Interaction of a cold cloud with a hot wind: the regimes of cloud growth and destruction and the impact of magnetic fields (2008.09118v2)

Abstract: Multiphase galaxy winds, the accretion of cold gas through galaxy haloes, and gas stripping from jellyfish galaxies are examples of interactions between cold and hot gaseous phases. There are two important regimes in such systems. A sufficiently small cold cloud is destroyed by the hot wind as a result of Kelvin-Helmholtz instabilities, which shatter the cloud into small pieces that eventually mix and dissolve in the hot wind. On the contrary, stripped cold gas from a large cloud mixes with the hot wind to intermediate temperatures, and then becomes thermally unstable and cools, causing a net accretion of hot gas to the cold tail. Using the magneto-hydrodynamical code AREPO, we perform cloud crushing simulations and test analytical criteria for the transition between the growth and destruction regimes to clarify a current debate in the literature. We find that the hot-wind cooling time sets the transition radius and not the cooling time of the mixed phase. Magnetic fields modify the wind-cloud interaction. Draping of wind magnetic field enhances the field upstream of the cloud and fluid instabilities are suppressed by a turbulently magnetised wind beyond what is seen for a wind with a uniform magnetic field. We furthermore predict jellyfish galaxies to have ordered magnetic fields aligned with their tails. We finally discuss how the results of idealised simulations can be used to provide input to subgrid models in cosmological (magneto-)hydrodynamical simulations, which cannot resolve the detailed small-scale structure of cold gas clouds in the circum-galactic medium.

Interaction of a Cold Cloud with a Hot Wind

The paper by Sparre et al. explores the complex dynamics between cold gas clouds and hot wind simulations, considering the implications of magnetic fields on these interactions. The paper is conducted within the context of astrophysical phenomena such as multiphase galaxy winds, cold gas accretion through galactic halos, and jellyfish galaxies experiencing gas stripping due to the intra-cluster medium (ICM). Using the magnetohydrodynamical code AREPO, the authors investigate regimes of cloud growth and destruction to resolve ongoing debates regarding the criteria for these transitions.

Core Findings and Methodology

The paper focuses on understanding the conditions under which a cold cloud either stabilizes and accumulates mass or is destroyed by the surrounding hot wind. Sparre et al. utilize cloud crushing simulations to model these interactions and leverage analytical criteria to assess the transition between regimes. The authors demonstrate that the cooling timescale of the hot wind, rather than that of the mixed phase gas, plays a pivotal role in determining these transitions.

• Kelvin-Helmholtz Instability and Cloud Fragmentation: Small clouds experiencing interactions with the hot wind tend to be destroyed due to instabilities that fragment the cloud into smaller parts, eventually mixing into the hot wind. In contrast, larger clouds induce mixing with the hot wind at intermediate temperatures, becoming thermally unstable, leading to further cooling and accretion.
• Impact of Magnetic Fields: The paper reveals that magnetic fields, particularly those present in hot winds, substantially modify the interactions. Magnetic draping enhances the magnetic field upstream of the cloud, reducing fluid instabilities beyond what is observed in uniform magnetic field scenarios. Consequently, jellyfish galaxies likely exhibit ordered magnetic fields that align with their tails.

Simulation Insights and Numerical Results

Utilizing AREPO, the researchers provide a detailed simulation setup presenting various magnetic configurations within cloud and wind interactions. Notably, simulations with turbulent magnetic fields in the wind show a suppression of instabilities, extending the lifespan of dense gas regions within the clouds. Additionally, these magnetic draping effects facilitate the formation of elongated filamentary tails downstream.

Numerical results from simulations show that in both the destruction and growth regimes, the magnetic field's configuration plays a critical role. The presence of a magnetic field modifies the wind-cloud dynamics, especially in dense clouds, providing resilience against Kelvin-Helmholtz instability.

Practical and Theoretical Implications

The findings hold significant implications for understanding multiphase interactions in astrophysical systems. Particularly, these insights are crucial for jellyfish galaxies where gas tails remain dense over vast distances. The presence of ordered magnetic fields in these tails could be detectable via radio synchrotron emissions, providing a potential observational indicator of magnetic field alignment.

On a theoretical level, these simulations offer critical insights for developing and refining subgrid models in cosmological magnetohydrodynamic simulations. Such models are paramount for accurately depicting small-scale structures in cold gas clouds, ultimately enhancing resolutions in simulations of galactic formation and evolution.

Future Directions

Future investigations could explore the magnetic field's influence on gas interactions by employing more diverse magnetic configurations in simulations. Moreover, enhancing the resolution in cosmological simulations could better resolve the circumgalactic medium's dynamics, refining our understanding of cold and hot accretion modes. Further exploration into observational techniques, such as polarized radio emission, can offer new pathways to test and validate theoretical predictions regarding magnetic field alignments in space phenomena.

In summary, Sparre et al.'s work not only contributes valuable insights into cloud dynamics within galactic formations but also opens doors for advanced simulation techniques and observational methods that provide a fuller comprehension of cosmic magnetic fields and their influence on gas interactions across the universe.