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Boosting galactic outflows with enhanced resolution

Our new work shows how better resolving the thermodynamics of galactic outflows boost their energetics, and thus their potential efficiency at regulating galaxy growth over cosmic time.

Published onFeb 23, 2023
Boosting galactic outflows with enhanced resolution

Resolving galactic outflows venting from a dwarf galaxy

This movie shows how simulations of multiphase galactic outflows improve when they are able to resolve the spatial structure of rapidly cooling gas. On that left is a poorly resolved simulation of a dwarf galaxy, and a better resolved simulation of the same galaxy is on the right.

Real galactic outflows are multiphase in nature and are detected ubiquitously in cold, warm, and hot gas across the galaxy population (e.g., [1], [2]). Numerical simulations of galaxies have traditionally relied on strong, mass-loaded outflows to regulate galactic star formation across cosmic time (e.g [3]), but the importance of the hot, energy-loaded phase to pressurise a galaxy’s CGM and prevent future inflows is becoming increasingly scrutinised (e.g., [4],[5], [6], [7]).

A detailed picture of how each gas phase contributes to regulating galactic star formation remains lacking. Achieving this feat is complicated by the Lagrangian nature of traditional galaxy formation simulations — by focussing numerical resolution in high-density regions (i.e. in the interstellar medium) and degrading it in lower-density environments (i.e. in outflows and the circumgalactic medium), such numerical simulations drastically under-resolve thermal and hydrodynamical instabilities in the diffuse gas and artificially suppress its multiphase structure (e.g. [8], see also this previous post for a similar idea).

Our recent work [9] shows how better resolving the diffuse gas of galactic outflows impacts their energetics. We implement a new resolution strategy forcing our adaptive-mesh-refinement code, Ramses-RTZ ([10]) to resolve the cooling length of the gas, in addition to the traditional Lagrangian approach. By also tracking the non-equilibrium chemistry of >60 ions, Ramses-RTZ allows us to predict the ion-by-ion, line-by-line cooling rate and to compute observable gas emission lines on the fly. The above movie highlights the combination of these two advances in galactic outflows modelling, showing the [OIII, 5007Å] (yellow) and Hβ\beta (white) gas emission venting out of a dwarf galaxy (108 M\odot) overlaid over the gas density (blue).

Our new approach allows us to better capture the shocks and cavities from supernovae-driven superbubbles expanding off the disc plane, and the outflowing turbulent, filamentary and multi-phase structure. We show that incrementally increasing the resolution in this way leads to increasingly multiphase outflows, but also systematically boosts their energetics with greater than factor-of-five increases in mass, energy and metal loading for higher-resolution outflows. This improvement occurs because of the improved treatment of thermodynamics and kinematics of each gas phase, rather than a change in the structure of star formation and feedback within the disc.

Boosting galactic outflow energetics in this way is highly significant, as the enhancement does not require modifications to the internal astrophysics within the galaxy. Better resolving the venting behaviour of outflows themselves could thus provide a way to increase their efficiency at regulating galaxy formation across cosmic time, without further increasing mass-loadings within the interstellar medium and disrupting the inner galaxy. our results strongly motivate future studies testing our findings in a cosmological context.

Comments
1
Ben Wibking:

There is a diffuse FUV emission component that is not included in Ramses-RTZ that may be quite significant for the correct hydrogen ionization: i) the diffuse FUV line emission from the hot ionized medium/SNRs, and ii) the two-photon continuum from the WIM. I am not aware of any estimates of the contribution of these components in dwarf galaxies, but for the hydrogen ionization rate in the diffuse Galactic ISM, they are quite significant. See in particular the review by Kulkarni (2022): https://ui.adsabs.harvard.edu/abs/2022PASP..134h4302K/abstract.