cold phase consists of small, dense cloudlets entrained in a faster hot outflowing phase. The paper shows that the cold gas is predominantly formed due to radiative cooling at phase boundaries, which is a critical mechanism for its survival over the simulation timescale. Interestingly, in homogeneous discs, the outflows form a cooling shell where the cold phase dominates the mass and kinetics, a contrast to the clump interaction scenario.

Phase and Energetics Analysis

The paper explores the energetics and phase distribution, providing a detailed account of how these properties differ with initial ISM conditions. In clumpy ISMs with radiative cooling, the cold outflow phase, despite carrying most of the mass, holds a relatively minor fraction of the kinetic energy, which predominantly resides in the hot phase. This is a crucial detail as it underscores the efficiency of energy transfer and momentum boosting through heterogeneous media, and contrasts sharply with the smooth ISM scenario where the outflow's kinetic energy is more evenly distributed across a cold shell structure.

Implications for Observational Studies

The authors provide an insightful discussion on the challenges faced by observations in accurately measuring outflow properties, primarily due to the complex nature of AGN outflows in clumpy media. They stress the importance of distinguishing different phases and accounting for varying structures to avoid misclassification in the momentum-driven and energy-driven paradigms. The paper also emphasizes that a thorough and accurate understanding of AGN-feedback requires multiwavelength observations to capture the entire spectrum of kinematics and phase constituents, which observationally is often challenging.

Effects of Luminosity and ISM Properties

The paper explores how variations in AGN luminosity affect the scaling relations of outflow properties. It is evident that luminosity changes significantly impact the mass outflow rates and kinetic coupling efficiencies, challenging the consistency of observational scaling relations due to inherent galaxy-to-galaxy variability in ISM structures and AGN properties. The research shows that scaling relations could inherently carry scatter due to these discrepancies, and care must be taken when associating observed outflows directly with theoretical models.

Future Directions

While the paper offers significant insights, it highlights the necessity for further exploration into additional processes like metal-line and low-temperature cooling, which could significantly impact the resulting outflow structure and apparent luminosity scaling relations. Future developments in this area could also benefit from incorporating additional physical mechanisms such as magnetic fields, cosmic rays, and radiation pressure.

In summary, this paper provides a substantive look into the dynamic interactions between AGN-driven winds and clumpy ISM, reinforcing the critical nature of phase resolution for understanding the impacts of AGN feedback in shaping galaxy evolution. The research underlines the complexities involved in translating numerical and theoretical advances into observable diagnostics, a facet pivotal for the advancement of astrophysical models of galaxy dynamics.