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

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.

💡 Research Summary

The GECKOS (Generalising Edge‑on galaxies and their Chemical bimodalities, Kinematics, and Outflows) survey exploits the VLT/MUSE integral‑field spectrograph to obtain deep, high‑resolution (≈50–200 pc) data cubes of nine edge‑on galaxies whose stellar masses lie within ±0.35 dex of the Milky Way (≈5 × 10¹⁰ M⊙). The sample spans a wide range of star‑formation rates (SFR ≈ 0.2–8 M⊙ yr⁻¹), allowing a mass‑controlled investigation of how star‑formation activity shapes extraplanar ionised gas.

Observations were carried out between late 2022 and mid‑2023, using 1–3 MUSE pointings per galaxy to cover the major axis out to at least μ = 23.5 mag arcsec⁻². For galaxies with SFR > 2 × the main‑sequence value, the tiling was extended to reach ≥10 kpc above the disk, ensuring that any large‑scale wind would be captured. Data reduction employed the pymusepipe wrapper around the ESO MUSE pipeline, followed by careful sky subtraction, astrometric alignment with DESI‑Legacy or Pan‑STARRS r‑band images, and mosaicking of the individual exposure cubes.

Continuum subtraction was performed with the nGIST pipeline, which fits a library of simple‑stellar‑population (SSP) models using pPXF, masks emission lines, and produces separate continuum‑only and line‑only cubes. Special attention was given to the Hβ and Hα regions (where stellar absorption must be removed) and to the