Supernova-driven winds in simulated dwarf galaxies

Chia-Yu Hu

Published 2018-05-17Version 1

We investigate galactic winds driven by supernova (SN) explosions in an isolated dwarf galaxy using high-resolution (particle mass $m_{\rm gas} = 1{\rm M_\odot}$) hydrodynamical simulations that include non-equilibrium cooling and chemistry, individual star formation, stellar feedback and metal enrichment. We find that the system reaches a quasi-steady state on a Gyr-timescale though with strong temporal fluctuations. Clustered SNe lead to the formation of superbubbles which break out of the disk and vent out hot gas, launching the winds. 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 at $m_{\rm gas}=5 {\rm M_\odot}$ (with an injection mass of $500 {\rm M_\odot}$), and the winds weaken dramatically once the cooling masses of individual SNe become unresolved. We demonstrate that injecting the terminal momentum of SNe (neglecting the residual thermal energy), a popular sub-grid model in the literature, fails to capture pressure-driven winds. The failure owes to its assumption that most thermal energy is radiated away right after injection even for highly clustered SNe.