The GECKOS Survey: Extraplanar ionised gas in star-forming galaxies from eDIG to galaxy-scale winds

Abstract: We map the extraplanar gas, with $\sim$50-200 pc resolution, in nine star-forming galaxies using Multi-Unit Spectroscopic Explorer (MUSE) observations from the GECKOS VLT Large Program targeting edge-on galaxies with similar stellar mass as the Milky Way. The narrow range in stellar mass ($\pm0.35$ dex) of the GECKOS sample makes it ideal for studying trends with star formation rate (SFR). We find strong extraplanar emission reaching $\sim$2-8 kpc from the disk midplane in all targets with $\rm{SFR}\geq$1 M${\odot}$ yr$^{-1}$. Targets with SFR$\,\geq\,$5 M${\odot}$ yr$^{-1}$ have brighter, more extended H$α$ emission compared to lower SFR targets. In high-SFR systems, the gas velocity dispersion ($σ{\rm Hα}$) shows a biconical morphology, consistent with the expectation of outflows. This agrees with previous works suggesting high velocity dispersion in a biconical shape is a good means to identify outflows. We find mixed results using line diagnostics ([OIII]${5007}$/H$β$ - [NII]/H$α$ and $σ_{\rm Hα}$ - [SII]/H$α$) to spatially resolve ionisation mechanisms across the extraplanar gas. The highest [NII]/H$α$ are the extraplanar gas of the highest SFR systems, yet main-sequence galaxies have the highest [OIII]/H$β$. While the morphology of [NII]/H$α$ may be useful to identify outflows, the absolute value of the line ratio alone may not distinguish strong outflows from extraplanar gas of main-sequence galaxies. The ubiquitous extraplanar emission can be interpreted as the result of feedback, in the form of large-scale winds for starbursts or smaller-scale galactic fountains for main-sequence galaxies. Moreover, shock-heating may ionise gas at the interface of the disk and the circumgalactic medium, independent of the source of the gas.