Boosting galactic outflows with enhanced resolution

Abstract: We study how better resolving the cooling length of galactic outflows affect their energetics. We perform radiative-hydrodynamical galaxy formation simulations of an isolated dwarf galaxy ($M_{\star}=10^{8}\, \mathrm{M}\odot$) with the Ramses-RTZ code, accounting for non-equilibrium cooling and chemistry coupled to radiative transfer. Our simulations reach a spatial resolution of $18 \, \mathrm{pc}$ in the interstellar medium (ISM) using a traditional quasi-Lagrangian scheme. We further implement a new adaptive mesh refinement (AMR) strategy to resolve the local gas cooling length, allowing us to gradually increase the resolution in the stellar-feedback-powered outflows, from $\geq 200 \, \mathrm{pc}$ to $18 \, \mathrm{pc}$. The propagation of outflows into the inner circumgalactic medium (CGM) is significantly modified by this additional resolution, but the ISM, star formation and feedback remain by and large the same. With increasing resolution in the diffuse gas, the hot outflowing phase ($T > 8 \times 10^{4} \, \mathrm{K}$) systematically reaches overall higher temperatures and stays hotter for longer as it propagates outwards. This leads to two-fold increases in the time-averaged mass and metal outflow loading factors away from the galaxy ($r=5\, \mathrm{kpc}$), a five-fold increase in the average energy loading factor, and a $\approx$50 per cent increase in the number of sightlines with $N{\text{OVI}} \geq 10^{13}\, \mathrm{cm}^{-2}$. Such a significant boost to the energetics of outflows without new feedback mechanisms or channels strongly motivates future studies quantifying the efficiency with which better-resolved multiphase outflows regulate galactic star formation in a cosmological context.