Supernova-driven winds in simulated dwarf galaxies

Abstract: We investigate galactic winds driven by supernova (SN) explosions in an isolated dwarf galaxy using high-resolution (particle mass $m_{\rm gas} = 1{\rm M_\odot}$, number of neighbor $N_{\rm ngb} = 100$) smoothed-particle hydrodynamics simulations that include non-equilibrium cooling and chemistry, individual star formation, stellar feedback and metal enrichment. Clustered SNe lead to the formation of superbubbles which break out of the disk and vent out hot gas, launching the winds. We find much weaker winds than what cosmological simulations typically adopt at this mass scale. At the virial radius, the time-averaged loading factors of mass, momentum and energy are 3, 1 and 0.05, respectively, and the metal enrichment factor is 1.5. Winds that escape the halo consist of two populations that differ in their launching temperatures. Hot gas acquires enough kinetic energy to escape when launched while warm gas does not. However, warm gas can be further accelerated by the ram pressure of the subsequently launched hot gas and eventually escape. The strong interactions between different temperature phases highlight the caveat of extrapolating properties of warm gas to large distances based on its local conditions (e.g. the Bernoulli parameter). Our convergence study finds that wind properties converge when the cooling masses of individual SNe are resolved, which corresponds to $m_{\rm gas}=5 {\rm M_\odot}$ with an injection mass of $500 {\rm M_\odot}$. The winds weaken dramatically once the SNe become unresolved. We demonstrate that injecting the terminal momentum of SNe, a popular sub-grid model in the literature, fails to capture SN winds irrespective of the inclusion of residual thermal energy.

Supernova-Driven Winds in Simulated Dwarf Galaxies: Insights from High-Resolution Simulations

The paper by Hu et al. presents a detailed investigation of supernova-driven winds in simulated dwarf galaxies using advanced smoothed-particle hydrodynamics (SPH) simulations. The researchers focus on understanding the dynamics of these winds, including their mass, momentum, energy, and metal content, while exploring the conditions under which they form and disperse throughout the galaxy and beyond its halo.

The simulations incorporate high resolution, with particle masses as low as (1 \, {\rm M_\odot}), and sophisticated models encompassing non-equilibrium cooling, individual star formation, stellar feedback, and metal enrichment. The authors find that clustered supernovae (SNe) generate superbubbles capable of breaking out of the disk and driving these galactic winds. Notably, their simulations reveal that the winds are significantly weaker than those typically assumed in cosmological models for galaxies of similar mass, with time-averaged loading factors at the virial radius being 3 for mass, 1 for momentum, and 0.05 for energy, while the metal enrichment factor is 1.5.

A critical finding of the study is that the galactic winds comprise two distinct gas populations based on their launching conditions. Hot gas, upon being launched, possesses sufficient kinetic energy to escape the galaxy's gravitational pull, while warm gas initially lacks this energy. The warm gas, however, can be accelerated by the ram pressure of the hot gas, allowing it to join the outflow eventually. This dynamic interaction highlights the limitations of extrapolating the properties of warm gas based solely on local conditions.

The study further emphasizes the importance of resolving individual SN events to accurately simulate wind properties. The convergence analysis indicates that the simulations achieve accurate results when the cooling masses of individual SNe are resolved, corresponding to particle masses of (5 \, {\rm M_\odot}). Interestingly, the researchers also demonstrate the limitations of popular sub-grid models that inject terminal SN momentum, finding them inadequate for capturing the dynamics of SN-driven winds, particularly when residual thermal energy is not considered.

The implications of this research are manifold. From a theoretical standpoint, the study challenges the assumptions of current cosmological models regarding the strength and characteristics of supernova-driven winds, suggesting a need to revise these models for more realistic predictions. Practically, the findings stress the importance of resolution in simulations to capture the multi-phase structure of the ISM accurately and the resultant wind properties.

Looking to the future, the research paves the way for more nuanced models that can incorporate the complex interactions between different gas phases, potentially improving the fidelity of galaxy formation simulations. As computational power and techniques continue to evolve, there is promise for even more precise examination of the microphysics driving galactic outflows, which will enhance our understanding of galaxy evolution in various environments. This deeper insight could eventually inform observational strategies aimed at detecting and characterizing these winds in actual dwarf galaxies.