The Structure and Composition of Multiphase Galactic Winds in a Large Magellanic Cloud Mass Simulated Galaxy

Abstract

We present the first results from a high-resolution simulation with a focus on galactic wind driving for an isolated galaxy with a halo mass of ∼1011 M ⊙ (similar to the Large Magellanic Cloud) and a total gas mass of ∼6 × 108 M ⊙, resulting in ∼108 gas cells at ∼4 M ⊙ mass resolution. We adopt a resolved stellar feedback model with nonequilibrium cooling and heating, including photoelectric heating and photoionizing radiation, as well as supernovae, coupled to the second-order meshless finite-mass method for hydrodynamics. These features make this the largest resolved interstellar medium (ISM) galaxy model run to date. We find mean star formation rates around 0.05 M ⊙ yr−1 and evaluate typical time-averaged loading factors for mass (η M ∼ 1.0, in good agreement with recent observations) and energy (η E ∼ 0.01). The bulk of the mass of the wind is transported by the warm (T < 5 × 105 K) phase, while there is a similar amount of energy transported in the warm and the hot phases (T > 5 × 105 K). We find an average opening angle of 30° for the wind, decreasing with higher altitude above the midplane. The wind mass loading is decreasing (flat) for the warm (hot) phase as a function of the star formation surface rate density ΣSFR, while the energy loading shows inverted trends with ΣSFR, decreasing for the warm wind and increasing for the hot wind, although with very shallow slopes. These scalings are in good agreement with previous simulations of resolved wind driving in the multiphase ISM.

SAGAbg. I. A Near-unity Mass-loading Factor in Low-mass Galaxies via Their Low-redshift Evolution in Stellar Mass, Oxygen Abundance, and Star Formation Rate

“…A mass-loading estimate based on Hα should trace only the η m of gas in the warm phase, whereas the present work does not distinguish between ISM phases. The concordance is not surprising, however, if we consider that the warm phase is predicted to dominate mass outflow (Kim et al 2020b;Steinwandel et al 2024). An estimate of η m as defined by Equation (12) could therefore be reasonably expected to be in agreement with measures of η m,warm , the mass-loading factor in the warm (ionized) phase.…”

“…However, the exact definition of outflowing gas varies significantly from work to work-a prediction for the mass-loading factor is typically set by computing the mass flux for some subset of the gas classified as outflowing through a slab or shell displaced a few kiloparsecs from the galaxy of interest. Outflows are either selected to be all gas that is moving away from the midplane (v z > 0 or v R > 0, depending on the coordinate system used; see, e.g., Muratov et al 2015;Kim & Ostriker 2018;Hu 2019;Nelson et al 2019;Steinwandel et al 2023Steinwandel et al , 2024 or a subset of the gas that is predicted to escape to a given midplane distance (Anglés-Alcázar et al 2017;Pandya et al 2021). Both the height and outflow selection criteria can change the estimate of η m by a factor of several (Nelson et al 2019;Pandya et al 2021); we will discuss the effect of these choices on our approach to simulation comparison below.…”

“…We consider two resolved-ISM, noncosmological galaxy simulations close to our mass range: the LMC-mass galaxy simulation of Steinwandel et al (2024), with a gas mass resolution of m gas = 4 M e , and the lower-mass (M å ≈ 2 × 10 7 M å ) simulation of Hu (2019), with a mass resolution of m gas = 1 M e . We find that these two simulations, which bound the stellar mass range we probe, also bound our prediction for η m .…”

“…Let us first compare to the Steinwandel et al (2024) results for the LMC-mass galaxy. We show the time-averaged estimate of η m at z = {3, 5, 10} kpc from the disk (where 10 kpc is approximately 0.1 R vir ).…”

“…Star formation and the (self-)regulation thereof are thought be crucial drivers of structure and evolution in low-mass galaxies (Kim & Ostriker 2018;Hu 2019;Steinwandel et al 2023Steinwandel et al , 2024, but quantifying the impact of the star formation cycle on individual galaxies is difficult due to the stochastic and temporally variable nature of star formation. A crucial component of the self-regulation of star formation is the ability of star formation to remove mass from the gas reservoir of the gas galaxy not only through the conversion of gas to stars, but via galactic-scale outflows driven by star formation feedback (see, e.g., Lynden-Bell 1975;Pagel & Patchett 1975;Carigi 1994;Lilly et al 2013;Muratov et al 2015;Anglés-Alcázar et al 2017;Kim & Ostriker 2018;Hu 2019;Nelson et al 2019;Pandya et al 2020Pandya et al , 2021Ostriker & Kim 2022;Carr et al 2023;Steinwandel et al 2023Steinwandel et al , 2024. The mass-loading factor, η m , is a key metric of star formation feedback that quantifies the amount of gas mass lifted out of a galaxy's interstellar medium (ISM) per unit star formation, i.e.…”