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Global simulations of galactic winds including cosmic ray streaming (1602.04856v1)

Published 15 Feb 2016 in astro-ph.GA and astro-ph.HE

Abstract: Galactic outflows play an important role in galactic evolution. Despite their importance, a detailed understanding of the physical mechanisms responsible for the driving of these winds is lacking. In an effort to gain more insight into the nature of these flows, we perform global three-dimensional magneto-hydrodynamical simulations of an isolated Milky Way-size starburst galaxy. We focus on the dynamical role of cosmic rays injected by supernovae, and specifically on the impact of the streaming and anisotropic diffusion of cosmic rays along the magnetic fields. We find that these microphysical effects can have a significant effect on the wind launching and mass loading factors depending on the details of the plasma physics. Due to the cosmic ray streaming instability, cosmic rays propagating in the interstellar medium scatter on self-excited Alfven waves and couple to the gas. When the wave growth due to the streaming instability is inhibited by some damping process, such as the turbulent damping, the cosmic ray coupling to the gas is weaker and their effective propagation speed faster than the Alfven speed. Alternatively, cosmic rays could scatter from "extrinsic turbulence" that is driven by another mechanism. We demonstrate that the presence of moderately super-Alfvenic cosmic ray streaming enhances the efficiency of galactic wind driving. Cosmic rays stream away from denser regions near the galactic disk along partially ordered magnetic fields and, in the process, accelerate more tenuous gas away from the galaxy. For cosmic ray acceleration efficiencies broadly consistent with the observational constraints, cosmic rays reduce the galactic star formation rates and significantly aid in launching galactic winds.

Citations (118)

Summary

Overview of Cosmic Ray Driven Galactic Winds

The paper presents an advanced paper of cosmic ray (CR) driven galactic winds, focusing on the role of CR streaming and anisotropic diffusion. The authors utilize global three-dimensional magnetohydrodynamical (MHD) simulations to understand the dynamics of galactic winds in an isolated Milky Way-size starburst galaxy, facilitating insight into the microphysical effects that significantly influence wind launching and mass loading.

The research highlights several important results:

  1. Cosmic Ray Influence on Star Formation: The inclusion of CRs in simulations reveals a significant suppression of star formation rates. This effect underscores the critical role of CRs as a feedback mechanism in galactic evolution.
  2. CR Streaming and Anisotropic Diffusion: It is shown that efficient CR streaming and anisotropic diffusion considerably affect the launching efficiency and the mass loading factors of galactic winds. When cosmic ray propagation speed exceeds the Alfvén speed due to wave damping (such as turbulent damping), the coupling of CRs to gas diminishes, enhancing wind driving efficiency.
  3. Moderately Super-Alfvénic Streaming: The presence of super-Alfvénic CR streaming results in more efficient galactic wind propulsion. CR pressure gradients accelerate the tenuous gas away from denser regions near the galactic disk, leveraging partially ordered magnetic fields in the process.
  4. Mass Loading and Energy Transfer: The simulations suggest mass loading factors are broadly consistent with other models while depending significantly on CR acceleration efficiency, streaming speed, magnetic field strength, and additional transport characteristics. The paper's models contribute to the understanding of how CR energy transfer can affect a galaxy's gas dynamics and star formation rates effectively.
  5. Modeling and Parameters: Multiple parameter choices were examined, including cosmic ray acceleration efficiency, streaming speed, and initial magnetic field values. These controlled simulations provide insight into how CR feedback significantly alters galactic wind characteristics.

Implications and Future Directions

The outcomes of this investigation into cosmic ray-driven winds have profound implications for understanding galactic evolution. The demonstrated ability of CRs to efficiently suppress star formation and drive winds provides a mechanism potentially essential in comprehending the baryon composition of galaxies and feedback processes during star formation.

Further research may involve exploring these dynamics in varying galactic conditions (e.g., different halo sizes), introducing live halo simulations, and assessing the longer-term evolution of mass loading factors during galaxy formation. The complex interactions between CRs, magnetic fields, and turbulent damping underscore an intricate balance affecting galactic dynamics, opening pathways for future exploration in high-resolution MHD simulations.

Computational and Methodological Approach

The use of adaptive mesh refinement (AMR) and the advanced MHD FLASH4.2 code enables handling of diverse, multidimensional flux interactions and dynamic field influences. Regularization techniques are applied to manage discontinuities in CR energy density, upholding computational efficiency during extensive simulation periods. The paper’s simulations underscore an integration of particle dynamics, CR physics, and fluid motions critical to examining CR-mediated galaxy evolution in contemporary astrophysical research.

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