The SAMI Galaxy Survey: understanding observations of large-scale outflows at low redshift with EAGLE simulations

Abstract

This work presents a study of galactic outflows driven by stellar feedback. We extract main sequence disc galaxies with stellar mass $10^9\le$ M$_{\star}/$M$_{\odot} \le 5.7\times10^{10}$ at redshift $z=0$ from the highest resolution cosmological simulation of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) set. Synthetic gas rotation velocity and velocity dispersion ($σ$) maps are created and compared to observations of disc galaxies obtained with the Sydney-AAO Multi-object Integral field spectrograph (SAMI), where $σ$-values greater than $150$ km s$^{-1}$ are most naturally explained by bipolar outflows powered by starburst activity. We find that the extension of the simulated edge-on (pixelated) velocity dispersion probability distribution depends on stellar mass and star formation rate surface density ($Σ_{\rm SFR}$), with low-M$_{\star}/$low-$Σ_{\rm SFR}$ galaxies showing a narrow peak at low $σ$ ($\sim30$ km s$^{-1}$) and more active, high-M$_{\star}/$high-$Σ_{\rm SFR}$ galaxies reaching $σ>150$ km s$^{-1}$. Although supernova-driven galactic winds in the EAGLE simulations may not entrain enough gas with T $<10^5$ K compared to observed galaxies, we find that gas temperature is a good proxy for the presence of outflows. There is a direct correlation between the thermal state of the gas and its state of motion as described by the $σ$-distribution. The following equivalence relations hold in EAGLE: $i)$ low-$σ$ peak $\,\Leftrightarrow\,$ disc of the galaxy $\,\Leftrightarrow\,$ gas with T $<10^5$ K; $ii)$ high-$σ$ tail $\,\Leftrightarrow\,$ galactic winds $\,\Leftrightarrow\,$ gas with T $\ge 10^5$ K.

18 pages, 12 figures, accepted for publication in MNRAS